Loss of control

Executive summary

What happened

On the morning of 28 October 2023, a SOCATA-Groupe Aerospatiale TB-20, registered, VH-JTY, departed Montpelier aircraft landing area, Queensland, for a visual flight rules private flight to Palmyra aircraft landing area, Queensland. The flight was to be just over one hour duration and the pilot and their passenger were familiar with the route.

During the flight, the pilot contacted a friend at the destination for an appreciation of the weather. After the friend advised them of the prevailing conditions including cloud, the pilot replied that they would need to go through some cloud before arriving.

Around 30 NM from the destination, shortly after commencing descent for the intended landing, the aircraft began a steep descending turn to the left towards mountainous terrain. During this descent, the aircraft exceeded the airframe’s designed maximum airspeed before pitching up and passing over the top of Bull Mountain. The aircraft then entered a second steep descending turn, this time to the right, before the recorded flight path data ceased.

The wreckage was located nearby in dense forest on the north-east face of Bull Mountain. The accident site indicated that the aircraft had collided with terrain at a steep angle, and with significant forward velocity. The aircraft was destroyed and both occupants received fatal injuries.

What the ATSB found

The ATSB found that, after encountering cloud en route, the pilot elected to continue along the intended flight path through cloud instead of diverting around or remaining on top of it. Shortly after, it is very likely the pilot entered weather conditions not suitable for visual navigation, leading to spatial disorientation and a descent into mountainous terrain.

Safety message

One of the key risk controls for a visual flight rules (VFR) pilot to avoid entering instrument meteorological conditions (IMC) is appropriate pre-flight preparation and planning. Pilots should always obtain up-to-date weather information before and during flight. While forecasts will assist in selecting the route to be flown, pilots should plan an alternate or be prepared to make necessary deviations from the planned route should actual weather conditions indicate the possibility of not being able to comply with the VFR.

For a non-instrument rated pilot, even with basic attitude instrument flying proficiency, maintaining control of an aircraft in IMC by reference to the primary flight instruments alone entails a very high workload that can result in narrowing of attention and loss of situational awareness. While autopilot can be used to reduce workload, it is not infallible and should not be relied upon or used by VFR pilots as a risk mitigator to decide to fly into unsuitable conditions. 

Unapproved mobile devices displaying charts and data from an approved data service provider are a useful supplement to navigation, but they cannot be used as a navigation device, and must not be the sole means of navigation when operating under the VFR. Pilots should use navigation equipment approved for aviation and maintain skills in navigating by reference to approved charts.

The investigation

The occurrence

On 28 October 2023, at about 0735 local time, a SOCATA-Groupe Aerospatiale TB-20 (TB-20) registered VH-JTY, departed from Montpelier (Antill Plains) aircraft landing area,[1] Queensland, for a private flight to Palmyra aircraft landing area,[2] Queensland. The flight was to track via Dalrymple Heights, near Eungella before descending into the Pioneer Valley to the west of Mackay, and then track direct to Palmyra (Figure 1). On board were the pilot and a passenger, who was also a licenced pilot.

Figure 1: VH-JTY fight path

Source: Google Earth and OzRunways, annotated by the ATSB

OzRunways[3] data showed the aircraft was flown to 5,500 ft above mean sea level (AMSL) where it maintained a steady track without any significant deviations during the cruise phase of flight. 

At 0814, approximately 20 minutes prior to commencing descent for the intended landing at Palmyra, the pilot made a phone call to a friend, another licenced pilot, in Mackay. It was reported that the pilot enquired about the weather at the destination. The friend recalled advising of the presence of cloud at Palmyra and blue sky to the south of the landing area. The pilot replied that they would have to go through some cloud. 

The same friend called the pilot at 0833 to confirm their estimated time of arrival so they could meet the pilot in Palmyra. They stated that the pilot reported being over Eungella at 5,500 ft and was about to commence their descent. The friend reported ending the phone call so as not to cause any distraction.

At 08:34, there was a change to the previously stable flight path. The aircraft initially climbed about 100 ft and entered a slight right turn before turning left towards Bull Mountain in a shallow descent. As the turn continued, the aircraft’s descent rate increased, descending about 1,600 ft in 13 seconds (average descent rate of 7,400 ft/min) (Figure 2).  

Figure 2: VH-JTY descent

Source: Google Earth with overlaid OzRunways data, annotated by the ATSB 

During this descent, the aircraft accelerated to about 218 kt, 29 kt above the aircraft’s published V_NE[4] (velocity never exceed) of 189 kt (Figure 3).

Figure 3: TB-20 airspeed limitations

Source: Aircraft manufacturer, annotated by the ATSB

In the 10 seconds that followed, the aircraft pitched up steeply, climbed approximately 600 ft, and passed overhead Bull Mountain at 4,200 ft AMSL. The aircraft then slowed to 132 kt before entering a steep descending right turn to the east. The recorded data stopped at 0835, 3,655 ft AMSL with the aircraft estimated to be descending at around 6,000 ft/min in a right turn with 36° angle of bank and increasing roll angle.

The aircraft collided with terrain in dense forest on the north-east face of Bull Mountain 1,900 ft AMSL, not far from where the recorded data stopped. The aircraft was destroyed, and the pilot and passenger were fatally injured.

Context

Pilot information

The pilot was qualified and authorised to fly VH-JTY. They held a valid Private Pilot Licence (Aeroplane), issued on 4 May 2004. In addition, the pilot held a single engine aeroplane rating, and endorsements for manual propeller pitch control and retractable undercarriage. Their last flight review was conducted in July 2023 and valid to 30 September 2025. At the time of the accident the pilot had about 2,100 hours total aeronautical experience of which about 1,500 hours were in VH-JTY.

The pilot held a valid class 2 medical certificate that was issued on 12 December 2022 and valid to 7 October 2024. The class 2 was issued with the restrictions that both distance and reading vision correction was available during flight. A review of the pilot’s medical records showed that a chronic medical condition was being appropriately managed by the pilot and their designated aviation medical examiner (DAME). It required annual review and there were no additional restrictions placed on the pilot’s class 2 medical certificate.

A copy of the coroner’s toxicology and pathology reports were not available to the ATSB at the time of publishing this report.

Passenger information

The passenger was familiar with VH-JTY. While a licensed pilot, they had not renewed their medical certification for flight and had not conducted a recent flight review. 

Aircraft information

General information

The TB-20 is an all-metal, 5-place, single engine aircraft with fully retractable landing gear. It was powered by a 6-cylinder Lycoming IO-540 fuel-injected engine, driving a 3-blade constant-speed propeller and equipped[5] for visual flight rules (VFR)[6] (Day) flying only. VH-JTY was manufactured in France in 1985 and was first registered in Australia in April 1987. The pilot had owned VH-JTY since April 2008 (Figure 4).

Figure 4: VH-JTY

VH-JTY as it appeared in 2015. The yellow lines and wingtips were later painted dark red.

Source: Simon Coates, modified by the ATSB

Maintenance history

The aircraft was maintained in accordance with the CASA system of maintenance defined in Civil Aviation Regulations 1988 - Schedule 5 and the last periodic inspection was conducted on 14 December 2022. A review of the maintenance records and history for VH-JTY did not show any outstanding defects. The active maintenance release was not recovered from the accident site.

The aircraft was equipped with a Bendix/King KLN 90 GPS navigation system and a Bendix/King KAP 150 2-axis (pitch and roll) autopilot system that had been installed since the aircraft was manufactured. A review of the maintenance history and comments of the approved maintenance organisation showed minor issues with the autopilot consistent with its age. The faults were reported to have resulted in the uncommanded disconnection of the autopilot in flight. The unit was repaired in March 2022 and there were no records of any further maintenance on the autopilot after this date.

In January 2023, a ground handling incident damaged VH-JTY’s vertical fin and rudder. Structural repairs were carried out and following inspection of the flight controls, the aircraft returned to service on 25 May 2023. At the time of the accident, VH-JTY had a total time in service of about 5,233.4 hours and had flown about 35 hours since return to service.

King KAP 150 Autopilot

The pilot information manual supplement for the autopilot did not provide a minimum activation airspeed for the TB-20 but did contain a maximum airspeed limitation of 175 kt for autopilot use. Automatic flight could be activated by pressing the AP ENG (autopilot engage) button on the control panel and disengaged by pressing the AP ENG button again, or by pressing the AP DISC (autopilot disconnect) button on the pilot’s control wheel. When the autopilot is disengaged, an aural alert sounds for 2 seconds to alert the pilot.

The supplement also provided maximum altitude losses that may be encountered following an autopilot malfunction. In a cruise, climb or descent configuration, the maximum altitude loss would be 450 ft.

Recorded data

ADS-B

The aircraft was equipped with a SkyEcho portable automatic dependant surveillance broadcast (ADS-B) antenna capable of receiving information from ADS-B equipped aircraft and transmitting positional information to nearby stations. A review of available flight track history revealed that this antenna did not transmit any positional information during the accident flight.

Mobile devices

There were almost certainly 2 iPads and 2 telephones onboard the aircraft. Queensland Police located and recovered one telephone from the accident site. ATSB data recovery specialists determined that the phone was too heavily damaged to recover the data from the phone’s internal memory. At least 2 mobile devices were running the OzRunways app.

OzRunways data

OzRunways is an electronic flight bag (EFB) provider in Australia that also provides a ‘TX’ service that tracks device location and uploads it to OzRunways servers over the cellular network every 5 seconds. This service displays the device’s location on a user‑selected chart and also plots location data from other aircraft utilising OzRunways. The pilot used the OzRunways application on an iPad for en route planning and navigation. The pilot’s passenger also carried a second iPad with OzRunways when they flew together.  

OzRunways is an approved source of aeronautical charts, but it must not be used as a primary means of navigation. The iPad GPS does not meet technical standard order[7] (TSO) specifications for aviation use (OzRunways, 2022). Additionally, there are limitations to the data provided via OzRunways. Altitude information has a resolution of 100 ft, a change in altitude from 190 ft to 210 ft will be displayed as a change from 100 ft to 200 ft. Additionally, filtering applied to smooth the data can affect the accuracy of analysis of small sections of data. 

Tracking data obtained from OzRunways provided latitude and longitude as well as time, speed and heading. The last data point was at an altitude of about 1,700 ft above ground level in the vicinity of the accident site (Figure 2). When the OzRunways data ended, the aircraft was shown heading east in a steep descending right turn. The lack of any further data points was probably due to a loss of cellular signal among the mountains.

Accident site

The aircraft wreckage was located in heavily vegetated steep mountainous terrain, north-east of Bull Mountain (Figure 5). A Central Queensland Rescue helicopter located the wreckage at 1113 on 28 October and first accessed the site by winch at about 1400 on the day of the accident. The rescue crew confirmed that both occupants were deceased and took preliminary photographs of the wreckage. 

The ATSB was unable to access the accident site. ATSB investigators provided a briefing to Queensland Police Service (QPS) forensic officers prior to them attending site on 27 November. With the support of specialists trained in high angle rescue operations the QPS officers collected evidence and provided it to the ATSB. 

Figure 5: Accident site terrain

Source: Central Queensland Rescue, annotated by the ATSB

Wreckage examination

The ATSB’s review of the accident site photographs, and evidence provided by QPS, showed that the aircraft impacted the terrain at a steep angle while tracking 036° (close to north-east). The aircraft was destroyed by the impact and consumed by a post-impact fuel‑fed fire. Several components were buried by shifting soil after the accident.

Figure 6: Propeller damage

Source: Queensland Police Service, annotated by the ATSB

The following observations were made from the wreckage examination:

• Impact marks on engine and propeller components indicated that the propeller was turning with power applied when it impacted terrain, indicating the engine was almost certainly operational at that time (Figure 6).
• The extremities of the aircraft, the wing tips and stabilator were found at the accident site indicating that the aircraft was complete at the time of collision.

Communications

Police contacted Mackay air traffic control tower following the report of a missing aircraft. The controller confirmed that they did not hear a radio transmission from VH-JTY and that aircraft operating in the area at the time did not hear a distress call from the aircraft. 

Operational information

General

Peers of the pilot reported that the pilot would often climb to operate VFR over the top of cloud. The pilot would generally fly the cruise portion of the flight on autopilot and would use the autopilot if they ever had to fly through cloud. While VFR over the top of cloud is permitted under the VFR, a minimum separation from cloud is required when flying under the VFR (see section Visual meteorological conditions). 

The Civil Aviation Authority of New Zealand safety publication Vector contained an article called Who’s really flying your aircraft? The article discussed the potential downfalls of relying on automation in the cockpit, and its prevalence in VFR into instrument meteorological conditions (IMC) accidents. 

VH-JTY was fitted with an approved GPS. The pilot was not known to program flight plans into the unit, instead relying on OzRunways for navigation. The CASA Visual Flight Guide provided the following note to VFR pilots utilising GPS in their aircraft: 

An approved GNSS system may be used under the VFR: 

• to supplement map reading and other visual navigation techniques

• to derive distance information for enroute navigation and traffic separation.

The positioning and navigational tools featured in the OzRunways application rely on equipment such as mobile phones or iPads that do not meet the required standard for operational use in aircraft. OzRunways acknowledges this limitation and advises users that the OzRunways application shall not be used as a primary means of navigation. It can, however, be used to supplement traditional visual navigation methods such as map reading.

Previous flights

Data obtained for the previous 3 months showed a number of previous flights between the two locations consistent with logged flight records (Figure 7). Each flight took about 1.2 hrs depending on the route taken. 

Flights would often depart Palmyra, tracking inland towards Townsville where they would descend outside the Townsville control zone steps and land at Montpelier Airfield. On return to Palmyra, the pilot routinely flew inland past Collinsville and would descend down the Pioneer Valley approaching Palmyra from the west below the Mackay control zone steps.

Figure 7: Previous flight data

Source: Google Earth and OzRunways, annotated by the ATSB

Selection of route

The pilot and their passenger were familiar with the route between Montpelier and Palmyra, completing the return journey as often as once a fortnight for the last few years. When they considered the weather unsuitable for flight, they would make the trip by car, and had a vehicle at their disposal in both locations.

The pilot was known to plan routes that were outside controlled airspace (Figure 8). Witnesses interviewed advised this was common practice among local pilots due to the potential of encountering delays in access to controlled airspace in that region.

Figure 8: Intended flight path

Source: Visual Navigation Chart, annotated by the ATSB

Visual meteorological conditions

Visual meteorological conditions (VMC) are expressed in terms of in-flight visibility and distance from cloud (horizontal and vertical) and are prescribed in the Civil Aviation Safety Regulations (CASR) Part 91 (General Operating and Flight Rules) Manual of Standards 2020: 2.07 VMC criteria. A VFR flight can be conducted above cloud provided VMC can be maintained for the entire flight, including climb, cruise, and descent.[8] The CASA Visual Flight Rules Guide included the following notes for VFR flight: 

Pilots should not initiate VFR flight on top of more than SCT [scattered][9] cloud when weather conditions are marginal. Before committing to operate VFR flight on top of more than SCT cloud, pilots should be confident that meteorological information used is reliable and current, and clearly indicates that the entire flight will be able to be conducted in VMC. 

And:

When navigating by visual reference to the ground or water, you must positively fix the aircraft’s position by visual reference to features marked on topographical charts at intervals not exceeding 30 minutes.

Figure 9, taken from the CASA Visual Flight Rules Guide, provides a visual depiction of the VMC criteria for aeroplanes below 10,000 ft.

Figure 9: VMC criteria below 10,000 ft

Source: Civil Aviation Safety Authority

Weather reports obtained by the pilot

According to CASR Part 91 (General Operating and Flight Rules) Manual of Standards 2020: 7.02 Forecasts for flight planning, an authorised weather forecast must cover the whole period of the flight, and include a wind and temperature forecast and, for a flight at or below 10,000 ft AMSL, a general aviation meteorological area forecast (GAF).

Airservices Australia conducted a review of the pilot’s national aeronautical information processing system (NAIPS)[10] account activity. Two accounts were linked to the pilot. On the day of the flight, one account had expired, and the other had not been accessed since a password update 2 months earlier. Weather requests through OzRunways use the user’s NAIPS login credentials to retrieve the requested information. A third account, linked to the passenger, was last accessed through OzRunways 5 days prior to the flight to obtain an area briefing.

While other third party information such as weather radar overlays, satellite imagery and wind observations are available without an NAIPS login,[11] NAIPS is the only source through which OzRunways obtains weather forecasts for flight planning. 

Notwithstanding the requirement of a forecast for flight planning, CASR Part 91 (General Operating and Flight Rules) Manual of Standards 2020: 7.03 Flights unable to obtain an authorised weather forecast before departure states that a flight can still depart without an authorised forecast provided other conditions are met. These include:

The pilot in command reasonably considers that the weather conditions at the departure aerodrome will permit the aircraft to return and land safely at the departure aerodrome within 1 hour after take-off.

The pilot in command of a Part 91 flight must return to the departure aerodrome if:

(a) the authorised weather forecast required for the planned destination aerodrome is not obtained within 30 minutes after take-off; and

(b) the pilot in command has not nominated a destination alternate aerodrome if required to do so by subsection 8.04 (3).[12]

CASA defined what constituted an approved weather report and a list of who could provide one.[13] Licenced pilots were included in the list. The pilot obtained a weather update for the destination from their friend en route. While the accuracy of the information could not be verified, as a licenced pilot, the weather report provided by the pilot’s friend would have been considered an approved weather report for the destination. Based on the pilot’s phone records, this report was obtained about 39 minutes after departure.

Meteorological information

Forecast weather

The planned flight from Montpelier to Palmyra was within the QLD north (QLD-N) Graphical Area Forecast (GAF)[14] region. Forecast weather conditions in the GAF, valid from 0300 to 0900 on 28 October 2023, included average conditions of greater than 10 km visibility with areas of scattered stratocumulus clouds between 2,000 and 4,000 ft. The cloud was forecast to lift and become scattered cumulus and stratocumulus between 4,000 and 8,000 ft with moderate turbulence below 8,000 ft in thermals later in the day.

The Bureau of Meteorology aerodrome forecast (TAF)[15] for Mackay Airport, issued at 0435 and valid at the time of the accident showed an expected visibility of greater than 10 km, few cloud at 3,000 ft and a wind of 17 kt from the south-east.

Based on the forecasts, local pilots interviewed by the ATSB described the conditions as being conducive to mechanical turbulence around the ranges.

Actual weather

The meteorological aerodrome report (METAR)[16] for Mackay Airport reported wind from the east at 17 kt (100°), visibility greater than 10 km and cloud scattered at 3,300 ft and broken[17] at 5,500 ft. There was no rainfall recorded in the previous 24 hours.

The ATSB requested an assessment of the weather from the Bureau of Meteorology who provided the following observations for the morning of the accident: 

• Cloud extended from the coast to the ranges and was clear west of ranges.

• South easterly winds persisted which often bring cloud between 2,000 – 3,000 ft, being lower earlier in the day and climbing as the day warms up.

• Over the accident site, cloud was overcast to broken.

• Cloud extended 25 NM south of accident site.

• Eungella Dam was visible to the west on satellite images.

• Closer to the coast cloud was scattered to broken. 

Bureau of Meteorology satellite images showed cloud building in the Pioneer Valley east of the ranges from 0600, covering the mountains to the north of the Pioneer Valley by 0800. Figure 10 shows cloud cover at 0830, 3 minutes before the accident.

Figure 10: Satellite image showing cloud formation on 28 October at 0830 local time

Source: Source: Bureau of Meteorology, annotated by the ATSB

Figure 11 shows the location of witnesses and CCTV recorded at the time of the accident.

• Eyewitness 1 located near Bull Mountain heard a low flying aircraft that stopped suddenly. They stated that the top of the mountain was visible around the time of the accident.
• Eyewitness 2 described the cloud as being very thick that morning from when they woke at 0500. The cloud started to rise around 0800 but still produced limited visibility of their paddock until around 1000. They also described a ‘good breeze’ on the ground during the morning with occasional gusts but did not recall strong winds on the day. CCTV footage from that location showed low cloud close to where the pilot commenced their descent (Figure 12).
• CCTV recording 1 obtained from a business 5 km south-west of the accident site showing diffuse light indicating cloud cover at the time of the accident.
• CCTV recording 2 facing north from an elevated position at the head of the valley. That footage showed low cloud to the east and tree movement equivalent to a fresh breeze. 

Additionally, a large bushfire around 70 km to the south-east had filled the Pioneer Valley with smoke which was yet to dissipate. The pilot of the rescue helicopter advised that during the search for VH-JTY at around 1000, the cloud base was scattered at 3,000 ft with poor visibility in smoke haze. 

Figure 11: Witness locations

Source: Google Earth, annotated by the ATSB

Figure 12: Closed circuit footage from Dalrymple Heights

Source: Supplied

Decision making

Flight under the VFR requires minimum conditions of visibility and distance from cloud. Variation from the expected weather conditions en route may prevent a pilot from reaching their destination under this ruleset. Flying into instrument meteorological conditions[18] (IMC) can occur in any phase of flight. However, a 2005 ATSB research publication – General Aviation Pilot Behaviours in the Face of Adverse Weather (B2005/0127) – concluded that the chances of a VFR into IMC encounter increased as the flight progressed, with the maximum chance occurring during the final 20 per cent of the planned flight. It stated:

This pattern suggests an increasing tendency on the part of pilots to ‘press on’ as they near their goal. To turn back or divert when the destination seemed ever closer became progressively more difficult.

The CASA Resource Booklet 7 Decision making contained the following:

A non-instrument rated pilot who proceeds with a flight in marginal weather and ends up in instrument meteorological conditions (IMC) decides to firstly, proceed with the flight and secondly, not turn back when the weather indicated visual flight rules were not able to be maintained

Dejoy (1992, cited in Hunter, 2002) suggests that a person’s propensity to engage in risky behaviour is the result of lower perceived risk in the outcome. Studies have shown that pilots who do not perceive the risks with adverse weather are more likely to engage in higher risk activities when dealing with weather (Cooper, 2003). 

A Transportation Safety Board of Canada report A23O0028 into a VFR into IMC accident looked at pilot decision making and the acceptance of unsafe practices.

Pilot decision making is a cognitive process used to select a course of action between alternatives. Several factors, circumstances, and biases can affect pilot decision making, including the flight objective or goal, and the pilot’s knowledge, experience, and training. These factors can lead to situations where pilots might prioritize the achievement of the goal over the management of threats, likely resulting in a reduced safety margin.

A focus on achieving a goal or outcome may lead to a reduced sensitivity to risk, especially when high-risk activities repeatedly result in no negative outcomes. Flight crew members may grow accustomed to these risks, altering their perception and acceptance of such risks over time (Hollenbeck and others 1994).

Spatial awareness

The ATSB publication Avoidable Accidents No. 4: Accidents involving Visual Flight Rules pilots in Instrument Meteorological Conditions (AR-2011-050) discusses the physiological limitations of the human body when trying to sense its orientation in space. 

In conditions where visual cues are poor or absent, such as in poor weather, up to 80 per cent of the normal orientation information is missing. Humans are then forced to rely on the remaining 20 per cent, which is split equally between the vestibular system and the somatic system. Both of these senses are prone to powerful illusions and misinterpretation in the absence of visual references, which can quickly become overpowering.

Pilots can rapidly become spatially disoriented when they cannot see the horizon. The brain receives conflicting or ambiguous information from the sensory systems, resulting in a state of confusion that can rapidly lead to incorrect control inputs and resultant loss of aircraft control.

For non-instrument rated pilots, statistics show they may not be able to recover at all. Research has shown the pilots not proficient in maintaining control of an aircraft with sole reference to the flight instruments will typically become spatially disoriented and lose control of the aircraft within 1 to 3 minutes after visual cues are lost.

The FAA Advisory Circular FAA AC60-4A Pilot’s spatial disorientation discussed the challenges associated with recovering from spatial disorientation. The results of a test conducted with qualified instrument pilots found that it took as much as 35 seconds to establish full control by instruments after the loss of visual reference with the ground or surface. 

The ATSB report AR-2011-050 was updated in 2019 and found that in the 10 years prior, there were 101 VFR into IMC occurrences in Australian airspace reported to the ATSB. Of these, 9 were accidents resulting in 21 fatalities. An almost 10% chance of the encounter ending in a fatal accident. 

A search of the ATSB Aviation Occurrence Database shows that in the 5 years since 2019, there have been 56 VFR into IMC occurrences reported to the ATSB. Of these, 10 resulted in accidents with 16 fatalities. The dangers of spatial disorientation following a loss of visual cues remains one of the most significant causes of concern in aviation safety. 

Similar Occurrences

The risks of visual flight rules (VFR) pilots flying from visual meteorological conditions (VMC) into instrument meteorological conditions (IMC) are well documented, and have been the focus of numerous ATSB reports and publications. VFR pilots flying into IMC is a significant cause of aircraft accidents and fatalities.

AO-2022-016 – VFR into IMC, loss of control and collision with terrain involving Airbus Helicopters EC130 T2, VH-XWD, near Mount Disappointment, Victoria, on 31 March 2022

On 31 March 2022, at about 0741 local time, 2 Microflite Airbus EC130 helicopters, registered VH‑WVV and VH-XWD, departed the Batman Park helicopter landing site in Melbourne, for the town of Ulupna, Victoria. Both helicopters were operated in accordance with the VFR and departed in VMC conditions. Cloud was forecast along the route, but the pilots elected to continue to the destination. The helicopters encountered instrument meteorological conditions (IMC) over Mount Disappointment and VH-WVV conducted a U-turn to avoid entering cloud. While also attempting to conduct a U-turn, VH-XWD entered cloud, developed a high rate of descent, and collided with terrain. The helicopter was destroyed, and the 5 occupants were fatally injured.

AO-2021-017 – VFR into IMC and in-flight break-up involving Van's Aircraft RV-7A, VH-XWI 90 km south of Charters Towers, Queensland, on 23 April 2021

On 23 April 2021, a Van’s Aircraft RV-7A, registered VH-XWI, was being operated on a private flight under the visual flight rules (VFR) from Winton to Bowen, Queensland. During the flight, the pilot most likely entered IMC and lost control of the aircraft several times. This led to the airspeed limitations for the aircraft being exceeded and the aircraft sustained an in-flight break-up. The pilot was fatally injured, and the aircraft was destroyed.

VFR into IMC resources

The 2011 ATSB publication, Accidents involving Visual Flight Rules pilots in Instrument Meteorological Conditions, updated in 2019, includes a selection of weather-related general aviation accidents and incidents that show weather alone is never the only factor affecting pilot decisions that result in inadvertent IMC encounters. The documented investigations consistently highlight that conducting thorough pre-flight planning is the best defence against flying into deteriorating weather.

CASA also released a collection of resources related to this type of occurrence on its website titled Preventing VFR into IMC and other related resources on its pilot safety hub under Weather and forecasting.

For more information on VFR into IMC occurrences, recognising inadvertent entry into IMC, and what to do to recover, refer to the following publications: 

• Civil Aviation Authority United Kingdom: Safety sense booklet VFR flight into IMC
• Aircraft Owners and Pilots Association: Encountering IMC

Safety analysis

Data collected by the OzRunways electronic flight bag (EFB) application indicated that shortly after commencing descent from 5,500 ft, VH-JTY made a series of turns that displayed excessive sink and climb rates before colliding with terrain. This manoeuvring indicates that the autopilot was not being used during this part of the flight. Site and wreckage examination indicated that the aircraft was complete, had significant forward velocity, a high angle of entry and the engine was producing power. Those items of evidence indicated that the aircraft was most likely in an uncontrolled state when it collided with terrain.

The analysis considers the limited evidence available and discusses possible explanations for the departure from controlled flight:

• pilot incapacitation
• technical failure or malfunction
• decision making
• spatial disorientation.

Pilot incapacitation

Pilot incapacitation following a medical event was considered unlikely based on the short timeframe between ending the phone conversation prior to descent and the apparent departure from controlled flight. Medical records indicated that the pilot’s pre-existing condition was being appropriately managed and in the absence of any additional evidence, that was excluded. 

Additionally, the passenger was also a pilot, although not current at the time, and they would have been capable of assuming control if the pilot had a medical episode. 

Technical malfunction

Structural failure was considered as a possible explanation for the departure from controlled flight. The propeller, wing tips, stabilator, and vertical stabiliser/rudder were all located in site photographs. This was significant in determining that the recent repair to the vertical stabiliser had not failed. With the main components identified at the accident site, the possibility of an in-flight break‑up was excluded. Further, the damage to the engine and propeller indicated the engine was producing power at the time of the accident, eliminating engine malfunction as a possible explanation for the rapid descent. 

The autopilot was installed in the aircraft from new and the pilot had owned the aircraft for close to 15 years. The pilot was almost certainly familiar with basic operation of the autopilot. The design of the autopilot servos that moved flight control surfaces incorporated a clutch so a pilot could override the autopilot in the event of a malfunction. The unit also incorporated a design whereby the autopilot would disengage if excessive pitch or roll rates were encountered. The autopilot had been repaired almost a year prior with no record of any additional maintenance, and the nature of the previous autopilot failures would not induce a loss of control. 

If a technical malfunction occurred that would have affected the immediate safe continuation of the flight, it is very likely the pilot would have made a radio call declaring an emergency. No radio call was detected.

With no evidence of a failure or malfunction that would have induced uncommanded control inputs, it is considered unlikely that the autopilot contributed to a loss of control. 

Pilot decision making

After excluding pilot incapacitation and technical failure, the ATSB considered pilot decision making and the flight path through cloud during the final minutes of the flight.

There was no record that the pilot obtained relevant aviation weather forecasts prior to the flight, and it could not be determined if additional weather information was obtained from other non‑approved sources. However, as the flight was a relatively short distance, and the pilot was very familiar with the route and destination, it is almost certain that they had knowledge and experience of weather behaviour and conditions to be expected. Additionally, the weather report obtained over the telephone while en route provided an accurate assessment of the prevailing conditions along the intended flight path.

Weather observations showed that at the cruising altitude of 5,500 ft, VH-JTY would have been in VMC above cloud before commencing descent. While there was variation in the witness reports concerning the lower level of the cloud, evidence showed there was significant cloud present, and that the pilot planned to pass through it. Witness statements related that it was common practice for the accident pilot to intentionally fly through cloud with the autopilot on. While this action would represent intentional non-compliance with aviation regulations, the main advantage of doing so would be to avoid loss of control following loss of visual references. The hazard being that if the autopilot or any of its input sensors failed or were inadvertently disengaged, loss of control would reasonably ensue.

This behaviour without previous consequence may have affected their perception of the associated risk of continuing through cloud and influenced their decision to continue along the intended flight path instead of diverting around the cloud en route (Cooper, 2003). Personal bias and misperception of the risks associated with VFR flight into IMC (Hollenbeck and others, 1994) is a factor frequently seen in aircraft accidents (Hunter, 2002). 

The Bureau of Meteorology assessment of the conditions and analysis of satellite imagery matched the report of the pilot’s friend on conditions at the destination. While cloud existed around to the east of Mackay and around the mountains of the Pioneer Valley, the west and south of the destination were clear of cloud. It was determined that in this context, it is highly likely that the pilot chose to fly through cloud rather than continue visual flight over the top of cloud, divert around weather, or plan flight through controlled airspace.

Spatial disorientation

The flight path from Montpelier to top of descent was compared to previous flights. It was determined to be consistent with the way the pilot would normally conduct cross-country flights in VH-JTY and was almost certainly being flown on autopilot during cruise. The sudden change in flight path prior to descent indicated disconnection of the autopilot and resumption of hand flying. 

The weather on the day of the flight was conducive to poor or absent visual cues and turbulence, factors which are known to contribute to spatial disorientation. The pilot was not trained or experienced in flying in low visibility. In these conditions, the pilot would have been required to reference the aircraft’s flight instruments to maintain control. In this case the aircraft was not equipped with the instruments required for flight in IMC.

The accident site showed that the aircraft collided with terrain tracking north-east, indicating that the turn to the right continued after the flight path recording ended. The instability of the flight path with excessive rates of descent and climb are markers commonly observed in spatial disorientation occurrences where pilots are aware of a departure from controlled flight and attempt to correct the unusual flight attitude. 

Due to the limited information available, it is not known whether the autopilot disconnect was intentional or why the pilot’s reported strategy of using autopilot when flying through cloud failed on this occasion. Following the commencement of the turn to the left, the significant deviation of pitch attitude during the turn was likely unintentional. The high rate of descent was consistent with the pilot becoming spatially disoriented after flying into weather conditions not suitable for visual navigation. As a result, the aircraft collided with terrain. 

Findings

From the evidence available, the following findings are made with respect to the VFR into IMC, loss of control and collision with terrain involving SOCATA-Groupe Aerospatiale TB-20, VH‑JTY, 65 km west of Mackay Airport, Queensland, on 28 October, 2023.

Contributing factors

• The pilot made the decision to descend through cloud rather than remain VFR over the top or divert around weather.
• The visual flight rules pilot very likely entered weather conditions not suitable for visual navigation, leading to spatial disorientation and collision with terrain.

Sources and submissions

Sources of information

The sources of information during the investigation included the:

• Queensland Police Service
• accident witnesses
• recorded data from the OzRunways navigation application
• CCTV video footage and other photographs taken on the day of the accident
• maintenance organisation for VH-JTY
• aircraft manufacturer
• Civil Aviation Safety Authority
• Airservices Australia
• Bureau of Meteorology.

References

Australian Transport Safety Bureau. (2005). General Aviation Pilot Behaviours in the Face of Adverse Weather. Aviation Research Investigation Report B2005/0127. 

Australian Transport Safety Bureau. (2007). An overview of spatial disorientation as a factor in aviation accidents and incidents.ATSB Aviation Research and Analysis Report B2007/0063.

Australian Transport Safety Bureau. (2011). Accidents involving Visual Flight Rules pilots in Instrument Meteorological Conditions.Aviation Research Investigation Report AR-2011-050.

Civil Aviation Authority of New Zealand. (2023) Vector: Who’s really flying your aircraft? Civil Aviation Authority of New Zealand.

Civil Aviation Safety Authority. (2019). Safety behaviours: human factors for pilots: Resource booklet 7 Decision making (2nd edition)

Cooper D. (2003). Psychology, Risk and Safety: Understanding how personality & perception can influence risk taking. Professional Safety. Journal of the American Society of Safety Engineers, November 2003, 39-46.

Federal Aviation Authority. (1983). Advisory Circular AC60-4A: Pilot’s spatial disorientation.

Hunter DR. (2002), Risk Perception and Risk Tolerance in Aircraft Pilots. Federal Aviation Administration, DOT/FAA/AM-02/17, 2002.

Hollenbeck, J. Ilgen, D. Phillips, J. Hedlund J. (1994) Decision risk in dynamic two-stage contexts: beyond the status quo. Journal of Applied Psychology, Vol. 79, Issue 4, pp. 592–598. 

OzRunways. (2022). How can OzRunways be used for navigation? On-line, Retrieved 19 August

UK Civil Aviation Authority. (2024). Safety Sense: VFR flight into IMC. UK Civil Aviation Authority

Submissions

Under section 26 of the Transport Safety Investigation Act 2003, the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. That section allows a person receiving a draft report to make submissions to the ATSB about the draft report. 

A draft of this report was provided to the following directly involved parties:

• maintenance organisation for VH-JTY
• aircraft manufacturer
• Civil Aviation Safety Authority
• Airservices Australia
• Bureau of Meteorology
• OzRunways.

Submissions were received from:

• the aircraft manufacturer.

The submissions were reviewed and, where considered appropriate, the text of the report was amended accordingly.

Purpose of safety investigations The objective of a safety investigation is to enhance transport safety. This is done through: identifying safety issues and facilitating safety action to address those issues providing information about occurrences and their associated safety factors to facilitate learning within the transport industry. It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action. Terminology An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue. Publishing information  Released in accordance with section 25 of the Transport Safety Investigation Act 2003 Published by: Australian Transport Safety Bureau © Commonwealth of Australia 2024 Ownership of intellectual property rights in this publication Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia. Creative Commons licence With the exception of the Commonwealth Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this report is licensed under a Creative Commons Attribution 4.0 International licence. The CC BY 4.0 licence enables you to distribute, remix, adapt, and build upon our material in any medium or format, so long as attribution is given to the Australian Transport Safety Bureau.  Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.

Investigation summary

What happened

On 6 October 2023, a Cirrus Design Corporation SR22 aircraft, registered VH-MSF, was being operated on a private instrument flight rules flight from Canberra, Australian Capital Territory, to Armidale, New South Wales. On board the aircraft were the pilot and 3 passengers.

About 12 minutes after take-off, at an altitude approaching 10,000 ft above mean sea level, the aircraft aerodynamically stalled, departed from controlled flight, entered a high vertical descent developing into a spin, and impacted with terrain. All occupants were fatally injured, and the aircraft was destroyed by a post‑impact fire. 

What the ATSB found

The ATSB found that the flight track data showed that, at about 8,000 ft, the aircraft had begun to deviate from its flight track, with heading, altitude and airspeed deviations. Those deviations coincided with reports from ear witnesses located below the aircraft’s flight path of sounds consistent with engine surging. 

The data also showed that the aircraft had a high rate of climb (up to 1,500 ft/min) coupled with a low and decreasing airspeed, which led to an aerodynamic stall and rapid descent. Recovery actions from the aerodynamic stall did not occur and the Cirrus aircraft parachute system was not deployed in-flight. It was also noted that no radio calls were received from the pilot to indicate there was a problem prior to the stall.

VH-MSF was not fitted with an anti-icing system and was prohibited from operating in icing conditions. Moderate icing conditions were forecast along the aircraft’s flight path from 7,000 ft to 10,000 ft when in cloud. It was likely that the aircraft had encountered icing conditions prior to the aerodynamic stall. However, the ATSB was unable to determine if these conditions were sufficient to have adversely affected the aircraft’s performance and/or handling.

The ATSB considered several scenarios to establish the reason for the deviations in flight track, subsequent stall and absence of recovery actions. These included in-flight icing, pilot incapacitation and possible aircraft issues. However, due in part to a significant post-impact fire, which limited the collection of evidence, the circumstances preceding the stall and impact with terrain could not be determined. 

Safety message

Although it could not be established that icing contributed to the accident, operating in these conditions in aircraft that are prohibited from doing so increases the risk of a loss of control event leading to an accident. Aircraft flying through cloud in sub-freezing temperatures are likely to experience some degree of icing. A pilot can reduce the chance of icing becoming an issue by selecting appropriate flight routes, remaining alert to the possibility of ice formation and knowing how and when to operate de-icing and anti-icing equipment if fitted.

The occurrence

Accident flight details

On 3 October 2023, a Cirrus Design Corporation SR22 aircraft, registered VH-MSF, was operated on a private flight from Redcliffe, Queensland, to Armidale, New South Wales, and then on to Canberra, Australian Capital Territory, the following day.  

On 6 October 2023, the return sectors were planned to operate from Canberra to Armidale, with a planned return to Redcliffe. On board the aircraft were the pilot and 3 passengers. 

At about 0648 local time, the pilot submitted an instrument flight rules[1] flight plan to Airservices Australia to fly from Canberra to Armidale with an estimated departure time of 1430. The flight planned track was via waypoint[2] ‘CULIN’ (about 31 km west of Goulburn) and Scone, New South Wales, at a cruising altitude of 10,000 ft above mean sea level (AMSL), using RNP 2[3] navigation performance. The lowest safe altitude from Canberra to CULIN was 4,600 ft. While there was no published instrument flight rules route from CULIN to Scone and from Scone to Armidale, the pilot’s flight planning software application provided a lowest safe altitude of 6,000 ft from CULIN to Scone and 6,300 ft from Scone to Armidale using RNP 2 performance. 

On contact with Canberra ground air traffic control at 1422, the pilot was provided an airways clearance to track to Armidale via their flight planned route at 10,000 ft.

At about 1437, the aircraft departed Canberra Airport. Soon after take-off, the pilot was transferred to, and established radio communication with, the approach controller, reporting that they were on climb through 3,400 ft (to their assigned cruise altitude) and turning left onto their assigned radar heading of 070°. A short time later, the controller instructed the pilot to turn left onto a heading of 010° and the pilot completed readback of the instruction. About 1 minute 30 seconds later (at about 1442), the controller cleared the pilot to resume their own navigation and track direct to waypoint CULIN. The pilot completed readback of that instruction, which was the last transmission received from the pilot. All transmissions made by the pilot were clear and concise. Flight data showed that the aircraft turned 5° to the left of the direct track to waypoint CULIN. 

During the flight, data was being transmitted by the aircraft’s automatic dependent surveillance broadcast (ADS-B) equipment.[4] A review of that data indicated that the aircraft was climbing through about 7,000 ft AMSL as it turned to track towards CULIN. During that turn, the ground speed increased, over a period of about 30 seconds, from about 110 kt (204 km/h) to 135 kt (250 km/h).

Climbing above 7,500 ft, the data indicated the aircraft’s ground speed had started to reduce, at an approximately linear rate, with a reduction of about 22 kt (41 km/h) over a 65‑second period. At that time, the data showed a relatively constant rate of climb generally between 550–750 ft/min.

Passing through 8,300 ft, the somewhat linear flight track altered to an onset of heading, altitude and airspeed variations. The ADS-B data indicated the ground speed then started to increase as the aircraft entered a slight descent. Over the next 4 minutes, the aircraft’s track varied up to 35° and the ground speed fluctuated between 93 kt and 121 kt (172–224 km/h). During this period, the altitude was generally increasing, although at a varying rate, with shorter periods where the altitude and reported rate of altitude change indicated that the aircraft had started to descend. 

The ADS-B data showed that, at about 12 minutes into the flight, the aircraft descended by about 250 ft, increased speed by about 13 kts and then climbed at a rate up to about 1,500 ft/min. While in that climb, the airspeed reduced significantly and from a calculated pressure altitude of 9,946 ft, at 1448:37, the aircraft departed controlled flight and descended rapidly towards the ground. For more details on the aircraft’s movements refer to the Recorded information section. About 44 seconds after the onset of the departure from controlled flight, the aircraft collided with terrain (at a ground elevation of about 2,250 ft) and was destroyed by impact forces and a post-impact fire. All occupants were fatally injured. An eyewitness was the first responder on the scene and notified emergency services. 

Figure 1 illustrates the ground track of the aircraft departing Canberra while assigned radar vectors and the direct track to CULIN. 

Figure 1: Ground track of VH-MSF (in blue) from take-off to the accident site

The aircraft ground track overlaid on this map is referenced to a latitude and longitude grid aligned to true north. The headings assigned by air traffic control are referenced to magnetic north. In the Canberra region, magnetic north is about 12° less than true north. An aircraft’s ground track relevant to the assigned heading can also be affected by wind. Source: OpenStreetMap with ADS-B data from Airservices Australia and aggregated ADS-B data from FlyRealTraffic.com, annotated by the ATSB

Witness observations

Figure 2 and Table 1 show the witness locations and observations along the aircraft’s flight path; earwitnesses reported hearing aircraft engine noises and 2 eyewitnesses reported seeing the aircraft in its final moments before the impact with terrain.

Four independent ear witnesses (1 through 4 in Table 1) in the local area where the aircraft was climbing through about 8,000 ft described hearing a rough running or surging (revs increasing and decreasing) light aircraft engine, which was likely to be VH-MSF. Another 2 earwitnesses located closer to the accident site reported hearing varying engine sounds (5 and 8 in Table 1).

Two eyewitnesses (6 and 7 in Table 1) in the local area of the accident site described seeing the aircraft at a low altitude, descending rapidly with its nose pitched down and rotating like a corkscrew (spiral descent). One of these witnesses stated that they heard the aircraft approaching with the engine noise fluctuating[5] and the engine running during the descent, but went quiet just before impact. The other eyewitness was seated on a tractor with the engine running and did not hear the aircraft engine. 

Table 1: Ear and eyewitnesses summaries with reference to Figure 2

Figure 2 shows the aircraft flight track, and the location of each ear/eyewitnesses summarised in Table 1.

Figure 2: Aircraft flight path with ear and eyewitnesses’ locations

Source: Google Earth, with ADS-B data from Airservices Australia, annotated by the ATSB

Context

Pilot information

The pilot held a valid private pilot licence (aeroplane), issued in 1985 (re-issued as a Civil Aviation Safety Regulations Part 61 licence in August 2016), and class ratings for single‑ and multi‑engine aeroplanes. The pilot was initially issued with a command instrument rating for single‑engine aeroplanes in 1987 and their most recent flight review, on 29 August 2023, was an instrument rating proficiency check with an endorsement for multi-engine aeroplanes. 

Insurance documentation indicated that the pilot had accumulated about 800 hours total flying experience, with about 180 hours in Cirrus SR22 aircraft, including 12.5 hours in VH‑MSF. The owner of VH-MSF had conducted several flights with the pilot and described them as being a good and careful pilot with no problems entering cloud and utilising the instruments. 

Aircraft information

General information

VH-MSF was a Cirrus Design Corporation SR22 low-wing aircraft with 4 seats and a fuel‑injected piston engine driving a constant speed 3‑blade propeller. It had a ballistic parachute system (Cirrus airframe parachute system – CAPS) fitted as standard. The aircraft was fitted with a cabin and windshield heating system, which utilised warm air ducted from the engine exhaust shroud.

The aircraft (S/N 0153) was manufactured in the United States in 2002 as a G1 model. It was purchased by the owner in the United States and first placed on the Australian aircraft register in 2017. It was issued a standard certificate of airworthiness in the normal category. Since then, it had been operated by its owner for private use, community service flights and had been leased to other private pilots (Figure 3).

Figure 3: Cirrus Design Corporation SR22, registered VH-MSF

Source: Aircraft owner

Aircraft maintenance

The current maintenance release was on board the aircraft and was destroyed. A carbon copy of the maintenance release was provided by the aircraft maintainer. It showed that the required 100‑hour/annual inspection was conducted and a maintenance release was issued on 9 November 2022 at an aircraft time-in-service of 2,558.9 flight hours. Inspection of the maintenance release copy, aircraft logbook and worksheet records showed there were 2 items of maintenance that were past their calendar due date. They were a standby compass calibration and an outside air temperature/clock back-up battery replacement, both due about 2 months before the accident. The aircraft owner advised that the overdue maintenance items were an oversight. 

The aircraft was certified for use in the private category and for instrument flight rules (IFR) operations. Maintenance documentation showed that the CAPS was inspected, and the parachute and rocket motor assemblies were replaced due to time expiry in January 2023. At the time of the accident flight, the airframe, engine and propeller had accumulated the following hours:

• airframe – 2,635.5 hours total time-in-service
• engine – 1,192.0 hours’ time since overhaul
• propeller – 133.3 hours’ time since overhaul. 

It was reported that, in September 2023, the aircraft was hard to start, and the starter motor was replaced, which resolved the issue. The aircraft owner also advised that, in the days prior to the accident flight the pilot had reported that the power lever was stiff to operate. In response to this, the owner checked the lever and determined that the reason for the stiffness was due to the friction adjustment, which had been wound up to a high friction setting. Following readjustment, when the friction was wound off, the power lever was free to move with no issues. 

Two and 3 days prior to the accident, post-flight at Armidale and Canberra, the pilot had reported to their family and the owner that the aircraft had operated with no issues. The ATSB did not identify any maintenance issues that may have contributed to the accident. 

Flight instrumentation

There were 6 primary flight instruments fitted to the aircraft. They were the airspeed indicator, attitude indicator, altitude indicator, turn coordinator, heading indicator, and vertical speed indicator (Figure 4). The aircraft was fitted with a Sandia SAI-340 model attitude indicator, which also incorporated airspeed, altitude and slip indicators. The unit contained a rechargeable battery capable of providing continued operation in the event of aircraft electrical failure. Maintenance documentation showed that the attitude indicator had been replaced in 2018 with a repaired model that had a software upgrade to include the addition of a vertical speed function. 

Figure 4: VH-MSF flight instruments with Sandia attitude indicator top centre

Source: Aircraft owner

The United States Federal Aviation Administration issued an emergency airworthiness directive (AD), AD‑2020‑18‑51, for Sandia attitude indicators in 2020. That directive stated the applicability as being for Sandia attitude indicator part number 306171‑10 and 306171‑20. These attitude indicators may be marked as Bendix King Model KI‑300 or Sandia Model SAI‑340A. They may be installed on aircraft certificated in any category. It also stated that the AD was prompted by reports of 54 failed attitude indicators, which produced erroneous attitude data to the pilot and autopilot, if equipped. The FAA issued the AD to prevent aeronautical decision-making based on erroneous attitude information, which may result in loss of control of the aircraft.

The attitude indicator fitted to VH-MSF was the Sandia SAI-340, part number 306171-00, which was outside the applicability of the emergency AD. Further, the aircraft owner reported that the attitude indicator had not had any issues since it was installed in 2018.

Electric trim control system 

The aircraft was fitted with an electric pitch, roll and rudder trim system. Electric trim buttons for pitch and roll were located on the top of each control yoke, while the rudder trim switch was mounted in the console next to the wing flap control switch. 

The SR22 Pilot’s Operating Handbook (POH) stated that the pitch trim could be controlled by manually moving the switch forward, which would initiate nose‑down trim and moving the switch aft would initiate nose-up trim. This occurred via an electric motor, which changed the neutral position of the spring cartridge attached to the elevator control horn. The pitch trim also provided a secondary means of pitch control in the event of a primary pitch control failure not involving a jammed elevator. The electric pitch trim was used by the autopilot.

For roll trim, moving the switch left would initiate a left-wing-down trim and moving the switch right would initiate a right-wing-down trim. The trim also provided a secondary means of roll control in the event of a failure with the primary roll control system not involving jammed ailerons. The electric roll trim was also used by the autopilot. The rudder trim was not connected to the autopilot.

Autopilot

The aircraft was equipped with a 2-axis (pitch and roll) S-TEC-55X autopilot system that received roll axis control inputs from an integral electric turn coordinator and altitude information from an altitude transducer connected to the pitot-static system. A multifunction control panel was fitted above the altitude indicator, which provided mode selection, disengage, and turn command functions. The autopilot controller or a button on each control yoke handle could be used to disengage the autopilot. The autopilot features included:

• roll stabilisation
• turn command
• navigation localiser and GPS tracking
• altitude hold
• vertical speed
• GPS steering (GPSS) for smoother turns onto a course or during course tracking.

The limitations section of the SR22 POH stipulated that the autopilot should be disconnected when moderate to severe turbulence was experienced.

According to the aircraft owner, when either the altitude hold or vertical speed modes were selected, the autopilot would not disengage automatically. Also, when in these modes and the flight controls were manually manipulated, the system would apply trim in the opposite direction to maintain the selected altitude or vertical speed. 

Electric trim and autopilot failure

The POH indicated that, any failure or malfunction of the electric trim or autopilot could be overridden by manually manipulating the control yoke. Further, if a trim runaway occurred, the pilot was to de‑energise the circuit by pulling the circuit breaker (PITCH TRIM, ROLL TRIM, or AUTOPILOT) and land as soon as the conditions permitted.

Icing protection system

The United States Federal Aviation Administration approved the Cirrus SR22 for flight into icing conditions in 2009 based on the introduction of an optional anti-ice system for the wings, windshield, propeller, vertical and horizontal stabiliser leading edges and the stall warning system. This was known as a FIKI (flight into known icing) approval. The accident aircraft was manufactured in 2002, which predated the FIKI approved modification, and the owner confirmed there was no anti-icing system fitted. Further, pilots were required to undergo an online Cirrus training course in icing awareness and use of the FIKI fitted to the SR22 aircraft before the system could be utilised. The manufacturer confirmed that the pilot of VH-MSF had not undergone that training. 

VH-MSF was fitted with windshield defrost, pitot heat[6] and an alternate induction air system, which offered some protection against windshield, pitot and engine air intake icing. Each of those items had to be manually selected on by the pilot as required. According to the SR22 POH, the pitot heat was to be turned on for flight into instrument meteorological conditions, flight into visible moisture, or whenever ambient temperatures were 5 °C or less. 

Stall warning system

The aircraft was equipped with an electro-pneumatic stall warning system to provide audible warning of an approach to aerodynamic stall.[7] When a slight negative pressure was sensed by the pressure switch from an inlet in the wing leading edge, a warning horn activated. The warning sounded at approximately 5 kt above the stall with full flaps and power off in wings level flight and at slightly greater margins in turning and accelerated flight.

Cirrus airframe parachute system 

The Cirrus airframe parachute system (CAPS) was designed to lower the aircraft and its occupants to the ground in the event of a life‐threatening emergency where activation was determined to be safer than continued flight and landing. The system consisted of the following primary components: 

• parachute
• solid-propellant rocket to deploy the parachute
• rocket activation handle and cable
• harness embedded in the fuselage structure. 

The parachute and rocket were located in the empennage behind the rear baggage compartment. The rocket activation handle was mounted in a cabin ceiling enclosure between the 2 front seats and the cable was routed through the cabin ceiling and angled towards the left side of the CAPS compartment. 

A safety pin with a remove before flight flag was fitted to the activation handle when operating on the ground. Part of a pilot’s pre-flight checks included a requirement to remove the safety pin prior to engine start. To initiate the CAPS, the pilot was to remove the access cover on the ceiling and pull the rocket activation handle out and down (Figure 5). Movement of the cable compressed the igniter steel spring and cocked the plunger. When one half inch of plunger travel was reached, the primary booster was ignited, which then ignited a secondary booster and the rocket motor. 

Figure 5: Activation handle (top left) and parachute system as fitted to the aircraft

Source: Cirrus Design Corporation, annotated by the ATSB

For aircraft with an electronic ignition for the booster (as was fitted to VH-MSF), both aircraft batteries were connected to the system and either could actuate the booster in response to cable movement. Once ignited, the rocket impacted and dis-bonded the parachute compartment cover situated behind the rear cabin window and pulled the deployment bag from the enclosure. The deployment bag then staged the suspension line deployment and inflation of the parachute.

Meteorological information

Accessing weather information

On the morning of the accident, the pilot submitted a location briefing request at 0615 to the National Aeronautical Information Processing System[8] (NAIPS), which included Canberra and Armidale Airports. This was followed by 5 area briefing requests between 0622 and 0635, which included meteorology information, notices and advisories (NOTAMs),[9] and charts. This would have provided the pilot with the New South Wales east (NSW-E) graphical area forecast (GAF) and the NSW grid point wind and temperature (GPWT) charts. 

At 1105, the pilot submitted another NAIPS area briefing request for the same weather information requested previously. This was the pilot’s last briefing request to NAIPS.

Bureau of Meteorology

Initial weather forecast

When the pilot submitted their flight plan at 0648, the current NSW GPWT chart was issued at 0454 and valid from 1100. Canberra was located near the intersection of 4 areas on the chart. Interpolation of the data between these 4 areas indicated the freezing level was forecast to be about 5,500 ft above mean sea level. The current NSW‑E GAF was issued at 0302 and valid from 1000 to 1600, which included the pilot’s planned departure time of 1430. Canberra was in subdivision C1 of area C on the GAF, which included the following weather:

• broken[10] stratocumulus cloud from 2,000 ft to 7,000 ft in C1 until 1600
• scattered drizzle in C1 with visibility reduced to 3,000 m and overcast stratocumulus cloud from 1,000 ft to 8,000 ft
• a freezing level[11] of 4,000 ft in the south and 7,000 ft in the north [Canberra was centrally located within the south region].

The remarks field on the GAF provided additional information of operational relevance and included:

• cloud above the freezing level implied moderate icing[12]
• stratocumulus cloud implied moderate turbulence.

The estimated freezing level of 5,500 ft at Canberra and forecast broken stratocumulus from 2,000 ft to 7,000 ft, indicated an icing layer of about 1,500 ft overhead Canberra. Armidale was in area B, which included broken cumulus/stratocumulus cloud from 6,000 ft to 9,000 ft from 1300, and a freezing level of 9,000 ft.

Subsequent weather forecast

When the pilot submitted their last NAIPS area briefing request at 1105, the current NSW GPWT chart was issued at 0559 and valid from 1400. It indicated the freezing level overhead Canberra was forecast to be at about 7,000 ft with south‑westerly winds increasing from 6 kt at 5,000 ft to 19 kt at 10,000 ft. The NSW-E GAF was issued at 0913 and valid for the period 1000-1600. Canberra and the accident site were in the south of subdivision D1 of area D. The forecast for area D included the following conditions relevant to the pilot’s departure time of 1430:

visibility greater than 10 km, scattered cumulus/stratocumulus cloud from 5,000 ft to 8,000 ft, with broken tops to 10,000 ft in D1/D2 – the Bureau of Meteorology advised the ATSB that this should be interpreted as being broken cumulus/stratocumulus cloud from 5,000 ft to 10,000 ft in D1/D2

• visibility reduced to 3,000 m in isolated showers of rain, with broken stratus cloud from 1,500 ft to 4,000 ft and broken cumulus/stratocumulus cloud from 4,000 ft to above 10,000 ft
• freezing level of 5,000 ft in the south and 8,000 ft in the north of area D.

The remarks field stated:

• cloud above the freezing level implied moderate icing
• cumulus and stratocumulus cloud implied moderate turbulence.

The estimated freezing level of 7,000 ft at Canberra and forecast broken cumulus/stratocumulus from 5,000 ft to 10,000 ft indicated the forecast depth of the icing layer overhead Canberra had increased to 3,000 ft. Figure 6 depicts the NSW-E GAF, current at the time of the pilot’s last NAIPS area briefing request. Information relevant to the flight is labelled and highlighted. This forecast included a freezing level of 9,000 ft for the area containing Scone (area B) and a freezing level of above 10,000 ft for the area containing the destination of Armidale (area A).

Figure 6: Graphical area forecast for New South Wales – East

Source: Bureau of Meteorology, annotated by the ATSB 

Assessment of the local conditions

The ATSB requested an analysis of the weather conditions by the Bureau of Meteorology applicable to the aircraft’s track. The following is a summary of that analysis:

Satellite observations at 0400Z [1500 local time] indicated that the IR [infrared] cloud top temperature was ‑7°C which corresponds to a cloud top height of 10,000 ft…Taking the base and cloud top estimates, this gives a depth of cloud of approximately 3,000ft.

The weather conditions observed for the area between Canberra and the accident site were consistent with the forecasts for the afternoon of 6 October 2023. The cloud cover was scattered to broken cumulus/stratocumulus, with a freezing level at approximately 7000ft. Cumulus and stratocumulus cloud were forecast on the GAF and observed on satellite imagery. Showers were forecast and observed over the ranges to the north and northeast of Canberra, including just north of the accident site at 0349Z [1449 local time]. 

These conditions would have been conducive to moderate icing conditions (most likely of the clear icing type) between approximately 7000ft to 10,000ft above mean sea level. The severity of any icing experienced would depend on how long the aircraft is in the cloud layer between these heights, however, the existence and/or type of airframe icing is very difficult to verify, particularly with the absence of any aircraft icing reports from the area at the time. 

Based on Himawari-9 satellite imagery and observations from the aerological diagram at Wagga there is high confidence of moderate icing within the cumulus and stratocumulus field that extended across the Canberra region on 6/10/2023. This is consistent with forecast cloud and weather referenced on the NSW-E GAFs and reinforces the GAF statement (noted on all GAF issued by the Bureau of Meteorology) of "CLD ABV FZLVL implies MOD ICE".

Weather observations

Canberra Airport automatic terminal information service

When the pilot contacted Canberra ground air traffic control for an airways clearance, they reported receipt of the automatic terminal information service[13] ‘Golf’. The recording for information ‘Golf’ stated the following regarding the current weather conditions at Canberra Airport:

Expect instrument approach runway 17, wind 200° 8 kt, visibility greater than 10 km, cloud few 3,000 ft scattered 3,500 ft,[14] temperature 18, QNH[15] 1025. 

Camera footage of cloud cover at Canberra Airport

A fixed camera was located about 1.3 km to the west of Canberra Airport showing an aspect to the north-north-east. The footage, coupled with other stills taken around the time the aircraft took off to the south and then tracked to the north-north-east, showed broken cumulous/stratocumulus cloud as per the forecast (Figure 7, taken at 1439).

Figure 7: Footage taken near Canberra Airport with a north-north-east aspect at 1439 

Source: Aus Web Cams/myairportcams.com  

First responders

Shortly after the accident occurred, first responders took a video around the aircraft in the hope that it could assist with the investigation into the accident. Consistent with the forecast, that video showed broken cloud overhead the accident site to the south-west, which was the direction the aircraft had come from (Figure 8).

Figure 8: Video image taken shortly after the accident viewed to the south-west

Source: Supplied  

Pilot report of weather

The pilot of a Cirrus SR22T aircraft fitted with a flight into known icing (FIKI) kit was conducting an IFR flight from Wagga to Moruya, New South Wales, via Canberra on the day of the accident, flight planned at 9,000 ft. The pilot recalled using the anti-icing fluid, first to the west of Canberra when the aircraft was in the tops of the clouds at 9,000 ft. The pilot requested and received a clearance from air traffic control to climb to 10,000 ft to clear the cloud. They turned the anti-icing off and passed overhead Canberra at about 1358 (about 50 minutes prior to the accident) where they entered higher level cumulus cloud. The pilot then turned the anti-icing on again for the leg from Canberra to Moruya. 

The pilot observed ice build-up on the aircraft in areas that did not receive the anti-icing fluid directly but did not notice any icing on the wings or any loss of engine performance. The pilot reduced the power and speed for turbulence penetration during the trip and assessed their aircraft was experiencing light icing. They also commented that it would have built up rapidly on an aircraft without an anti-icing system. 

The pilot was able to recall one prior experience of icing when on descent into Moruya from Dubbo in their previous SR22, which was not fitted with a FIKI kit. The pilot reported that the ice built up quickly on the wings for about 2,000 ft but dissipated rapidly when the aircraft entered warmer air. They did not notice any loss of engine performance but acknowledged they were using a low power setting for the descent.

Airline flight data

A Virgin Australia Boeing 737-800 aircraft, callsign Velocity 1690, transited and descended through airspace and altitude bands close to the outbound flight track of VH‑MSF and at a similar time.

Velocity 1690 was being operated on a flight from the Gold Coast, Queensland, to Canberra with a landing time of 1445:26, about 4 minutes prior to the accident. Runway 17 was the active runway and Velocity 1690 approached Canberra from the north. On descent, at 1437:03, the aircraft data recorded the engine anti-icing system (ENG COWL ANTI-ICE) being turned on by the flight crew at an altitude of 10,144 ft and an outside air temperature (OAT) of −6.2°C (6°C total air temperature (TAT)).[16] The operator reported that the flight crew could not provide a detailed recollection of the approach but that the selection of the anti-ice would be consistent with the aircraft in cloud above the freezing level during the descent. At 1439:35, the flight crew turned the engine anti-icing off at 6,848 ft and an OAT of 0.2°C (10.5°C TAT). Figure 9 depicts the approach flight path of Velocity 1690 with the times, altitudes and temperatures when the engine anti-icing was turned on and off. The flight path of VH-MSF includes the positions that corresponded with the altitude band (shown in red) in which Velocity 1690 used engine anti‑icing.

Figure 9: Relative positions of Velocity 1690 and VH-MSF

Source: Google Earth and Virgin Australia, annotated by the ATSB

ATSB review of satellite imagery

Introduction

The Bureau of Meteorology provided the ATSB access to various satellite imagery of the Canberra region during the afternoon of the accident, which had been processed from the geostationary satellites Himawari-8 and 9, operated by the Japanese Meteorological Agency. Imaging sensors carried on board the satellite would make progressive scans of the Earth’s full disk at 10-minute intervals. 

Cloud coverage

Figure 10 is the enhanced visible satellite image showing cloud coverage for the Canberra region for the 10-minute acquisition period commencing 1440, which was the closest image relative to the time of the accident. The areas of cloud are represented by the lighter pixels, where the darker pixels are areas without (or with less) cloud coverage. This image is overlaid with VH-MSF’s flight track, the direct track to waypoint CULIN, the estimated position and time the aircraft climbed through the 0°C estimated freezing level (at about 14:43 and 7,000 ft).[17] It also showed the aircraft’s position at 8,000 ft where flight path variations commence in relation to cloud coverage. 

While this shows cloud coverage along parts of the aircraft’s track, it does not provide information about the depth of the cloud or the height of the cloud tops, and whether those tops were above or below the aircraft’s operating altitude. 

Figure 10: VH-MSF track (in red) overlaid on an enhanced visible satellite image, taken close to the time of the accident

Source: Bureau of Meteorology and Japan Meteorological Agency, modified by the ATSB

Temperature of reflective surfaces

To assist with the estimation of cloud heights in the vicinity of the aircraft’s flight path, the Himawari‑8 and 9 RGB infrared enhancement image[18] was reviewed for the same period commencing at 1440. For the infrared images, the colour of the pixels is an indicator of the average temperature of the reflecting surface measured by the infrared imaging sensors. 

The lighter coloured pixels represent lower temperature (colder) reflecting surfaces and could reasonably be used to infer higher cloud tops in those regions. The darker pixels were consistent with warmer average temperatures of the reflecting surfaces and could suggest lower cloud tops in those areas. Based on the infrared images, the Bureau of Meteorology assessed the cloud conditions as being scattered to broken with a 3,000 ft cloud band between 7,000 and 10,000 ft, with moderate clear icing likely when in cloud from 7,000 ft.

Figure 11 shows a temperature scale, the flight path, times and altitudes with the position of the aircraft at an estimated freezing level of 7,000 ft. It also shows the point at which the aircraft flight path variations commence and the lighter grey areas indicating cloud tops at 10,000 ft. Based on the infrared image, it was considered likely that the aircraft entered cloud above the freezing level. However, the exact amount of time the aircraft spent in cloud could not be determined.

Figure 11: VH-MSF track overlaid on RGB infrared enhancement image, taken close to the time of the accident

Source: Bureau of Meteorology and Japan Meteorological Agency, modified by the ATSB

Comparison with the airline flight data

The infrared imagery for VH-MSF (Figure 11) was compared with the imagery applicable to the Boeing 737-800 (acquisition period commencing 1430) and the corresponding altitude and temperature data for when this aircraft likely entered cloud during descent into Canberra. This indicated that the lightest coloured pixels in the image represented average temperatures of about −6 °C, with the tops of the reflecting surfaces (clouds) being about 10,000 ft as per the forecast.

Recorded information

General information

The aircraft’s Avidyne multi-function display and EMax engine monitoring system had been significantly damaged by the post-impact fire. Technical assessment and X-ray images of the unit’s compact flash data storage card showed considerable thermal damage, which precluded recovery of onboard recorded data. The aircraft was not equipped with a data transfer unit and recoverable data module, which was available as a fitted option in later models of Cirrus aircraft. Therefore, no onboard recording devices were available for data download to assist the investigation.

Flight data performance assessment

The ATSB obtained digital data that had been broadcast by the aircraft’s automatic dependent surveillance broadcast (ADS-B)[19] equipment and which had been recorded by Airservices Australia and other flight tracking websites.[20] That data included information about the aircraft’s position, ground track, ground speed and altitude.

The ADS-B data transmitted by the aircraft did not include parameters such as airspeed, altitude rate of change, heading or temperature. However, the transmitted data could be integrated with other sources of information (such as wind velocity, air temperature and atmospheric pressure), to derive estimates for other relevant performance data. For the purpose of analysing the ADS-B data and to derive estimates of the aircraft’s calibrated airspeed (CAS)[21] during the accident flight, wind and temperature data was obtained from several sources, which included: 

• the Bureau of Meteorology’s NSW GPWT chart, valid from 1400, providing wind and temperature forecast data in 1.5 by 1.5° grids
• the Bureau of Meteorology’s vertical wind profiler observations (averaged over the preceding 30-minute observation period) for Canberra Airport, issued 1430 and 1500
• wind and temperature information from the Boeing 737-800 (Velocity 1690) flight data when transiting through the airspace north of Canberra and passing close abeam the accident site about 10 minutes prior
• the United States National Centres for Environmental Prediction global forecast system and global data assimilation system in 0.25 by 0.25° grids, valid at 1400.

Evaluation of those sources demonstrated a reasonable correlation between datasets, particularly during the latter stages of the climb and immediately prior to the departure from controlled flight. For the main analysis task, the investigation used the Canberra Airport vertical wind profiler observations, and the wind velocity and temperature data recorded for Velocity 1690. The estimate for CAS was derived from ADS-B recorded ground speed and ground track, using the sources for wind velocity (from Velocity 1690 data and vertical wind profiler observations), and recorded atmospheric pressure at Canberra Airport and temperature (from Velocity 1690 data). 

Further, this information, along with published aircraft performance data was used to determine the required engine power and propellor thrust to meet the performance seen in the recorded data. However, due to the limitations of ADS-B broadcast data (such as position errors, recording resolution, and broadcast dropouts) and at times dynamic manoeuvring of the aircraft, the aircraft trajectory and power required analysis was indeterminate for much of the aircraft’s flight.

Figure 12 depicts the ADS-B data for the accident flight, together with an estimate of the aircraft’s airspeed. The initial climb was conducted on reasonably stable headings and climb rates at airspeeds that were estimated to be generally between 85 kt and 105 kt CAS, to an altitude of about 7,000 ft above mean sea level. At one point during this climb, at about 1441 when approaching 6,000 ft, the aircraft appeared to have passed through an area of rising air (Figure 12 ‘vertical air movement’). This was evidenced by the aircraft substantially exceeding the POH published maximum rate of climb performance while the aircraft additionally accelerated slightly. 

At 1442:08, the air traffic controller cleared the pilot to resume their own navigation and track direct to CULIN, where the estimated airspeed increased to about 115 kt. At 1443, the airspeed began to progressively reduce as the aircraft continued to climb. For a full‑page view of Figure 12 refer to Appendix A.

The following provides a summary of the data (in sections A to G, as annotated on Figure 12 and Figure 13) from just prior to passing through 8,000 ft until the departure from controlled flight:

• A: Over a period of about 90 seconds, the airspeed reduced by about 25 kt at a relatively linear rate.
• B: Climbing through 8,300 ft, the airspeed continued to reduce, with a reduction of about 20 kt occurring over a 15‑second period. The aircraft was estimated to have slowed to around 72 kt, which was 5 kt above the calculated stall speed for the flight.
• C: The airspeed recovered slightly but, 40 seconds later, the airspeed reduced again to an estimated 70 kt. During this time, the aircraft was passing overhead several witnesses who had reported hearing unusual revving or stuttering from an aircraft engine.
• D: The aircraft then accelerated to the best rate of climb speed for about 45 seconds, and the altitude increased by almost 800 ft. This also included what appeared to be a controlled turn (based on a relatively constant turn radius) to the right, changing heading by about 35° (as shown on Figure 1 and Figure 2).
• E: At 1447:20, the aircraft entered a final period of unstable flight. The aircraft decelerated from 100 kt to 94 kt while the climb rate reduced to zero.
• F: The airspeed then further decreased by 11 kt, before the aircraft descended 250 ft and recovered to an estimated airspeed of 96 kt. A power required analysis suggested the speed loss and descent were possibly conducted with low or idle power, or due to a downdraft.
• G: Over the next 30 seconds, the recorded data showed that the aircraft then climbed above the best rate of climb to about 1,500 ft/min while the airspeed reduced. 

At 1448:31–33, about 12 minutes after take-off from Canberra, the aircraft reached a maximum altitude of 9,946 ft at an airspeed of 71 kt. Following this, the flight data showed the aircraft’s airspeed and altitude declined, and rapidly so from 1448:37. The descent rate increased to 13,000 ft/min, the ground speed reduced to less than 32 kt and became erratic, and the aircraft track aligned somewhat with the estimated wind direction, all of which indicated the aircraft had likely entered a spin.[22] As the aircraft passed through about 8,000 ft, the rate of descent started to reduce, which was indicative of the increased drag from an increasing air density as the aircraft descended. When the aircraft had reached ground level the descent rate had reduced to around 10,000 ft/min.

Figure 12: Aggregated ADS-B altitude data for VH-MSF, together with estimated airspeed (CAS)

Source: ATSB, using ADS-B data aggregated from Airservices Australia and FlyRealTraffic.com 

Figure 13 depicts the aircraft’s flight track looking back along the flight path with A through G labelled to the relevant sections of the flight as shown in Figure 12. As the aircraft climbed through 8,300 ft, the somewhat linear flight track changed, with heading, altitude and airspeed variations commencing.

Figure 13: Aggregated ADS-B data for VH-MSF, looking back along the flight path

Source: Google Earth, with ADS-B data from Airservices Australia and aggregated ADS-B data from FlyRealTraffic.com, annotated by the ATSB

Performance comparison between flights

Figure 14 illustrates ADS-B altitude data from initial climb to about 10,000 ft for the accident flight and the 2 prior flights (on 3 and 4 October 2023)[23] conducted by the pilot in VH-MSF. The data showed the aircraft climb performance for the accident flight was initially similar or better than the prior flights. During the period of flight path variations, the aircraft performance reduced, potentially due to manoeuvring, but then momentarily returned to comparable performance after passing 9,000 ft.

Figure 14: Comparison of ADS-B altitude data for the accident and 2 prior flights

Flight start times have been adjusted to allow for comparison. Source: ATSB, using ADS-B data aggregated from Airservices Australia and FlyRealTraffic.com 

The manufacturer was provided a copy of the flight track data for assessment. That assessment was conducted by one of their senior investigators and a senior test pilot. While no definitive conclusions were able to be made based on the data provided, the manufacturer indicated that the aircraft had slowed, aerodynamically stalled and, after a short period of time, entered into a spin.   

Wreckage and impact information

Site and wreckage

The aircraft came to rest in an open field adjacent to a dam wall with a 10° downward slope towards the right wing. Although post-impact fire damage precluded examination of a significant proportion of the aircraft, inspection of the site and wreckage showed (Figure 15 and Figure 16):

• The impact marks and wreckage distribution indicated that the aircraft impacted with terrain upright, with a slight nose low attitude and no forward momentum. Although the impact evidence was indicative of a spin, it was difficult to ascertain the spin direction.
• All of the aircraft extremities (wings and tail section) were accounted for and there was no evidence of an in-flight break‑up.
• There were no identified structural defects in the evidence available.
• All flight controls systems were inspected to the degree possible with no pre-accident defects identified.
• The flap actuator was identified within the wreckage and was in the flap zero position.
• The fuel selector was tested and assessed to be in the right tank position.
• The fuel tank caps were located and found secured in the filler point opening of each fuel tank.
• The engine cowl was located forward of the aircraft outside the fire zone. It did not have any residue to indicate an in-flight loss of oil.
• Due to the destruction of the wreckage, cockpit switch settings and circuit breakers, flight/engine control, autopilot or trim positions were unable to be determined.
• The engine power lever and mixture control positions could not be determined. 

Figure 15: Overview of the accident site and remaining wreckage

Source: ATSB

Figure 16: Aircraft wreckage viewed from the rear showing downslope to the right 

Source: ATSB

The cabin heat position was unable to be ascertained, and the engine exhaust and shroud that was utilised for cabin heat was removed so that the exhaust could be examined for pre‑impact defects. No cracks or pre-impact defects were identified in the exhaust that may have led to carbon monoxide[24] being introduced into the cabin by an exhaust leak.

Cirrus aircraft parachute system

The CAPS fuselage cover was located adjacent to the wreckage, but outside the fire zone. Inspection of the cover showed an impact mark on the internal surface at the rocket head location. The cover did not display any thermal or smoke damage (Figure 17). The parachute deployment rocket was not in its original position and was located about 3 m to the right of the fuselage and had dispensed its propellent. The cover and rocket position indicated that the rocket had deployed due to ground impact forces before the post-impact fire had initiated.

The parachute was located in its normal fitted position, remaining in its pack. After an extensive search throughout the remaining wreckage, the parachute deployment handle and safety pin could not be located. Therefore, the ATSB could not establish if an attempt was made to deploy the parachute in-flight.

Figure 17: CAPS external cover showing internal impact mark

Source: ATSB

Propeller and engine examinations

Propeller 

The propeller was partially buried at the front of the aircraft. The propeller flange had separated from the engine crankshaft and remained attached to the propeller hub. Two of the 3 propeller blades (Figure 18, blades A and B) were undamaged and showed no signs that they had passed through the ground during the impact with terrain. 

Figure 18: Propeller as found at the accident site

Source: ATSB

Propeller blade C was buried in the earth directly under the hub and showed some signs of rotational scoring, some leading-edge gouges, and slight chordwise twisting. A fracture surface at the base of the blade was from back bending overload as a result of the impact with terrain (Figure 19).

Site photographs of the propeller and fractured crankshaft were examined further and blade C was physically examined at the ATSB’s technical facilities in Canberra to determine the level of engine power being produced at the time of impact. The materials failure analysis identified that the propeller hub had fractured from the engine crankshaft at the propeller flange, in a manner consistent with ductile overstress due to bending. The examination determined that propeller blade C exhibited minor compound bending through its section and had a slight twist at the blade tip. Chordwise gouging observed on the front face of the blade was a characteristic of propeller rotation.

With respect to the engine power output, typically, windmilling or an engine at idle power will stop very rapidly when the propeller blades contact the ground. There is often minimal ground entry and little to no distortion to the blade sections. In this case, when blade C entered the ground, the propeller stopped suddenly. Therefore, the ATSB’s analysis concluded that, while there were some signatures that would indicate that the propeller was rotating at the time of impact, there was no evidence of appreciable power being produced by the engine. Rather, the damage to the propeller blades indicated that the engine was operating at low power when it impacted terrain.

Figure 19: Blade C as recovered from the accident site

Source: ATSB 

Propeller governor

The propeller governor remained attached to the engine. Impact and fire damage precluded functional testing. The governor was disassembled and inspected at the ATSB’s technical facilities with no pre-impact defects identified.

Engine 

The engine was removed from the accident site and taken to an approved engine overhaul facility for disassembly and inspection under the supervision of the ATSB. Sections of the engine were consumed by the intense post-impact fire, which precluded functional testing of specific areas such as the ignition and fuel systems. 

The oil filler cap was secured to the crankcase fill adapter. The engine and accessories were completely disassembled. The engine was found to be mechanically sound, with the crankcase section intact and all the cylinders present and securely mounted to the crankcase. No defects were identified in any of the cylinder assemblies that may have provided an indication of a malfunction contributing to a loss of engine power. There was no distress of the main or connecting rod bearings due to oil starvation or loss.

The engine sump was pushed upwards during the impact with terrain, bringing it in contact with the cam shaft drive gear. That contact perforated the sump with gear teeth impressions, indicating that the engine camshaft was not rotating and the engine had stopped by the time the imprints were made (Figure 20).

Figure 20: Camshaft drive gear and impressions made in the engine sump

Source: ATSB

Medical and pathological information

General information

The pilot held a class 2 aviation medical certificate valid to 22 October 2023, with 2 restrictions. These were a requirement for reading and distance vision correction to be worn while flying and that a continuous positive airway pressure (CPAP) machine be used for the sleep period before flying. 

The pilot’s last Civil Aviation Safety Authority (CASA) required medical assessment was conducted on 22 October 2021. That documented assessment showed that the pilot had:

• been prescribed medication for high cholesterol for over 10 years
• an electrocardiogram stress test (heart trace) in 2016 with nil issues reported
• their appendix removed in 1980
• a computed tomography (CT) angiogram and CT calcium test in 2019 with nil issues reported
• a CT chest X-ray in 2019, which was all clear
• blood tests in 2016, 2017, 2019 and 2021
• a sleep study performed in 2016, which resulted in the use of a CPAP machine (details below). 

In 2016, the pilot was identified with moderately severe obstructive sleep apnoea and used a CPAP machine to manage that condition. Downloaded CPAP data showed that the pilot was consistently using the CPAP machine. It was reported that the pilot had their CPAP machine with them during the trip to Canberra and given the previous continuous use it was concluded that the pilot likely utilised the machine during the trip, including the night prior to the accident. 

The pilot was reported to have been well rested and had consumed a salmon bowl meal from a local restaurant about 1 hour before the flight. In general, the pilot was reported by their family to be fit and healthy with no known illnesses.

Post-mortem and toxicology results

A full post-mortem[25] of the pilot was conducted. The pilot received extensive thermal injury as a result of the post-impact fire and multiple other injuries from the accident. 

The pathologist noted that the pilot had a right coronary artery angulation with an ostium (opening of the artery) that had a slit-like appearance. They stated that it is a rare congenital coronary artery anomaly that, in most cases, does not present with clinical symptoms and may be considered an incidental post-mortem finding in asymptomatic patients. In approximately 20% of cases, the anomaly may result in symptoms such as angina (chest pain), dyspnoea (shortness of breath), syncope (fainting), myocardial ischaemia (reduced blood flow to the heart), ventricular fibrillation (irregular heart rhythm), and sudden death. According to the literature, symptoms generated by congenital coronary artery anomalies are predominantly associated with athlete patients or after intense physical exercise and are rarely present in sedentary individuals.

Toxicology testing was conducted to detect common therapeutic medicines and illicit drug use, and these tests were found to be negative for all substances. The toxicology report noted that a low, insignificant blood alcohol concentration was detected that may have been attributed to post‑mortem decomposition changes (0.006 g per 100 mL). A carbon monoxide saturation level of 2% was also detected in the pilot’s blood, but the report stated that this did not suggest that carbon monoxide poisoning contributed to the accident and death, nor did it suggest a significant survival period after the impact. As previously discussed in ATSB investigation AO‑2017‑118, the physical symptoms and cognitive effects of carbon monoxide exposure generally start to occur at levels of around 10%. 

In their concluding remarks the pathologist stated that:

No definite answer can be provided based on the post-mortem findings alone regarding whether the possible sudden incapacitation of the pilot may have contributed to the aviation fatalities. The post‑mortem findings must be correlated carefully with all other available evidence, not least the findings arising from examination of the scene and other relevant evidence as unearthed by detective officers and other investigative authorities.

Specialist medical assessment

Due to the circumstances of the accident and the indeterminate results of the pilot’s post‑mortem, the ATSB requested the assistance of a specialist doctor of forensic pathology to assess the information obtained by the ATSB, which included:

• post-mortem and toxicology report
• the sequence of events detailed in the ATSB preliminary report
• CASA medical records relating to the pilot
• Medicare and pharmaceutical benefits scheme records relating to the pilot
• compliance and therapy report for the pilot’s ResMed Airsense 10 Elite CPAP machine.

A summary of the specialist’s assessment of the information provided was as follows:

• The pilot sustained fatal injuries due to the impact with terrain prior to the post-impact fire.
• The blood alcohol level detected was from a sub-optimal sample taken from the chest cavity (often the only choice with severe trauma). Although alcohol consumption could not be ruled out, it was entirely possible that the alcohol was produced post-mortem and could be expected under the given circumstances.
• The pilot was known to take rosuvastatin medication for high cholesterol treatment. The drug was not detected in the pilot’s toxicology results and is generally not detectable in routine screening. While it could not be determined if the medication was in the pilot’s blood, the drug would not be expected to have a psychoactive effect or cause incapacitation.
• Regarding the identified 3 heart abnormalities in the pilot’s post-mortem, the specialist indicated that:
  • In the case of the right coronary artery angulation, the specialist indicated that it is an anatomical abnormality in 2% of hearts where one of the 2 main blood vessels suppling the heart muscle is abnormally angled at its origin from the aorta and often presents as a slit‑like opening (as was the case with this pilot), as opposed to the normal opening, which has a round profile. In the majority of cases, it is considered an incidental finding of no clinical significance. In a small percentage of cases, this abnormality is determined to be a cause for heart muscle abnormalities, including the development of cardiac arrhythmias, scarring of heart muscle, and potentially incapacitation and death. It was noted that the pilot had a CT coronary angiogram performed in January 2019, which was reported to be normal. It was considered likely that had a coronary artery abnormality been a clinically significant issue at the time it would have been identified. Also, an absence of fibrosis (scarring) or other changes typical of chronic ischaemia in the distribution of the right coronary artery argues against this being clinically significant in this case.   
  • The second heart abnormality identified was the narrowing of the left anterior descending coronary artery without obvious atherosclerosis. It was considered likely that post‑accident heat effect caused the change rather than natural disease. It was noted again that the pilot had a CT scan in 2019 that was reported as normal, and it would be unlikely that coronary artery disease would have progressed to the extent of being capable of causing incapacitation in that time period.
  • The third heart abnormality identified was contraction banding in association with lacerations of the heart muscle and was seen in areas supplied by widely patent coronary arteries such as the right coronary artery. In the absence of other indicators, it was considered likely that the contraction banding was a result of the aircraft accident rather than incapacitation due to cardiac disease.
• The specialist advised that many natural medical conditions that can result in pilot incapacitation would generally not be detectable from a post-mortem, especially where there have been very extensive injuries, and therefore could not be ruled out. Examples of such conditions include the pilot losing their corrective eyewear at a critical time, the pilot having a coughing fit, the development of many gastrointestinal illnesses including diarrhoea, vomiting, and stomach cramps, and diverse conditions such as fainting spells, kidney stone passage and cardiac arrhythmias.

In conclusion, the specialist stated that it was unlikely that natural disease caused or contributed to the events leading up to the accident. There were no indications of toxicological abnormalities causing incapacitation and/or death. In common with many aircraft accident fatalities, a definitive comment in relation to cause of death could not be made in this case.

Operational information 

Icing conditions

The limitations section of the POH stated ‘Flight into known icing conditions is prohibited’. The abnormal procedures section stipulated that, if a pilot inadvertently entered icing conditions, the following abnormal checklist procedure for Inadvertent Icing Encounter was to be applied:

1. Pitot Heat…ON

2. Exit icing conditions. Turn back or change altitude.

3. Cabin Heat…MAXIMUM

4. Windshield Defrost…FULL OPEN

5. Alternate Induction Air…ON

The use of alternate induction air was described further in the emergency procedure for Engine Partial Power Loss as follows:

A gradual loss of manifold pressure and eventual engine roughness may result from the formation of intake ice. Opening the alternate engine air will provide air for engine operation if the normal source is blocked or the air filter is iced over.

Aerodynamic stall

A wing generates lift as a result of the pressure differential created by airflow over the wing’s surface. The angle between the incoming or relative air flow and wing chord is known as the angle of attack (AoA). As the AoA increases, lift increases up to a certain angle, known as the critical AoA. At this point, the airflow over the upper surface of the wing becomes separated. This condition is referred to as an aerodynamic stall (or simply a stall) and results in a significant loss of lift and an increase in drag. Due to the sudden reduction in lift from the wing and rearward movement of the centre of lift, typically an uncommanded aircraft nose-down pitch results. 

Most general aviation aircraft typically have a critical AoA of around 16°. This critical AoA can be exceeded at any airspeed, any (pitch) attitude and any power setting. However, as most small aircraft are not fitted with an AoA indicator, the AoA at which the stall occurs may be referenced to an airspeed.

A loss of altitude also occurs during the recovery from a stall and it is possible to stall with insufficient height above the ground to recover. The POH stated that the altitude loss during a wings level stall may be 250 ft or more.

The Cirrus SR22 performance data showed that, at the maximum weight of 3,400 lbs (1,542 kg) with 0° bank angle and flaps full up, the power-off stall speeds at the forward and aft centre of gravity limits were 70 kt and 68 kt (indicated airspeed) respectively. The calibrated airspeed (CAS) for each limit was 1 kt less than the indicated.

The stall speed was calculated for a mid-centre of gravity position and corrected for an operating weight of 3,300 lb (1,497 kg), generally representative of the accident flight is mid centre of gravity given the take-off weight was close to the maximum take-off weight. The estimated stall speed was 68 kt (indicated) and 67 kt CAS. 

The POH normal procedure for stalls stated:

SR22 stall characteristics are conventional. Power-off stalls may be accompanied by a slight nose bobbing if full aft stick is held. Power-on stalls are marked by a high sink rate at full aft stick.

…

When practicing stalls at altitude, as the airspeed is slowly reduced, you will notice a slight airframe buffet and hear the stall speed warning horn sound between 5 and 10 knots before the stall. Normally, the stall is marked by a gentle nose drop and the wings can easily be held level or in the bank with coordinated use of the ailerons and rudder. Upon stall warning in flight, recovery is accomplished by immediately reducing back pressure [on the control yoke] to maintain safe airspeed, adding power if necessary and rolling wings level with coordinated use of the controls.

Spins

A spin can result when an aircraft simultaneously stalls and yaws.[26] A spin is characterised by the aircraft following a downward, corkscrew path and requires significantly more altitude for recovery compared to a wings level stall (Federal Aviation Administration, 2021).

The limitations section of the POH stated ‘Aerobatic manoeuvres, including spins, are prohibited’. The emergency procedures stipulated that the SR22 was not approved for spins and had not been tested or certified for spin recovery characteristics. The only approved and demonstrated method of spin recovery was the activation of the CAPS (refer to the section titled Cirrus aircraft parachute system deployment). Specifically, the POH stated:

If, at the stall, the controls are misapplied and abused accelerated inputs are made to the elevator, rudder and/or ailerons, an abrupt wing drop may be felt and a spiral or spin may be entered. In some cases, it may be difficult to determine if the aircraft has entered a spiral or the beginning of a spin. 

…

In all cases, if the aircraft enters an unusual attitude from which recovery is not expected before ground impact, immediate deployment of the CAPS is required. 

…

The minimum demonstrated altitude loss for a CAPS deployment from a one turn spin is 920 feet. Activation at higher altitudes provides enhanced safety margins for parachute recoveries. Do not waste time and altitude trying to recover from a spiral/spin before activating CAPS.

Wood and Sweginnis (2006), Aircraft Accident Investigation – 2nd edition, provides the following description of the wreckage from an aircraft that had spun into the ground, with reference to Figure 21:

There is little or no evidence of forward motion. Although the fuselage probably impacted at a steep nose down attitude [spins can be anywhere between nose up, flat, but most commonly nose down], it is likely that there is evidence of a wing tip striking the ground before the nose. The down-going wing will normally strike the ground before the up-going wing, providing one clue as to the direction of the spin. Both the fuselage and the wings will probably have damage which reflects both a high sink rate and yaw. Tall thin objects on the ground, like trees and fence posts, are likely to penetrate the airplane almost from bottom to top, reflecting the almost vertical trajectory of the airplane. Undamaged objects may be found immediately behind the trailing edges, again indicating the vertical path of the airplane.

Figure 21: Example wreckage pattern from a spin 

Source: Wood and Sweginnis (2006)

Cirrus aircraft parachute system deployment 

Procedures for deployment

For the deployment of the CAPS, the POH stated:

*Warning*

CAPS deployment is expected to result in loss of the airframe and, depending upon adverse external factors such as high deployment speeds, low altitude, rough terrain or high wind conditions, may result in severe injury or death to the occupants. Because of this, CAPS should only be activated when any other means of handling the emergency would not protect the occupants from serious injury. 

*Caution*

Expected impact in a fully stabilized deployment is the equivalent to a drop from approximately 13 feet.

*Note*

Several possible scenarios in which the activation of the CAPS would be appropriate are discussed in section 10 – Safety information of this handbook. These include:

 - Mid-air collisions

 - Structural failure

 - Loss of control

 - Landing in inhospitable terrain

 - Pilot incapacitation.

The POH also noted that the maximum demonstrated deployment speed was 133 kt (indicated airspeed). Once a decision was made to deploy the CAPS, the airspeed should be reduced to the minimum possible, the mixture should be moved to cutoff, the activation handle cover should be removed and the handle pulled down with both hands. Pull forces up to, or exceeding, 45 lbs (20 kg) may be required. After deployment, the fuel selector, fuel boost pump, battery and alternator master switch and ignition switches were to be turned off and the emergency locator transmitter turned on.

In regard to a CAPS deployment altitude, the POH indicated that:

No minimum altitude for deployment has been set. This is because the actual altitude loss during a particular deployment depends upon the airplane’s airspeed, altitude and attitude at deployment as well as other environmental factors. In all cases, however, the chances of a successful deployment increase with altitude. As a guideline, the demonstrated altitude loss from entry into a one-turn spin until under a stabilized parachute is 920 feet. Altitude loss from level flight deployments has been demonstrated at less than 400 feet. With these numbers in mind it might be useful to keep 2,000 feet AGL in mind as a cut-off decision altitude. Above 2,000 feet, there would normally be time to systematically assess and address the aircraft emergency. Below 2,000 feet, the decision to activate the CAPS has to come almost immediately in order to maximize the possibility of successful deployment. At any altitude, once the CAPS is determined to be the only alternative available for saving the aircraft occupants, deploy the system without delay.

Cirrus, in its guidance document CAPS Guide to the Cirrus Airframe Parachute System, advised that, while the POH noted a maximum demonstrated deployment speed, it was possible for the parachute to withstand deployments at higher speeds. The guide provided examples where the CAPS had been deployed at speeds up to 187 kt (indicated airspeed) with a successful outcome. The guidance reiterated that the maximum demonstrated speed was not intended to be a limitation. 

Cirrus also encouraged pilots to conduct a take-off briefing that incorporated when to activate the CAPS, as well as the inclusion of a passenger briefing that included the use of the CAPS. The briefing should include: 

 - Engage the autopilot using the level button (if equipped)

 - Attempt to revive the pilot

 - Follow the deployment procedures detailed on the CAPS placard 

 - Prepare for CAPS touchdown

 - Follow egress procedures

The ATSB could not confirm what take-off or passenger briefings were undertaken by the pilot on the day of the accident. Further, nor could it be determined with certainty that the passenger seated adjacent to the pilot would have had the physical capability to undertake the required actions if they had received the briefing on the use of the CAPS. 

Deployment history

At the time of writing this report, the aircraft manufacturer reported that there had been 126 in‑flight CAPS deployments. They also stated that there had been 3 CAPS anomalies where the parachute failed to deploy. A recent issue where the rocket did not deploy was related to a batch of rocket motor initiating devices (squibs) manufactured in 2015 and 2016 that would not fully ignite. There was a mandatory service bulletin to have those squibs replaced. The squib on VH‑MSF was replaced when the parachute assembly was replaced in its entirety in January 2023.

The ATSB reviewed several aircraft accident reports, which indicated that there had been a number of CAPS deployments above the maximum recommended indicated airspeed of 133 kt resulting in an overload and separation of the chute from the aircraft. Further, there have been a number of documented accidents where the parachute had not been deployed in‑flight but had ground impact initiations of the rocket. 

Loss of control

The POH safety information section listed potential reasons for a loss of control and an associated response to such a situation:

Loss of control may result from many situations, such as: a control system failure (disconnected or jammed controls); severe wake turbulence, severe turbulence causing upset, severe airframe icing, or sustained pilot disorientation caused by vertigo or panic; or a spiral/ spin. If loss of control occurs, determine if the airplane can be recovered. If control cannot be regained, the CAPS should be activated. This decision should be made prior to your pre-determined decision altitude (2,000’ AGL).

Engine issue in-flight

In the event of an engine failure in-flight, the POH emergency procedure checklist stipulated:

If the engine fails at altitude, pitch as necessary to establish best glide speed. While gliding toward a suitable landing area, attempt to identify the cause of the failure and correct it. If altitude or terrain does not permit a safe landing, CAPS deployment may be required. 

The emergency procedures section of the POH detailed that, for a partial engine power loss, indications of such include fluctuating revolutions per minute, reduced or fluctuating manifold pressure, low oil pressure, high oil temperature, and a rough-sounding or rough-running engine. 

The procedure required that, if a partial engine failure permitted level flight, land at a suitable airfield as soon as the conditions allowed. If the conditions did not permit safe level flight, use partial power as necessary to set up a forced landing pattern over a suitable landing field. It was also advised that a pilot should be prepared for a complete engine failure and consider CAPS deployment if a suitable landing site was not available. 

To troubleshoot, the POH advised to select the fuel boost pump on, switch fuel tanks, check the engine controls, and cycle the ignition switch left and right to ensure both magnetos were working. Select alternate induction air on, as a gradual loss of manifold pressure and eventual engine roughness may result from the formation of intake ice. Opening the alternate engine air would provide air for engine operation if the normal source was blocked or the air filter was iced over.

Fuel uplift

The aircraft had a total fuel capacity of 318 L (159 L per wing tank) as stipulated in the POH. According to fuel company records, the aircraft was refuelled on 5 October 2023 (one day prior to the accident) at about midday with 110 L of Avgas from a fuel bowser at Canberra Airport. The fuel remaining in each tank before the refuelling commenced was unable to be determined. However, the fuel uplift was close to the estimated fuel consumption of 118 L for the previous flight from Armidale to Canberra. The estimated fuel consumption from Canberra to the accident site was 22 L. 

As part of the Canberra Airport fuel company procedures, a sample of fuel was tested for clarity and water content on the morning the aircraft was refuelled and on the afternoon of the accident, with no issues identified. Several other aircraft utilised the same batch of fuel with no issues reported. Therefore, fuel quality and quantity was not considered to be a factor in the accident. 

Weight and balance 

The aircraft load data sheet indicated that the empty weight was recorded as 1,045 kg and the gross weight limit for the SR22 was 1,542 kg. For the purpose of calculating the weight and balance for the accident flight, the ATSB assumed full fuel and used average weights for each of the occupants and their luggage, based on 4 separate estimates provided by their relatives. This produced an estimated engine start weight of 1,494 kg, which was 48 kg below the gross weight limit. The centre of gravity was within limits for the entirety of the flight. 

Flight into icing

Bureau of Meteorology pilot guidance on icing conditions

The accumulation of ice on an aircraft is ‘one of the most significant hazards to the safe and efficient operation of aircraft as it can reduce aircraft performance in a number of ways’ (Bureau of Meteorology, 2015). This includes:

• increased stall speed of the aircraft by increasing its weight with the accumulation of ice
• difficulty operating control surfaces and landing gear
• increased drag and decreased lift due to ice accumulation on the airframe (tests have shown that icing no thicker or rougher than a piece of coarse sandpaper can reduce lift by 30% and increase drag by 40%)
• engine power reductions (intake and carburettor icing)
• propeller vibrations due to ice accumulation on the blades
• errors in instrument readings of airspeed, altitude and vertical speed due to ice contaminated pitot static systems
• interference with communications systems (icing on antennas)
• reduced visibility due to icing on the windshield and side windows.

The Bureau of Meteorology aviation weather services brochure titled Hazardous Weather Phenomena – Airframe Icing has informative content for pilots. Included in that brochure was a depiction of the icing environment and the various levels of icing risk based on temperature and water content. As shown in the icing environment depiction (Figure 22), aircraft operating within the 0 to −10°C higher risk range if/when in cloud could experience clear ice conditions. 

Figure 22: Icing environment depiction

Source: Bureau of Meteorology 

The Bureau of Meteorology classifies icing severity as trace, light, moderate or severe. Moderate icing (as identified on the VH-MSF flight route) means the rate of accumulation is such that even short encounters become potentially hazardous, and the use of de‑icing/anti‑icing equipment or a diversion is necessary. An area forecast will include any expectation of moderate or severe icing, while a SIGMET[27] is only required when severe icing is predicted.   

There are 4 types of icing which are clear, rime, mixed ice (a combination of clear and rime icing) and hoar frost. Clear ice is formed when supercooled water droplets impact the aircraft. As the droplets freeze, heat is released, slowing the freezing process. This causes some of the water droplets to flow back over the exposed surfaces and freeze as clear ice. Therefore, clear ice tends to cover a large area of the aircraft and can disrupt the airflow and affect the performance of the aircraft. Clear ice forms most readily in temperatures between 0 ºC and −10ºC but can occur, with reduced intensity, at lower temperatures.

Impact of icing on aircraft performance

Baars et al. (2010) conducted research titled A review on the impact of icing on aircraft stability and control. The research stated that:

Structural ice formation on leading edges of wings and control surfaces initiate significant regions of unsteady flow. This change in performance of the lifting surfaces can result in a major change in the handling of aircraft; the aircraft may stall at higher speeds, the stall angle of attack may decrease and irreversible upset events can be initiated.

In the period of 1990-2000, a total of 3,230 aircraft accidents were recorded by the Air Safety Foundation. Twelve percent of those were related to icing.

…

Studies on ice-related accidents of small general aviation aircraft have revealed that in many cases even the most experienced pilots have less than 5 to 8 minutes to escape the harmful icing conditions before their aircraft experience violent upsets. This suggests that in cruise the accumulation of ice, and its effect on stability of aircraft, remain mostly unobserved. Upon changing the attitude of the aircraft, the formation of ice induces unsteady flow phenomena capable of upsetting the aircraft in a catastrophic manner.

United States Federal Aviation Administration – Pilot Guide: Flight in Icing Conditions

The purpose of the United States Federal Aviation Administration’s advisory circular AC 91‑74B, Pilot Guide: Flight in Icing Conditions, was to provide pilots with a convenient reference guide on the principal factors related to flight in icing conditions and the location of additional information in related publications. It included the following information:

Flight planning

If an aircraft is not certificated for flight in icing conditions, each flight should be planned carefully so that icing conditions are avoided…In the event of an inadvertent icing encounter, the pilot should take appropriate action to exit the conditions immediately, coordinating with ATC [air traffic control] as necessary, and declaring an emergency.

Effects of icing on unprotected wings

…The ice causes an increase in drag, which the pilot detects as a loss in airspeed or an increase in the power required to maintain the same airspeed. (The drag increase is also due to ice on other parts of the aircraft). The longer the encounter, the greater the drag increase; even with increased power, it may not be possible to maintain airspeed. If the aircraft has relatively limited power (as is the case with many aircraft with no ice protection), it may soon approach stall speed and a dangerous situation. 

Effects of icing on critical systems

Because contamination of the wing reduces lift, even an operational, ice-free stall warning system may be ineffective because the wing will stall at a lower AOA [angle of attack] due to ice on the airfoil. Heated or unheated, if the wing is contaminated in any way, an AOA will become unreliable. The stall onset would occur prior to activation of stall warning devices leading to a potential pitch or roll upset. It is imperative that pilots maintain airspeed and monitor AOA closely when in icing conditions.

Induction icing

Fuel-injected aircraft engines usually are less vulnerable to icing, but still can be affected if the engine’s air source becomes blocked with ice. Manufacturers provide an alternate air source that may be selected in case the normal system malfunctions.

Moderate icing accretion rate

…

The rate of accumulation is such that anything more than a short encounter is potentially hazardous. A representative accretion rate for reference purposes is 1 to 3 inches (2.5 to 7.5 cm) per hour on the unprotected part of the outer wing. The pilot should consider exiting the condition as soon as possible.

General advice 

Avoidance - The pilot of an aircraft that is not certificated for flight in icing conditions should avoid all icing conditions. This guide provides guidance on how to do this, and on how to exit icing conditions promptly and safely should they be inadvertently encountered. 

Vigilance - The pilot of an aircraft that is certificated for flight in icing conditions can safely operate in the conditions for which the aircraft was evaluated during the certification process, but should never become complacent about icing. Even short encounters with small amounts of rough icing can be very hazardous. 

Guidance - The pilot should be familiar with all information in the AFM [airplane flight manual] or POH concerning flight in icing conditions and follow it carefully. Of particular importance are proper operation of ice protection systems and adherence to minimum airspeeds during or after flight in icing conditions. Monitor airspeed, pitch attitude, and do not rely on the airplane’s autopilot or stall warning system in icing conditions. There are some icing conditions for which no aircraft is evaluated in the certification process, such as SLD [supercooled large droplets] conditions within or below clouds, and flight in these conditions can be very hazardous. The pilot should be familiar with any information in the AFM or POH relating to these conditions, including aircraft-specific cues for recognizing these hazardous conditions.

Cirrus SR22 flight in known icing conditions information

Although not fitted to VH-MSF, the approval and specifications for FIKI (flight into known icing) were reviewed as they provided specific guidance for icing on the Cirrus SR22.

The approved POH and airplane flight manual supplement for the FIKI system recommended that the minimum airspeed for flight into known icing conditions was 95 kt (indicated airspeed). The emergency procedures section contained the following information when discussing an observed or suspected failure of the anti-ice system:

An unobserved failure may be indicated by a decrease in airspeed, anomalous handling characteristics, or airframe vibrations.

Note: Significant loss in cruise or climb performance may be an indication of propeller ice accretions that are not visible to the naked eye. Operation of the engine at 2700 RPM [revolutions per minute] will help shed ice in severe icing conditions.

The performance section of the FIKI supplement further stated:

Airplane performance and stall speeds without ice accumulation are essentially unchanged with the installation of the Ice Protection System. Significant climb and cruise performance degradation, range reduction, as well as buffet and stall speed increase can be expected if ice accumulates on the airframe.

Propeller icing

The adverse effects of propeller icing have been explored for several decades, which included the United States National Advisory Committee for Aeronautics producing a report in 1950 (NACA TN 2212), on the subject of The effects of ice formation on propeller performance. Its report included the following observations:

 - when a propeller accumulates ice, the resulting changes in propeller performance are reflected in corresponding changes in aircraft performance

 - the combined action of centrifugal force and kinetic heating resulting from an increase in propeller rotational speed is often effective in reducing the extent of the ice accumulation

 - thus, it appears that in operation of unprotected or inadequately protected propellers in icing conditions, periodic attempts should be made to throw off the accretions by increasing propeller speed.

The propeller manufacturer for VH-MSF, Hartzell, stated on its website that ‘ice typically appears on propeller blades before it forms on the wings, so it’s important to address propeller icing as quickly as possible’. While the NACA (1950) report and the Cirrus FIKI supplement both indicated that increasing the propeller speed was a technique to address propeller icing, another similar technique, published as an online instructional video, was to cycle the propeller lever forwards and backwards. This would vary the propeller blade angle and propeller speed to promote shedding of ice. As the Cirrus aircraft combine the propeller pitch control and engine power control in one lever, the use of propeller blade angle and rotational speed changes to shed ice would be accompanied by associated engine power changes. The ATSB was unable to establish if the pilot was aware of these techniques to remove ice accumulation on the propeller. 

Related occurrences

There have been a number of loss of control accidents involving Cirrus SR22 aircraft with contributors including flight in icing conditions, autopilot control issues, pilot incapacitation and loss of control during stall demonstration to name a few. A varied sample of those events is listed below from the United States National Transportation Safety Board (NTSB) and the ATSB.

NTSB investigation (ATL06LA035) 

While climbing on autopilot, the airplane entered clouds at 5,000 ft at an airspeed of 120 kt. Upon reaching 7,000 ft, the airplane encountered icing conditions. The pilot informed air traffic control and requested a clearance to climb to 9,000 ft, which was approved. As the airplane reached the cloud tops at 8,000 ft when in visual flight conditions, the airplane began to buffet. The pilot looked at the airspeed indicator and it showed 80 kt. The airplane subsequently aerodynamically stalled, started to spin and re‑entered instrument flight conditions. The pilot deployed the ballistic parachute system and informed the air traffic controller of his actions. The airplane descended under the parachute canopy into the trees.

The NTSB determined the probable cause of the accident to be:

The pilot’s inadequate pre-flight planning, failure to obtain a current weather briefing, and his decision to operate the airplane into known icing outside the airplanes certification standards resulting in the aircraft accumulating ice, loss of airspeed, an inadvertent stall/spin and subsequent collision with trees. 

NTSB investigation (ERA20LA129) 

While conducting an instrument landing system approach, the airplane flew through the localizer course, and as it passed outside of the outer edge of the localizer, the autopilot turned off. The pilot could not recall turning the autopilot off, and the reason for the autopilot turning off could not be determined from the available evidence. Over the next minute, a series of altitude excursions occurred during which the airplane repeatedly climbed and descended. The pilot reported that, when he added power, he had difficulty maintaining control of the airplane and that it was unstable. Subsequently, the pilot sensed that he was fighting the airplane and in an unusual attitude, he deployed the airframe’s parachute system. The airplane descended under canopy and touched down in the backyard of a house.

While off course with the autopilot engaged and the vertical speed mode selected, the pilot likely applied and held pitch control input that was sensed by the autopilot auto trim system as an out‑of-trim condition. The autopilot auto trim system responded by trimming the airplane, resulting in the corresponding altitude excursions.

The NTSB determined the probable cause of the accident to be:

The pilot’s incorrect use of the autopilot while approaching the initial approach fix and his subsequent improper primary pitch control input while a pitch mode of the autopilot was engaged, which resulted in pitch excursions and subsequent departure from controlled flight.

NTSB investigation (NYC05LA110)  

The airplane was in cruise flight at 3,000 ft when the pilot experienced a seizure and lost consciousness. When the pilot awakened, the airplane was in a high-speed descent. In addition, the pilot felt disoriented and numbness in his right leg. The pilot recovered from the descent at an altitude of about 1,700 ft and elected to deploy the CAPS. The airplane descended via the parachute and impacted in a river. The airplane sustained substantial damage to the underside of the composite fuselage. The pilot sustained a fractured vertebra and was able to egress from the airplane before it sank. Subsequent medical examinations on the pilot revealed the presence of a brain tumour.

The NTSB determined the probable cause(s) of this accident to be:

The pilot's physiological condition, which resulted in his incapacitation during the flight, and subsequent loss of aircraft control.

ATSB investigation (AO-2013-126) 

The aircraft was being operated on a private flight from Archerfield to Kingaroy, Queensland, with the pilot and one passenger on board. On approach to Kingaroy, at about 500 ft above ground level, the pilot extended the flaps and, shortly after, disconnected the autopilot (AP). Upon disconnecting the autopilot, the pilot reported that the aircraft pitched-up violently due to trim runaway. 

The AP pitch trim was trimming the aircraft for a nose-up position, even though the AP was disconnected. This required the pilot to use a large amount of forward physical force to maintain stable flight. The pilot attempted to resolve the problem several times by pressing and holding the autopilot disconnect switch located on the control yoke, however, this had no effect. 

The pilot then conducted a go-around. They then used the manual electric trim (MET) hat switch located on the control yoke, in an attempt to trim the aircraft nose-down. As the pilot was using the MET to trim the aircraft, which was going against the AP pitch trim runaway, the trim adjusted at a slow rate. 

The pilot was able to regain sufficient control of the aircraft and land safely at Kingaroy. The pilot reported that, upon parking the aircraft and after releasing the MET, the pitch trim was at full nose‑up deflection.

ATSB investigation AO-2014-083

When at about 6,000 ft above ground level, the pilot in command (PIC) was demonstrating the aircraft stall and recovery to a prospective purchaser of the aircraft. They selected 50% flap, rolled the aircraft into a left turn at about 25° angle of bank, reduced the power to idle, and raised the nose. As the aircraft approached the stall, the PIC pointed to the vertical speed indicator. As they did this, the right wing dropped rapidly, and the aircraft entered a spin to the right. The PIC reported that, at this time, they performed their normal recovery procedure for this manoeuvre. 

The passenger in the front seat reported that, on about the third rotation of the spin, the PIC said ‘I’m sorry’, and realised that the PIC had lost control of the aircraft. 

When at about 2,000 ft, the PIC was unsure whether they had enough height remaining to recover control of the aircraft, so they successfully deployed the CAPS, and the aircraft came to rest in a residential backyard. All 3 occupants were uninjured.

Downloaded flight data indicated that the aircraft stalled at an indicated airspeed of 62 kt and the vertical descent rate in the spin increased to a maximum of 14,000 ft/min before the parachute was deployed. 

Safety analysis

Introduction

Flight track data showed that, about 12 minutes after take-off and during the climb phase of the flight from Canberra, Australian Capital Territory, to Armidale, New South Wales, VH‑MSF departed controlled flight and entered a rapid descent just prior to reaching the planned cruising level of 10,000 ft. The aircraft subsequently impacted with terrain. The 4 occupants were fatally injured, and a post-impact fire destroyed the aircraft.

This analysis will consider the events leading up to the departure from controlled flight and the possible explanations for this. It will also consider why the pilot did not recover the aircraft from the rapid descent and the forecast and actual meteorological conditions along the aircraft’s flight track.

Aerodynamic stall 

Consistent with the 2 previous flights, the aircraft’s flight tracking data showed a normal, stable take-off and climb out of Canberra Airport towards Armidale until about 7,000 ft above mean sea level. This suggested that the pilot may have been using the aircraft’s autopilot system. Also, up to this point, all radio exchanges between the pilot and air traffic control were clear and readback correctly. 

Climbing through about 8,300 ft, the flight track data changed from a relatively steady state to variations in heading, altitude and airspeed. This suggested that the aircraft had likely changed from operating with the autopilot on to manually controlled flight. Potential reasons for this change may have included the avoidance of cloud, turbulence or issues with the autopilot. It was around this time that 4 independent witnesses located below the aircraft’s flight track reported that an aircraft obscured by cloud could be heard making engine surging sounds (see Possible explanations for the contributing factors below for further explanation).

Over the next couple of minutes, while the general trajectory of the aircraft remained in a climb, the aircraft slowed to almost the stall speed on 2 occasions. If working as designed, the stall warning system should have sounded when the aircraft’s airspeed deteriorated to about 5 kt above the stall speed, alerting the pilot to an impending stall condition. Additionally, as a precursor to the stall, a slight buffet might have been felt by the pilot through the airframe. The pilot’s operating handbook (POH) stipulated that, when the stall warning sounds, recovery was accomplished by immediately reducing back pressure on the control yoke to reduce the angle of attack, maintain a safe airspeed, and add power as required. Following these 2 occasions, the flight data showed a slight descent and an increase in airspeed, which may have been representative of a possible pre-stall recovery and then the climb continued.

Following a descent, the performance data indicated a climb rate of up to about 1,500 ft/min and the airspeed decreased from an estimated 96 kt to 70 kt past the point of a pre-stall recovery. At a maximum altitude of 9,946 ft, the airspeed and altitude rapidly decreased, which was consistent with the aircraft aerodynamically stalling and departing controlled flight. 

Recovery actions

The POH procedure for a recovery from an aerodynamic stall required the pilot to reduce back pressure on the control yoke to un-stall the wings and apply power, as necessary, to accelerate the aircraft. However, the flight data showed that, following the stall at about 9,900 ft, the rate of descent increased to about 13,000 ft/min, which was inconsistent with a stall recovery. While descending through around 8,000 ft, the ground speed reduced while the variations in the track became larger, and the rate of descent started to reduce towards 10,000 ft/min by ground level. This, combined with the witness observations, wreckage examination, and manufacturer’s assessment of the flight data, indicated the aircraft had likely entered a spin before the impact with terrain.

The POH stipulated that, following a loss of control when recovery may not be possible, the Cirrus airframe parachute system (CAPS) should be used. The POH further indicated that the only method of recovery from a spin was to deploy the CAPS. The decision to activate the CAPS should be made prior to an altitude of 2,000 ft above ground level. The POH also suggested that when no other survivable options were available, the CAPS should be activated regardless of altitude. That said, the ATSB considered there was adequate time (about 44 seconds) to deploy the CAPS following the departure from controlled flight. However, the inspection of the wreckage indicated that the CAPS had not deployed in-flight, but rather due to ground impact forces. That examination also found that the pre‑deployment procedure of shutting down the engine was not conducted.

It was also determined that a deployment failure was unlikely given the system’s recent replacement, high reliability and the ground impact initiation of the rocket. Therefore, the ATSB was unable to ascertain why the aircraft was not recovered from the stall or if an attempt was made to deploy the CAPS in-flight.

Possible explanations for the contributing factors 

The flight data showed aircraft performance and handling that was beyond what was considered normal, particularly the maintained climb at reducing airspeed leading to the stall. As such, the following section will discuss several scenarios that were considered by the ATSB, which may explain the stall and subsequent loss of control, with no recovery action taken. Those factors include whether there was an aircraft issue, if the pilot had some level of incapacitation, or if in‑flight icing was experienced. 

Aircraft issue

There were no reported problems with the aircraft on the 2 flights in the days that preceded the accident. A review of the maintenance documentation revealed 2 items of maintenance that were overdue, which were the standby compass calibration and an outside air temperature/clock back-up battery replacement. However, neither of those items were of significance and should not have contributed to the loss of control. All major aircraft components were identified in the general area of the accident site with an in-flight failure of the airframe structure ruled out. While the post‑impact fire prevented examination of a significant proportion of the aircraft, an inspection of the remaining aircraft structure and flight controls did not identify any pre-accident anomalies. 

There have been previous occurrences related to the autopilot and pitch trim systems. However, in this case, the position of the relevant switches and trim could not be established due to the extent of damage. 

Witnesses reported hearing surging or a rough running engine along the aircraft flight path in the minutes prior to the departure from controlled flight. If the sound heard was from VH‑MSF, this could potentially suggest an engine issue or alternatively, the pilot manipulating the engine power lever. There were also short periods in the flight track that indicated possible power reductions and loss of altitude, but the general trajectory of the aircraft remained in a climb until the aerodynamic stall. 

The engine was disassembled and inspected by the ATSB with no pre-impact mechanical defects identified. The inspection of the propeller damage and crankshaft fracture indicated evidence that the engine was running at low power when it impacted with terrain, although the ATSB was unable to ascertain if the engine controls were set at a low power setting (matching the observed propeller damage). It was also noted that no radio call was received from the pilot advising of a problem, nor had they attempted a diversion to a nearby airfield or return to Canberra, which would be expected if an aircraft issue was experienced. 

Therefore, while there were no observable indications of an issue, due to the limited remaining aircraft structure and systems that were available for inspection, an unidentified mechanical failure or anomaly could not be discounted. 

Pilot incapacitation

Partial or complete incapacitation can adversely affect a pilot’s psychological and/or physiological capacity to operate an aircraft. Research has shown that pilot incapacitation occurs for a variety of reasons including acute medical conditions (such as food poisoning and gastroenteritis) and pre‑existing medical conditions (such as heart disease, leading to a heart attack). While pilot incapacitation in general aviation accounted for only 13% of all reported occurrences between 2010 and 2014, 70% of those influenced flight operations, namely a return to the departure aerodrome or in the worst case, a collision with terrain (ATSB, 2016). In this accident, indicators of a potential incapacitation were:

• the absence of radio calls to indicate a problem or phase of distress
• the lack of stall recovery actions with ample altitude and time to recover
• the non-use of the CAPS as a procedural recovery action when there was sufficient altitude for deployment. 

The pilot’s post-mortem identified a heart anomaly, however, it was noted that in most cases symptoms do not present. Overall, the post-mortem report concluded that the cause of death was undetermined and that an assessment should be made in consideration of the other available evidence to determine if sudden incapacitation may have contributed to the accident. 

Therefore, to further examine the possibility of an incapacitating event, the ATSB requested the assistance of an independent doctor of forensic pathology to undertake an assessment of the pilot’s post‑mortem, toxicology and medical history. However, that assessment did not identify any underlying medical conditions, natural disease or toxicological abnormalities that could have led to an incapacitation event. 

In addition, records indicated the pilot consistently used a continuous positive airway pressure machine to manage sleep apnoea. As the pilot had taken the machine on their trip, it was likely that they had used it the night before the accident. Also, it was noted that the pilot had lunch just prior to departure, and as the research has shown, gastroenteritis related incapacitation can occur and therefore could not be discounted. 

Further, there was no evidence to suggest that the pilot's general health on the day of the accident was degraded. Similarly, the pilot was reported to be fit and healthy and had no identified health conditions that were not being appropriately treated. Consequently, there was insufficient evidence to determine if incapacitation was a contributing factor. That said, medical incapacitation can result for many reasons that may have been undetectable in the post-mortem, toxicology and review of the available medical information.

Icing conditions

The subsequent graphical area forecast accessed by the pilot, which was valid for the flight, indicated broken cloud was expected from 5,000 ft to 10,000 ft along the aircraft’s flight path after departing Canberra. The Bureau of Meteorology’s post-accident analysis concluded that the actual conditions experienced were consistent with the forecast conditions. This analysis estimated a cloud depth of 3,000 ft, with a top height of 10,000 ft, which was the pilot’s nominated cruising altitude. The ATSB’s analysis of the satellite imagery also showed cloud coverage along parts of the aircraft’s track. Likewise, the camera footage and automatic terminal information service at Canberra, and video from first responders at the accident site also noted cloud in the vicinity.

The Bureau of Meteorology’s analysis also determined an approximate freezing level of 7,000 ft. On that basis, it was concluded that the conditions would have been conducive to moderate icing between about 7,000 ft and 10,000 ft, when in cloud. The presence of icing was consistent with pilot observations and data from other aircraft operating in the vicinity of Canberra. The pilot of another Cirrus aircraft reported using the icing protection system when operating at about 9,000 ft. Likewise, the flight data from Velocity 1690 showed that, on descent, the engine anti-ice system had been used from about 10,000 ft down to 7,000 ft, indicating the aircraft was operating in cloud above the freezing level during that time. Therefore, considering the flight path and cruising altitude, VH-MSF likely entered cloud at some point during the flight and was subject to icing conditions. 

Operations in icing conditions can lead to performance degradation and changes in aircraft handling due to ice accretion on the wings and control surfaces, and a reduction in engine power due to blocked engine air intakes and ice‑affected propeller blades. It can also result in erroneous airspeed and altitude information due to blocked pitot static systems, loss of visibility due to ice on the windshield, render a stall warning system ineffective, and weaken radio signals due to ice accretion on antennas. 

The propeller will likely accumulate ice faster than the airframe and there are techniques for shedding propeller ice, which involve increasing the propeller speed and varying the propeller blade angle. In the Cirrus SR22 aircraft, the propeller lever is combined with the engine power lever and therefore the use of propeller speed and blade angle variations to shed ice would be accompanied by engine power changes. While this technique might have produced the engine power fluctuations heard by witnesses, who were located where the aircraft’s flight path was above the freezing level, the ATSB was unable to determine if the pilot was aware of this technique.

Consistent with the United States Federal Aviation Administration’s guidance, the POH stipulated that, when icing was encountered, the pilot should immediately exit icing conditions by turning back or changing altitude. However, the flight data showed that, overall, the aircraft continued to climb toward the cruising altitude. Also, while there were some variations in the aircraft’s track with a more observable change up to 35° later in the flight, there was no indication of a turnback towards Canberra. Likewise, there was no radio call received from the pilot advising of an intention to change altitude, divert from track or turnback due to icing. Although it was noted that icing has the potential to interfere with communication systems.

The POH also stated that a gradual loss of engine manifold pressure and eventual engine roughness due to intake icing could result, like what was heard by witnesses. However, the ATSB was unable to ascertain if the change in engine sound was from the pilot manipulating the engine control or uncommanded surging of the engine. Despite this, and as previously noted, there was no radio call received from the pilot advising of an engine issue nor was there an attempted diversion or return.

The Federal Aviation Administration’s guidance also indicated that, in moderate icing, a representative accretion rate for reference purposes was about 2.5 to 7.5 cm per hour on the outer wing. If the aircraft was in cloud for the entire period above the freezing level, the maximum amount of time spent in icing conditions before the loss of control would have been about 5 minutes. Therefore, a worst-case scenario was that the aircraft’s outer wings accumulated between 2.1 mm to 6.2 mm of ice. That said, satellite imagery with a flight track overlay showed some flight above the freezing level was likely to have been clear of cloud. Therefore, it was likely that the amount of time spent in icing conditions was less than 5 minutes. However, the exact amount of time spent in these conditions was unable to be determined due to the dynamic nature of the cloud on the day of the accident and the 10‑minute capture between local area satellite images. 

When about 1,000 ft above the freezing level, the heading, altitude and airspeed variations had commenced, which might suggest performance effects from icing or cloud avoidance. However, the general trajectory of the aircraft was a climb up to the point of the stall. Also, the aircraft went through a period of about 45 seconds where it achieved the best rate of climb, which was about 1 minute and 30 seconds before the stall. That rate of climb would likely be unachievable if the aircraft had significant icing accretion.  Further, as shown in the SR22 ice accretion accident example in this report, if an aircraft was affected by ice the stall speed for the aircraft would likely be much higher than the normal stall speed.  

In summary, icing may explain the rough running engine, the variations observed in the flight data, and reduction in airspeed to the point of a stall. However, from the available evidence, it could not be established with a reasonable degree of probability that the aircraft experienced icing for a duration sufficient to result in performance degradation or other known icing issues and, therefore, contributed to the accident. Also, experiencing icing did not explain why the CAPS was not deployed following the loss of control and entry into a spin.  

Flight plan

On the morning of the accident, the pilot lodged an instrument flight rules plan with a cruising altitude of 10,000 ft. Canberra Airport was within an area that had a forecast for broken cloud with a layer of moderate icing present below the pilot’s planned cruising altitude. At the time the pilot submitted the flight plan, the cloud tops along the planned route from Canberra to waypoint CULIN were forecast to be 7,000 ft and the layer of icing was expected to be about 1,500 ft deep with the top 3,000 ft below the planned cruising level. However, when the pilot checked the weather later in the morning the cloud tops were forecast to reach 10,000 ft and the icing layer was expected to be about 3,000 ft deep, which could only be avoided if the aircraft remained clear of the broken cloud. The aircraft was not fitted with anti‑icing equipment and was prohibited from operating in icing conditions. Therefore, the only way for the pilot to ensure that icing conditions would be avoided (if they did not amend their planned flight track) was to avoid flying the aircraft in cloud at those levels where icing was forecast.

Noting the lowest safe altitude from Canberra to CULIN was 4,600 ft, the pilot had the opportunity to amend their flight plan to fly this sector at 6,000 ft, below the freezing level. Alternatively, before departing Canberra, the pilot could have requested from air traffic control either a change of cruising altitude and/or a change in track. Likewise, a clearance to manoeuvre left or right of the planned track, or to climb or descend clear of cloud if icing became an issue after take-off, was a possibility. Neither of those options occurred and while the satellite imagery, and recorded images from Canberra and the accident site, indicated there were patches of clear sky, it was considered unlikely that the pilot was able to avoid all cloud above the freezing level. 

Noting that the aircraft would have likely been subject to moderate turbulence during the climb, it was possible the pilot was expecting smooth and clear flying conditions on top of the cloud at 10,000 ft. While this might have been a consideration for the pilot’s plan, it could not be confirmed if that was the reason. Despite this, and as discussed above, the ATSB was unable to determine if the aircraft experienced icing to an extent that affected performance and handling. 

However, aircraft flying through cloud in sub-freezing temperatures are likely to experience some degree of icing. Operating in these conditions in aircraft that are prohibited from doing so increases the risk of a loss of control event leading to an accident. A pilot can reduce the chance of icing becoming an issue by selecting appropriate routes during the flight planning stage. 

Findings

From the evidence available, the following findings are made with respect to the loss of control and collision with terrain involving Cirrus SR22, VH-MSF, near Gundaroo, New South Wales, on 6 October 2023. 

Contributing factors

• When approaching 10,000 ft above mean sea level, the aircraft climb rate increased significantly combined with a decreasing airspeed, resulting in an aerodynamic stall and departure from controlled flight.
• Following the loss of control, for undetermined reasons, an aerodynamic stall recovery did not occur nor was the Cirrus aircraft parachute system deployed before the impact with terrain.

Other factors that increased risk

• The flight was planned and flown through forecast moderate icing conditions from about 7,000 ft in an aircraft that was prohibited from operating in those conditions. It was therefore likely that the aircraft encountered icing, however, there was insufficient evidence to determine if it was at a level sufficient to affect aircraft performance and/or handling.

Glossary

Sources and submissions

Sources of information

The sources of information during the investigation included:

• Cirrus Design Corporation
• the aircraft owner
• the maintenance organisation
• witnesses
• Airservices Australia
• Bureau of Meteorology
• Civil Aviation Safety Authority
• forensic pathology specialist
• NSW Police Force
• United States National Transportation Safety Board.

References

Australian Transport Safety Bureau. (2016). Pilot incapacitation occurrences 2010-2014 (AR‑2015-096). https://www.atsb.gov.au/sites/default/files/media/5768970/ar-2015-096
final.pdf

Baars, W.J., Stearman, R.O. & Tinney, C.E. (2010). A review on the Impact of Icing on Aircraft Stability and Control. ASD Journal (2010), 2(1), 35-52.

Bureau of Meteorology. (2015). Hazardous weather phenomena – airframe Icing. www.bom.gov.au/aviation/knowledge-centre

Cirrus Aircraft Corporation (2013) CAPS Guide to the Cirrus Airframe Parachute System. 

Federal Aviation Administration. (2021). Airplane Flying Handbook (FAA-H-8083-3C). https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/airplane_handbook/00_afh_full.pdf

National Transportation Safety Board. (2022). Investigation ERA20LA129 - Autopilot issue and loss of control - Cirrus SR22 – Conway, South Carolina USA – March 17, 2020. https://data.ntsb.gov

National Transportation Safety Board. (2006). Investigation ATL06LA035 - Icing conditions and loss of control - Cirrus SR22 – Childersburg, Alabama USA. https://data.ntsb.gov 

National Transportation Safety Board. (2006). Investigation NYC05LA110 - Pilot incapacitation and loss of control - Cirrus SR22 - Haverstraw, New York USA – June 30, 2005. https://data.ntsb.gov

Wood and Sweginnis. (2006). Aircraft Accident Investigation – 2nd edition. Endeavour Books.

Submissions

Under section 26 of the Transport Safety Investigation Act 2003, the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. That section allows a person receiving a draft report to make submissions to the ATSB about the draft report. 

A draft of this report was provided to the following directly involved parties:

• Cirrus Design Corporation
• aircraft owner
• maintenance organisation
• Bureau of Meteorology
• Airservices Australia
• Civil Aviation Safety Authority
• forensic pathology specialist
• United States National Transportation Safety Board.

Submissions were received from the:

• aircraft owner
• Civil Aviation Safety Authority
• Bureau of Meteorology.

The submissions were reviewed and, where considered appropriate, the text of the report was amended accordingly.

Appendix A

VH-MSF aggregated ADS-B altitude data for the accident flight, together with the estimated airspeed.

This image depicts selected ADS-B data and derived estimates of calibrated airspeed and altitude for VH-MSF during the accident flight. The airspeed has been estimated using data from a Boeing 737 descending into Canberra, Australian Capital Territory, a short time prior to the accident. Source: ATSB, using ADS-B data aggregated from Airservices Australia and FlyRealTraffic.com 

Purpose of safety investigations The objective of a safety investigation is to enhance transport safety. This is done through:  identifying safety issues and facilitating safety action to address those issues providing information about occurrences and their associated safety factors to facilitate learning within the transport industry. It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action. About ATSB reports ATSB investigation reports are organised with regard to international standards or instruments, as applicable, and with ATSB procedures and guidelines. Reports must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue. Publishing information Released in accordance with section 25 of the Transport Safety Investigation Act 2003 Published by: Australian Transport Safety Bureau © Commonwealth of Australia 2025 Ownership of intellectual property rights in this publication Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia. Creative Commons licence With the exception of the Commonwealth Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this report is licensed under a Creative Commons Attribution 4.0 International licence. The CC BY 4.0 licence enables you to distribute, remix, adapt, and build upon our material in any medium or format, so long as attribution is given to the Australian Transport Safety Bureau.  Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.

Executive summary

What happened

On the afternoon of 20 September 2023, the pilot of a Bell Helicopter Company 204B, registered VH‑EQW, was tasked with firefighting operations utilising a 1,230 L (Bambi Max) water bucket with a 5 m line. The helicopter departed on a 25-minute flight from a private property near Amberley, Queensland, and tracked to another property in Tarome, about 48 km to the south‑west.

While picking up a full bucket of water from the dam, the helicopter lost control, impacted the water, and subsequently sank to the bottom of the dam. The pilot extricated themselves with only minor injuries, however, the helicopter was destroyed. 

What the ATSB found

The ATSB found that the Bambi Bucket suspension cables were caught over the left rear skid when the helicopter was on approach to the dam and during the water collection into the bucket. As the load of water was lifted, it was almost certain that the helicopter’s centre of gravity moved aft and left due to the tethered weight over the left rear skid. This resulted in asymmetric lift loads, loss of control and collision with water. 

The ATSB’s examination of the wreckage did not identify any pre-impact defects with the helicopter. Also, the pilot had completed helicopter underwater escape training (HUET) about 2.5 years prior to the accident. 

Safety message

Conducting helicopter external load operations over water is a complex task, with the risk of an accident shown to be over twice as high as private helicopter operations. There can be a lack of visual references, visual illusions over water, limited visibility and vertical reference of the hook and external load through mirrors and bubble windows. 

As shown in this accident, fouling of external load suspension cable(s) on the airframe can lead to rapid changes in weight distribution, asymmetric lift and loss of control. This investigation reinforces that correct cable positioning is vital to the safety of external lift operations. Further, this accident highlights the importance of conducting HUET to increase the occupants’ chances of post-accident survival in the event of impact with water.

The investigation

The occurrence

Summary of events

On the afternoon of 20 September 2023, the pilot of a Bell Helicopter Company 204B, registered VH‑EQW, was tasked with firefighting operations utilising a 1,230 L water bucket (Bambi Max) with 5 m cables. 

The helicopter departed from a private property near Amberley, Queensland, on a 25-minute flight to another property near Tarome, about 48 km away (Figure 1) with the intention of uplifting water from a dam (dip site) with the water bucket slung under the helicopter for firefighting operations. The flight track data indicated that the helicopter had an average cruise ground speed of about 85 kt (78 kt indicated airspeed (IAS)), with a maximum of 93 kt (85 kt IAS).[1] 

After arriving overhead the property, the pilot aligned the helicopter with the dam, descended over the water to the dip site and submerged the bucket. As the pilot began to initiate the bucket lift, control of the helicopter was lost, and it impacted the water surface and sank. The pilot sustained minor injuries, but exited and swam to shore, and the helicopter was destroyed. 

Figure 1: VH-EQW flight track from the take-off point to the accident site

Source: Google Earth, modified by the ATSB

Pilot account of events

The pilot was operating the helicopter from the left seat for increased visibility from the bubble window while conducting the external load operation. The pilot recalled that, after arriving at the property at Tarome, they commenced filling their first load of water from a dam. The pilot reported that, during water collection, they heard an unusual noise and that the helicopter ‘kicked a bit’. Remaining in the hover, the pilot checked that all engine indications were normal and that the bucket and line were in the appropriate place. However, the pilot reported that something still did not feel right. As a result, they elected to dump the water from the bucket and initiate a climb out. The pilot stated that, within about 10–15 seconds, as engine power was being applied and the water was being released (dumped) from the bucket, the pilot heard what they described as a ‘loud roaring’ sound and the helicopter pitched up, yawed, rolled left, and impacted the water at low speed. 

Witness observations

The accident was observed by 2 witnesses (Figure 2). Witness 1 observed the helicopter circle, move towards the dam on their property to collect water, and observed the entire accident sequence. They also photographed and videoed the helicopter’s movements up until moments before the accident. The witness did not see or hear anything unusual before the helicopter impacted the water. Witness 2 was on an adjacent property; they noted a definitive increase in what they thought may have been engine noise just before the accident occurred.

Figure 2: VH-EQW flight track with accident site and witness locations

Source: Google Earth, modified by the ATSB

Witness video 

Recorded video taken by witness 1 just prior to the accident showed the helicopter on approach to the dam. It captured the water bucket suspension cable caught over the rear of the left skid when on the approach (from the start of the video – Figure 3 top) until the helicopter was initiating lift‑off with the external load of water (Figure 3 lower). The video ended as the helicopter started to take the weight of the bucket, which contained a large quantity of water. The helicopter was recorded starting to pitch up and roll left before the video stopped. There was no discernible change in sounds emanating from the helicopter for the duration of the video. 

Figure 3: Sequence of water pickup from dam showing bucket cable position from approach (top) to lifting off (lower)

Source: Still photographs taken from witness video, annotated by the ATSB

Pilot egress

The pilot recalled that, after the surface impact, the helicopter almost immediately became inverted, filled with water, and sank to the bottom of the dam. The pilot stated that they removed their seatbelt and helmet and attempted to open the front left door but could not open it with either the normal or emergency release handles. The helicopter was almost fully submerged when the pilot swam to the rear of the cabin and tried to open the rear right door. They made further unsuccessful attempts to egress by kicking the helicopter windows. 

The pilot then moved to the rear left door, and applying considerable force, was able to successfully open it. The pilot recalled that, when they initially attempted to open the emergency exits, they may have been trying to operate the door handles in the incorrect (opposite) direction due to the helicopter being inverted. 

The pilot escaped the sinking helicopter and swam a few metres to the surface and then to the side of the dam. 

Context

Pilot information

The pilot held a Commercial Pilot Licence (Helicopter) with ratings for single and gas turbine engine helicopters. Prior to the accident flight, the pilot had accumulated 2,599.4 hours of total flying experience. They had 220.8 hours total on the Bell 204/205/UH-1 helicopter. Of this, 22.8 hours was pilot in command of the Bell 204, which was accrued in the 2 months prior. 

The pilot was qualified to conduct helicopter firefighting operations and had low‑level and sling operation ratings. The pilot last completed an aerial application proficiency check on 29 June 2023, which was valid until 20 June 2024, and a low-level helicopter flight review on 12 August 2023. 

The pilot held a Class 1 Aviation Medical Certificate, valid to 12 June 2024, with no restrictions.

Helicopter information

General information

The Bell Helicopter Company 204B (and 205) is the civilian version of the UH-1 Iroquois. It was designed in the mid‑1950’s as a utility helicopter. The helicopter had a 2-blade main rotor and 2‑blade tail rotor and was powered by an Ozark Aeroworks T53-L-13B turboshaft engine. The helicopter was manufactured in the United States in 1965 and first registered in Australia in 2014 as VH-EQW. It had accumulated about 23,515 flight hours total time in service and had a current certificate of airworthiness and registration. The helicopter’s technical log had no outstanding defects at the time of the accident. 

VH-EQW was fitted with an external load hook located directly underneath the main rotor transmission, in line with the helicopter’s centre of lift.

Bucket and suspension cable information

The Bambi Max water bucket fitted to VH-EQW was manufactured by SEI industries and weighed 67 kg empty and 1,300 kg when full, with a capacity of 1,230 L. The bucket was connected to the helicopter’s external load hook by several stainless steel suspension cables, separated fore and aft by a triangular spreader bar (Figure 4). A dump switch on the collective[2] was connected through a black electrical cable to control the dump valve located in the base of the bucket. The suspension cables used on the day had a length of 5.05 m and the total length including the bucket was 6.27 m.

Figure 4: Photo of exemplar Bambi Max bucket in the stored configuration showing the triangular spreader bar and suspension cables

Source: SEI industries, annotated by the ATSB

External load visibility

The helicopter was fitted with 2 rear vision mirrors that were located under each of the Perspex chin bubbles to provide visibility of the external hook, suspension cables and bucket. The bubble window fitted to the pilot’s left door also allowed for better visibility downwards and, to a limited extent, the rear of the helicopter (Figure 5). In response to this draft report, the pilot reported that the mirrors provided a full and clear view of the external hook and bucket. 

Source: Operator, annotated by the ATSB 

Bear paw modification

The helicopter had a pad like modification to the skids called ‘bear paws’, which are supplied as a kit of 2. The pads fit under and to the rear of each of the skids and are designed for landings off airport, on uneven or unstable terrain, helping with overall landing stability and to prevent the rear of the skids from sinking into soft surfaces. The bear paws are made from a polymer plastic and feature high impact resistance, durability and flexibility. They are secured to the skid utilising 4 metal clamps (Figure 6).

Figure 6: VH-EQW left skid with bear paw fitted

Source: ATSB 

Weight and balance

The helicopter was within weight and balance limits during the transit flight to Tarome. However, when lifting the load, with the suspension cables caught over the left rear skid and a full bucket of water (weighing 1,300 kg), the load shifted significantly to the rear and to the left. In this configuration, ATSB calculations showed that the helicopter was outside its balance limitations with the addition of just a 300 kg external load and well outside the balance limitations with the addition of a full bucket of water. 

Meteorological information

The weather at the time of the accident was described by the pilot as clear and calm. The Bureau of Meteorology forecast showed visibility was greater than 10 km and the wind was from the north‑west at 11 kt. 

The flight was to the south-west and had a calculated tail wind component of about 3 kt. The meteorological conditions were not considered a factor in this event. 

Wreckage examination

General engine and airframe examination

The helicopter was retrieved from the dam and taken to a secure facility for detailed examination. The rotor systems, drive shafts, transmissions, flight controls, exits, and engine were visually examined by the ATSB. The fuel control and overspeed governor units were removed from the engine and sent to the engine type certificate holder for functional testing. That testing did not identify any issues with the unit.

The engine drive to main rotor transmission shaft had broken out of its retaining couplings likely due to the impact. The engine manufacturer stated that the damage to the drive couplings was indicative of significant engine power driving the main rotor transmission at the time when the main rotors impacted with water, creating a sudden stoppage. 

The pilot’s left front door emergency jettison system was tested and worked as designed by releasing the door from its hinges. 

No pre-impact defects in the engine, flight controls or emergency exits were identified.

Skid examination

The left and right skid had their bear paw pads removed to facilitate the transport of the wreckage to the storage facility. The right skid did not have any notable damage. The left skid had several striation type wear marks at the rear of the skid (Figure 7).

Figure 7: Rear of left and right skid sections with the left skid showing wear marks

Source: ATSB

The left bear paw was refitted to the left skid to facilitate inspection as an assembly. It was noted that the bear paw had permanent deformation damage that indicated that it had rotated counterclockwise (viewed from the rear) until it had come into contact with the rear skid support. There was also damage to one of the attachment clamps, which had been forced forward at its upmost point. That clamp was directly in front of the forward set of wear marks on the rear of the skid. There was further abrasion damage that indicated the left bear paw had flexed downward under significant load on its outboard side (Figure 9).

Figure 8: Left rear skid showing corresponding skid and bear paw damage

Source: ATSB

The ATSB conducted testing with string lines and a spreader bar configured in a similar manner to the water bucket suspension cables. It was identified that, if both sets of cables were caught over the left skid, the position of the forward and rear cable positions was consistent with the locations of the striation type wear damage found on the left skid[3].

Figure 9 shows the left rear skid, viewed from an outboard direction, showing projected alignment with the hook, cable spreader and multiple bucket suspension cables. Detail A and B show close‑up wear patterns consistent with numerous stainless-steel cable wear striations that aligned with the direction of the external cargo hook attachment point. 

Figure 9: Outboard of left rear skid showing hook position and likely position of spreader with forward and rear cables aligned with wear damage

Source: ATSB

Pre-flight checks with an external load

Wagtendonk (1996), in Principles of Helicopter Flight, stated that:

When a cable or strap has been attached to the helicopter hook it is most important to ensure that the cable does not pass over the skid or undercarriage leg. As the aircraft rises and the strain is taken on the load, this could cause a serious rolling sequence. Use the mirror or look directly at the cable. Sadly, non-compliance with this simple rule continues to cause problems.

Common best practice is for the bucket to be positioned at the front of the helicopter and for the suspension cables to be routed from the hook between the skids to the front. This gives the pilot the best view of the bucket during take-off and reduces the chances of the suspension cables fowling. If the cables are routed and connected from the back of the helicopter, there is potential that the cables can be caught by the skid and not seen during take-off. 

The pilot reported that, during their pre-flight check, the bucket was placed at the front of the helicopter and was functionally tested.  

Helicopter underwater escape training 

Helicopter underwater escape training (HUET) has been in use around the world since the 1940s and is considered best practice in the overwater helicopter operating industry. HUET is designed to improve survivability after a helicopter ditches or impacts into water. Research of such accidents has shown that occupants who survive the initial impact will likely have to make an in‑water or underwater escape, as helicopters usually rapidly roll inverted post-impact due to the position and mass of the engine/s, transmission and main rotor system. The research has also shown that drowning is the primary cause of death following a helicopter accident into water.

Fear, anxiety, panic, and inaction are the common behavioural responses experienced by occupants during a helicopter accident. In addition to the initial impact, in-rushing water, disorientation, entanglement with debris, unfamiliarity with seatbelt release mechanisms and an inability to reach or open exits have all been cited as problems experienced when attempting to escape from a helicopter following an in-water accident (Rice & Greear, 1973). 

HUET involves a module (replicate of a helicopter cabin and fuselage) being lowered into a swimming pool to simulate the sinking of a helicopter. The module can rotate upside down and focuses students on bracing for impact, identifying primary and secondary exit points, egressing the wreckage and surfacing. HUET is normally part of a program of graduated training that builds in complexity, with occupants utilising different seating locations, exits and visibility (via the use of ‘blackout’ goggles). This training is conducted in a controlled environment with safety divers in the water.

HUET is considered to provide individuals with familiarity with the crash environment and confidence in their ability to cope with the emergency situation (Ryack et al., 1986).  Interviews with survivors from helicopter accidents requiring underwater escape frequently mention they considered that HUET was very important in their survival. Training provided reflex conditioning, a behaviour pattern to follow, reduced confusion, and reduced panic (Hytten, 1989). 

The pilot conducted HUET training in March 2021, with a renewal due in 2024. The pilot reported that familiarity with the helicopter, the open area in the cabin (all seats removed) and HUET assisted with their ability to successfully escape from the sinking helicopter.

Helicopter water bucket operation accidents

The helicopter manufacturer stated that, between 1974 and 2017, there were 6 accidents involving Bell Helicopters conducting external lift operations where the suspension cable became entangled with the skids and the helicopter lost control during water uplift. Two of those accidents involved the Bell 204/205 helicopter and 4 were Bell 206s.

The Flight Safety Foundation conducted a study titled External loads, powerplant problems and obstacles challenge pilots during aerial fire-fighting operations. The study utilised data from helicopter accident reports in the United States between 1974 and 1998. 

The study showed that, it was over twice as likely for a firefighting helicopter to be involved in an accident when compared to private helicopter flights. Of the 97 accidents studied, 4 instances were due to water bucket cables being caught on the skids leading to a loss of control during uplift. Further, there were 2 instances where the unloaded water bucket or external load cable came into contact with the tail/tail rotor due to excessive speed, turbulence and manoeuvring. 

Two recent accidents, one in 2017 (New Zealand Transport Accident Investigation Commission report AO-2017-001) and one in 2019 (French Bureau d'Enquêtes et d'Analyses report 2019‑0023) also involved helicopter buckets coming into contact with the tail rotor. Those accidents were partly attributed to operating at airspeeds above the manufacturers’ velocity never exceed speeds.  

Safety analysis

Suspension cable caught on left rear skid 

Video evidence showed the Bambi Bucket suspension cables were caught over the left rear skid when the helicopter was on approach to the dam and remained attached to the skid during the water uplift. This evidence was consistent with:

• The ATSB’s wreckage examination, which identified that the left rear skid and bear paw showed multiple areas of damage including striation marks indicative of contact with the bucket suspension cables.
• Testing of the striation marks alignment with the fore and aft cable positions when attached to the external cargo hook with the triangular load spreader. 

The video ruled out the capture of the cables over the skid during the water collection phase. However, the ATSB was unable to identify if the cables had become captured due to pre-flight bucket and cable positioning, take-off manoeuvring or during the transit flight to the dam.

Just before the video ceased, it showed the initiation of the full water bucket uplift followed by the helicopter pitching up and rolling left slightly. The ATSB weight and balance calculations concluded that any bucket weight above 300 kg (full is 1,300 kg) acting over the left rear skid, would be sufficient to move the centre of gravity outside of the helicopter’s balance limit. Therefore, it was almost certain that as the load of water was lifted, the helicopter’s centre of gravity moved aft and left as a result of the suspension cables being caught over the skid. The tethered weight created an asymmetric lifting point, which resulted in a rapid loss of control and subsequent collision with water.

Helicopter underwater escape training

The pilot conducted HUET about 2.5 years prior to the accident. This likely assisted their ability to egress the helicopter through a rear door while inverted and underwater. HUET has been shown to significantly increase the chances of survival in the event of collision with water. 

No pre-impact defects

The ATSB wreckage examination did not identify any pre-impact mechanical issues with the helicopter. Further, the engine manufacturer concluded that the engine was supplying significant power to the transmission at the time of the impact with water.

Findings

From the evidence available, the following findings are made with respect to the loss of control and collision with water involving Bell Helicopter Company 204B helicopter, near Tarome, Queensland, on 20 September 2023. 

Contributing factors

• The Bambi Bucket suspension cables were caught over the left rear skid. Consequently, as the load of water was lifted, it was almost certain that the helicopter’s centre of gravity moved aft and left, the tethered weight over the skid created an asymmetric lifting point. This resulted in a loss of control and the helicopter collided with water.

Other findings

• The pilot conducted helicopter underwater escape training 2.5 years prior to the accident. This training increased the pilot's chances of survival when the helicopter became submerged in the dam.
• There were no pre-impact defects identified with the helicopter.

Sources and submissions

Sources of information

The sources of information during the investigation included:

• pilot
• operator and chief pilot
• Civil Aviation Safety Authority
• Queensland Police Service
• aircraft manufacturer
• aircraft maintenance organisation
• Airservices Australia
• witnesses
• video footage of the accident flight and other photographs and videos taken on the day of the accident.

References

Hytten, K. (1989). Helicopter crashing in water: Effects of simulator escape training. Acta Psychiatrica Scandinavica, Suppl. 355: 73-78. Cited in Coleshaw, S. (2010). Report for the Offshore Helicopter Safety Inquiry. Report No SC176.

Rice, E, V., & Greear, J. F. (1973). Underwater escape from helicopters. In Proceedings of the Eleventh Annual Symposium, Phoenix, AZ: Survival and Flight Equipment Association, 59-60. Cited in Brooks C., (1989). The Human Factors relating to escape and survival from helicopters ditching in water. AGRAD.

Ryack, B. L., Luria, S. M., & Smith, P. F. (1986). Surviving helicopter crashes at sea: A review of studies of underwater egress from helicopters. Aviation, Space, and Environmental Medicine, 57(6), 603-609.

United States Federal Aviation Administration, (2019). Helicopter Flying Handbook. https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/hfh_front.pdf 

Wagtendonk, W.J. (1996). Principles of Helicopter Flight, Aviation Supplies & Academics, Inc. Washington, USA.

Veillette, P. R. (1999). External loads, powerplant problems and obstacles challenge pilots during aerial fire-fighting operations (Flight Safety Foundation). https://flightsafety.org

Submissions

Under section 26 of the Transport Safety Investigation Act 2003, the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. That section allows a person receiving a draft report to make submissions to the ATSB about the draft report. 

A draft of this report was provided to the following directly involved parties:

• pilot
• chief pilot  
• aircraft maintenance organisation
• National Transportation Safety Board
• aircraft and engine manufacturers
• Civil Aviation Safety Authority.  

Submissions were received from the pilot and chief pilot. The submissions were reviewed and, where considered appropriate, the text of the report was amended accordingly.

Purpose of safety investigations The objective of a safety investigation is to enhance transport safety. This is done through: identifying safety issues and facilitating safety action to address those issues providing information about occurrences and their associated safety factors to facilitate learning within the transport industry. It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action. Terminology An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue. Publishing information  Released in accordance with section 25 of the Transport Safety Investigation Act 2003 Published by: Australian Transport Safety Bureau © Commonwealth of Australia 2024 Ownership of intellectual property rights in this publication Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia. Creative Commons licence With the exception of the Commonwealth Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this report is licensed under a Creative Commons Attribution 4.0 International licence. The CC BY 4.0 licence enables you to distribute, remix, adapt, and build upon our material in any medium or format, so long as attribution is given to the Australian Transport Safety Bureau.  Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.

Executive summary

What happened

On 6 September 2023, following departure from Brisbane, Queensland and while approaching cruise altitude, the flight crew of a B737 registered VH‑YQR, received a call from the cabin crew requesting entry to the flight deck. The aircraft captain, who was the pilot monitoring (PM), reached across the centre aisle stand to activate the flight deck door switch.

Immediately after, the aircraft appeared to momentarily roll and/or yaw, which drew the crew’s attention but, as nothing abnormal was apparent, the PM continued to maintain the switch selection while looking at the door and waiting for it to open. After about 5 seconds, the aircraft began to roll to the left. The first officer, who was the pilot flying (PF), unsuccessfully attempted to correct the roll with autopilot input, and subsequently applied a large manual corrective roll input to bring the wings back to level while the PM released the switch. The aircraft’s bank angle peaked at about 42° left angle of bank and the bank angle alert was triggered. 

As the flight crew sought to determine the cause of the inflight upset, the PF needed to maintain significant right wing down aileron input to maintain an approximate wings level attitude. At the PF’s suggestion, the PM checked the aircraft’s rudder trim which was identified as being displaced to the left by about 5°. The trim was returned to neutral and the aircraft continued the flight without further incident, landing at Melbourne, Victoria about an hour later. A cabin crew member sustained a minor injury as a result of the upset.

What the ATSB found

The ATSB investigation found that, after visually identifying the flight deck door unlock switch, the PM diverted their attention to the door, and instead of grasping the door switch, the rudder trim control was selected. The PM then activated that control, and inadvertently applied full left rudder trim for about 8 seconds instead of unlocking the door.

The autopilot responded to the resultant left yaw and induced left roll by applying increasing right wing down aileron input, which was replicated on the pilots’ control wheels. While the autopilot was initially able to maintain an approximate wings level attitude, it reached the limit of its authority after 5 seconds of left rudder trim application and the aircraft began to bank left, with the rate of bank increasing rapidly and resulting in an inflight upset.

Despite the large right wing down aileron input required to recover and maintain the aircraft in an approximate wings level attitude, the flight crew were unable to promptly identify the significant left yaw as the primary initiator of the upset, which delayed the restoration of balanced flight.

What has been done as a result

Following the incident, Virgin Australia implemented changes to the flight deck door entry procedures that limited the time that the door unlock switch was to be held in the unlock position. It also provided a briefing on the event to flight crews and made changes to the non-technical skills program addressing this type of occurrence.

Safety message

When selecting and activating any control or switch, it is critical that flight crew ensure that the intended control or switch is positively identified and actually selected before activating it. Further, it is important that any mis-selection of switches be reported not only to the operator, but also to the manufacturer, as a continuing record of switch mis-selection across a fleet type may indicate a design error that needs correcting.

The investigation

The occurrence

At 1605 local time on 6 September 2023, a Virgin Australia Boeing 737-8FE (B737) aircraft registered VH‑YQR departed Brisbane, Queensland for Melbourne, Victoria. The flight crew consisted of the aircraft captain, who was performing the pilot monitoring duties (PM) from the left seat, and the first officer (FO), who was performing the pilot flying (PF) duties from the right seat.^[1]

Inadvertent application of rudder trim

Following an uneventful take-off, the aircraft was being controlled through the autopilot for the climb to the intended cruise altitude of flight level (FL) 380.[2] The PF did not have their hands and feet on the flight controls but was in a seating position that enabled full access to those controls.

As the aircraft approached FL 370, with the flight crew engaged in conversation, they received a call from the cabin crew requesting entry into the flight deck. Following completion of security procedures, the PM proceeded to enable entry into the flight deck using the flight deck door switch (FLT DK DOOR). The PM looked to the centre aisle stand, identified the FLT DK DOOR switch, and simultaneously reached across the stand to activate that switch.

However, just prior to grasping the switch, the PM transferred their gaze from the aisle stand to the rear of the flight deck and to the door. In doing so, they inadvertently grasped and, at 1625:22, activated the rudder trim control to the full left position instead of selecting the FLT DK DOOR switch.

On activation of the rudder trim control, both pilots felt the aircraft briefly roll and/or yaw and queried each other on what had occurred. The PM immediately looked forward and outside the aircraft, and then scanned the instruments, while continuing to maintain the input on the rudder trim control. The PF scanned the flight instruments and noted that the position trend vector[3] on the navigation display had begun to indicate a slight left turn. As neither pilot identified anything abnormal, the PM looked back to the cabin door, waiting for it to open, while maintaining the full‑left rudder trim control selection, and the PF continued to monitor the flight instruments.

Autopilot response

The autopilot responded to the increasing left rudder trim and resultant left yaw with an opposing and increasing right wing down aileron input. This was replicated on the control column’s control wheel as an increasing right wing down control wheel displacement. However, a slight left bank began to develop.

After 5 seconds of full left rudder trim input, the autopilot’s application of aileron input to counter the increasing rudder trim and yaw reached the limit of its authority – that is, the autopilot had applied the maximum aileron input available to it. This was also replicated on the control wheel, which by then was displaced to a 22° right wing down position. The aircraft, however, still had a left roll bank angle of about 5°. As the left rudder trim input continued, and in the absence of any further autopilot counter-input, the aircraft responded with an increasing left roll.

Inflight upset and recovery

Identifying the increasing left roll and turn, the PM again looked forward and queried whether the aircraft was supposed to be turning. The PF responded in the negative. At about the same time, about 8 seconds after first applying the unintended input, the PM released the rudder trim control. About 5° of left rudder displacement had been applied and the aircraft was now banked about 10° to the left.

Due to the significant rudder displacement, the aircraft’s left turn bank angle began to rapidly increase. As it passed about 25°, the PF attempted to counter the turn using the heading mode and heading changes on the mode control panel. This had no effect, and with the bank angle increasing past 35°, the PF announced and disconnected the autopilot and autothrottle, taking manual control of the aircraft. Almost simultaneously, the PF briefly applied about two-thirds deflection of the control wheel right wing down input to oppose the left roll.

Almost immediately after, the aircraft’s ground proximity warning system (GPWS) bank angle alert triggered, which the PM responded to by calling ‘upset’. The PF acknowledged the upset call and responded by verbalising and appropriately actioning the upset recovery procedure. A large application of opposite (right) right wing down roll stopped and then reversed the increasing left roll, but not before the aircraft had attained 42° left bank angle. The aircraft was recovered to an approximate wings level attitude of less than 10° bank angle about 18 seconds after the rudder trim input was first applied. Shortly thereafter, the aircraft was banked to the right with the intent to regain tracking.

The flight crew immediately initiated troubleshooting to determine the cause of the uncommanded roll, with the initial focus on an engine‑related issue. The aircraft had not lost any height during the upset, and the required tracking was quickly regained. However, during this period of troubleshooting, the PF needed to hold about 35° of right wing down control wheel displacement to maintain an approximate wings level attitude. The PF stated that, in recovering manual control after disconnecting the autopilot, both hands and feet were returned to the manual flight controls.

While the PM was checking for the cause of the upset, the PF called for the rudder trim to be checked, as there were no alerts or other apparent sources causing the large roll input. The PM checked the trim indicator and identified the inadvertently applied trim. At 1626:34, right rudder trim was then applied to neutralise the rudder position.

Events in the cabin

At the time of the occurrence, the cabin crew had commenced a food service, with service carts moving through the cabin. Due to the movement of the aircraft during the upset, a cabin crew member at the rear of the aircraft sustained a minor injury while stabilising a cart.

Context

Personnel information

The captain held an air transport pilot licence (aeroplane), while the FO held a commercial pilot licence (aeroplane). Both pilot licences included appropriate aircraft ratings, operational ratings and endorsements for operating the B737 aircraft type. Both pilots held a current Class 1 aviation medical certificate. The ATSB found no indicators that increased the risk of the flight crew experiencing a level of fatigue known to affect performance.

The captain had about 19,500 hours of flight experience, of which 13,500 hours were on the B737 type. The FO had about 2,700 hours of flight experience, of which about 350 hours were on the B737. The captain had flown 165 hours, and the FO 178 hours, on the B737 type in the previous 90 days.

Aircraft information

Flight controls

The B737 flight control system uses a conventional control wheel, column and rudder pedals (Figure 1) at each pilot’s station, linked mechanically to hydraulically‑powered control units at each flight control surface. These control units move those flight control surfaces in response to inputs from either pilot or the autopilot. The 2 sets of pilot flight controls are manually linked, such that an input on one control is replicated at the other station.

Figure 1: B737-800 flight deck layout

An image of the flight deck of a generic B737-800 type aircraft. Source: Copyright © Boeing. Reproduced with permission, annotated by the ATSB

Control of the aircraft along its 3 axes (Figure 2) is achieved through:

• ailerons supplemented by flight spoilers for roll control on the longitudinal axis
• rudder for yaw on the vertical axis
• elevators for pitch on the lateral axis.

Figure 2: B737 flight control surfaces

A 3-dimensional depiction of the B737 aircraft identifying the 3 axes of motion and the relevant control surfaces for those axes. Source: JTSB investigation AI2014-4, modified by the ATSB

The ailerons/flight spoilers are controlled by the pilots' control wheel. The 4 flight spoilers on the upper surface of each wing supplement roll control when the control wheel is displaced by more than about 10°. The flight spoilers on the up-aileron wing rise with the aileron, while those on the down-aileron wing remain faired. The rudder is controlled through the pilots’ rudder pedals. Rudder displacement is restricted at airspeeds greater than about 135 kt by reducing the amount of hydraulic pressure available to control the rudder.

Rudder trim

The rudder trim control (Figure 3), located on the aft electronic panel (Figure 1), adjusted the rudder’s neutral position by electrically positioning the rudder. The rudder pedals are also displaced proportionately to any rudder trim adjustment. The rudder trim indicator displayed the rudder trim position in non‑dimensional units.

Figure 3: Rudder trim and door lock switch

An image of the normal aisle stand configuration for the operator’s B737 aircraft, with the rudder trim control and position indicator, and the flight deck door switch identified. Source: Virgin, annotated by the ATSB

The rudder trim control was spring‑loaded to return to the neutral (centre) position and activation was through rotating the control in the direction of required trimming. The trim control was a circular rotary switch with segmented straight knurling.

Autopilot flight control

The aircraft was fitted with 2 autopilots (or flight control computers) that could be engaged using controls on the mode control panel (MCP) (Figure 1). Only one autopilot was able to be engaged at a time (except when the approach mode was selected on the MCP). The engaged autopilot controlled the aircraft’s flight path through commands to pitch and roll control units, which then moved the relevant flight control surfaces.

Boeing advised that the autopilot had limited flight control input in certain modes. In particular, during single autopilot operations, there was limited roll input authority, and therefore there was a limit to the maximum aileron input that could be applied. During the occurrence, the recorded data showed the autopilot input for the roll control surfaces reached the allowable limit, while rudder trim input and the resultant roll continued to increase. Upon disconnecting the autopilot, the aileron displacement rapidly increased with the pilot’s input.

The operator’s B737 flight crew operations manual (FCOM) did not document the limitation to the autopilot’s control surface inputs in single autopilot operation. The FCOM did, however, indirectly indicate an autopilot control input limitation in the section on the Roll/Yaw Asymmetry Alert.

Flight deck door lock

The flight deck door switch was a spring‑loaded, elongated, hexagonally (blade) shaped, rotary switch with 3 positions:

• UNLKD, which unlocked the door while the selector was maintained in this position
• AUTO, which locked the door automatically when closed
• DENY, which overrode the alternate method of opening the door.

The switch was spring‑loaded to the AUTO position and had to be pushed in before rotating from AUTO to UNLKD.

Primary flight display with bank angle and slip/skid indicators

The outboard display unit for both pilots (Figure 1) is normally used as the primary flight display (PFD). It provides the information and parameters necessary to monitor and control the aircraft’s flight path. Central to the display is the attitude indicator, which provides an indication of the aircraft’s pitch and roll attitude referenced to the horizon (Figure 4). The following features of the attitude indicator are relevant to this occurrence:

• the pitch scale is in 2.5° increments
• a bank angle pointer indicates bank angle, and always points to the vertical (a white-outlined triangle in the left panel of Figure 4 and a solid amber triangle in the right panel)
• the bank angle pointer turns solid amber when the bank angle is 35° or more
• a roll scale is marked to indicate bank angle increments of 10°, 20°, 30°, 45° and 60°.

Figure 4: PFD with bank angle pointer and slip/skid indicator highlighted

A split image showing the primary flight display bank angle pointer and the slip/skid indicator in a wings level and balanced flight state on the left, and on the right, a reproduction of the display at the time of the maximum bank angle during the occurrence. Source: Copyright © Boeing. Reproduced with permission, annotated by the ATSB

Immediately below, and adjacent to, the bank angle pointer is the slip/skid indicator (Figure 4). It is normally represented by a white-outlined rectangle. The slip/skid indicator will displace to the left or right of the bank angle pointer to indicate lateral acceleration (g), with maximum displacement of the indicator occurring at 0.21 g or greater of lateral acceleration.

The outline of the slip/skid indicator will turn amber when the aircraft is banked to 35° or more (see right panel of Figure 4). The indicator turns solid white when at full scale deflection and the bank angle is less than 35°, and solid amber when at full scale deflection and bank angle is 35° or more.

Boeing provided a simulated recreation of the occurrence event’s PFD indications at the maximum bank angle of 42° (right panel of Figure 4). The recreation showed that the bank angle indicator and outline of the slip/skid indicator had turned amber, but the slip/skid indicator was not at its maximum displacement and therefore not solid amber.

GPWS bank angle alert

The aircraft’s ground proximity warning system (GPWS) provided an aural BANK ANGLE, BANK ANGLE alert when roll angle exceeded 35°, 40°, and 45°. Once sounded, the alert was silent for that respective bank angle (35°, 40°, or 45°) until the system was reset by the bank angle decreasing to 30° or less.

Roll/yaw asymmetry alert

Seven of the operator’s B737 aircraft were fitted with a roll/yaw asymmetry (R/YA) alert, although the occurrence aircraft was not. The R/YA alert notified flight crew of an asymmetry issue that had led to yaw-induced roll, through the provision of alerts that identified the level of autopilot roll authority that had been used to counteract the yaw. These alerts were:

• the ROLL/YAW ASYMMETRY alert displayed at 75% of the autopilot’s roll authority limit
• the ROLL AUTHORITY alert displayed when the autopilot’s roll authority limit reached 100%, which was also accompanied by an aural ROLL AUTHORITY alert.

The asymmetry alerts also caused the bank pointer and slip/skid indicator to become outlined in amber. The slip/skid indicator would also become solid amber when it was displaced by more than 25% of its width (Figure 5).

Figure 5: Roll/yaw asymmetry alert

A split image showing an updated primary flight display where conditions exist that trigger the ROLL/YAW ASYMMETRY alert, on the left, and the ROLL AUTHORITY ALERT, on the right. Source: Copyright © Boeing. Reproduced with permission, annotated by the ATSB

Operating procedures

Use of rudder trim

The captain’s preflight procedure included checking all trim controls for trim’s freedom of movement, and then ensuring that the aileron and rudder trims were set to zero units. The flight crew training manual contained a section on recommended rudder trim technique. This provided guidance and procedures to ensure that the rudder trim was set for minimum drag and zero roll/heading change. Trimming the rudder for minimum drag was a normal and regularly practiced procedure, mostly used early in the cruise phase of flight.

Operation of switches on the flight deck

The operator’s operating policies and procedures (OPP) manual required specific procedures be applied when changes were made to a safety critical system’s switch or control. A critical control or switch was defined as one that controls or alters the configuration, operating mode or function of an aircraft system. A safety critical system was one where mis-selection may lead to an undesired aircraft or system state, incident or accident. The flight deck door lock switch did not fall into these categories and were therefore not subject to the relevant procedures in the OPP. However, the OPP also stated that controls and switches must not be changed or activated prior to positive visual identification.

Flight deck door lock

The OPP manual included a procedure for entering the flight deck, which included a method of communicating and then coordinating entry through the locked flight deck door. The procedure required the use of the flight deck door switch and did not contain any restriction or limitation on the use of that switch.

Bank angle

The OPP manual specified policies for passenger comfort and wellbeing, which limited bank angle to a maximum of 30°.

Inflight upset

The OPP manual defined an ‘upset’ as:

an undesired aircraft state characterised by unintentional divergences from parameters normally experienced during operations.

There was no specific procedure for upset recovery. Instead, the flight crew operating manual (FCOM) quick reference handbook (QRH) provided:

…actions that represent a logical progression for recovering the airplane. The sequence of actions is for guidance only and represents a series of options to be considered and used dependent on the situation.

The upset recovery sequence of actions was included within the non-normal manoeuvres section of the QRH, and contained a preliminary statement that ‘flight crews are expected to do non‑normal maneuvers from memory’. Similar guidance material was also contained in the flight crew training manual (FCTM).

Information on sideslip

The operator published a flight crew information manual, the purpose of which was to provide a consolidated source of training, reference or flight technical information for flight crew. That manual contained the following guidance on pilot-commanded sideslip:

The rudders on modern jet transport aircraft are sized to counter the yawing moment associated with an engine failure at very low take-off speeds and to ensure yaw control throughout the flight envelope, using up to maximum pedal input. This very powerful rudder is also capable of generating large sideslips. An inappropriate rudder input can produce a large sideslip angle, which will generate a large rolling moment that requires significant lateral control input to stop the aircraft from rolling. The rudder should not normally be used to induce roll through sideslip because the transient sideslip can induce very rapid roll rates with significant time delay...

Recorded data

Recorded data from the aircraft’s quick access recorder (QAR), which contained data from the aircraft’s flight data recorder, enabled a detailed examination and recreation of the occurrence event. The ATSB also sought Boeing advice on the aerodynamics of the occurrence event, which stated the following:

Analysis of the QAR data indicates that a roll to the left from wings-level to a peak of -42 °s (left wing down) with the autopilot B channel engaged was the result of a left rudder trim input that persisted for approximately 8 seconds. The rudder trim input remained for approximately 90 seconds during which time an average control wheel deflection of approximately 35 °s (right) was maintained along with a sustained, non-zero lateral acceleration (uncoordinated flight) of around -0.06 g’s (left). As the autopilot reached its maximum control wheel authority to the right with the airplane continuing to increase bank to the left, the flight crew intervened and commanded the control wheel further to the right, causing the autopilot to disconnect and resulting in bank angle returning back towards wings-level. Margin to stall warning activation was generally reduced as a result of elevated normal load factor from the non-zero bank angle and sustained right-wing-down control wheel deflection sufficient to raise the flight spoilers, leading to reduced lift on the wing and elevated angle of attack while the non-zero rudder trim input was maintained. When the rudder trim was returned to near zero °s (neutral), the airplane returned to normal flight; the rudder deflection and control wheel deflection subsequently reduced leading to reduced angle of attack and increased margin to stall warning activation. The airplane systems functioned as expected with no observed anomalies.

Table 1 contains data extracted from the recorded data for specific parameters covering the period from the commencement of the trim application until the aircraft was recovered and stabilised at an approximate wings level attitude.

Table 1: Selected DFDR data for the occurrence event

Boeing advice on the effect of rudder

In May 2002, Boeing published a flight operations technical bulletin[4] (FOTB) on the use of rudder in transport category aircraft. The FOTB provided both generic information applicable to all of its swept wing jet transport aircraft, and specific information relevant to the B737:

Maneuvering an airplane using the rudder will result in a yaw and roll response. The roll response is the result of sideslip. For example, if the pilot applies left rudder the nose will yaw left ... This yawing response to the left will generate a sideslip (right wing forward). The resulting sideslip will cause the airplane to roll to the left (i.e., roll due to sideslip). The actual force on the vertical tail due to the rudder deflection tends to roll the airplane right, but as the sideslip moves the right wing forward, the net airplane roll rate is to the left.

It is difficult to perceive sideslip and few modern transport airplanes have true sideslip indicators. In older transport instrument panels the “ball” was an indicator of side force or acceleration, not sideslip angle. Some newer models have electronic flight displays with a slip/skid indication, which is still an indication of side force or acceleration; not sideslip. As the pilot applies more rudder, more sideslip is generated and a greater roll response will result...

...Because sideslip must build up to generate the roll, there is a time lag between the pilot making a rudder input and the pilot perceiving a roll rate. This lag has caused some pilots to be surprised by the abrupt roll onset and in some cases to interpret the rapid onset of roll as being caused by an outside element not related to their rudder pedal input...

On [the Boeing 737], as the airplane speeds up, the rudder authority is limited, but the gearing between the rudder and the rudder pedal does not change. Since rudder authority is limited, rudder pedal travel is also limited; i.e., full rudder pedal deflection is not required to get full available rudder deflection. Rudder pedal force is a function of rudder pedal deflection, so less force will be required to achieve maximum available rudder deflection as airspeed increases.

Included within the FOTB was a table detailing rudder deflection and force required at various airspeeds (Table 2).[5]

Table 2: Rudder movement parameters for Boeing aircraft

The PF stated that, immediately on disconnecting the autopilot, they placed their feet on the rudder pedals. However, the displacement of those pedals due to the inadvertent trim input was not detected. This was most likely the result of that displacement being less than about 2 inches (5 cm) despite that displacement corresponding to a significant rudder position change from the neutral. This relatively small pedal displacement in proportion to rudder position is a unique feature of the B737. This factor as well as the PF’s limited experience on the aircraft type likely influenced the rudder pedal displacement associated with the inadvertent rudder trim not being detected.

History of inadvertent rudder trim application events on B737 aircraft

JTSB investigation AI2014-4

On 6 September 2011, a B737-700 operating into Tokyo, Japan experienced an inflight upset during which it deviated significantly from track, reached a bank angle of 131°, lost about 6,000 ft in altitude, and exceeded the aircraft’s load factor limitation during the recovery. The subsequent Japanese Transport Safety Board (JTSB) investigation found that, as the aircraft approached Tokyo at FL 410, the captain briefly left the flight deck and, on notifying the first officer (FO) to allow re‑entry, the FO inadvertently operated the rudder trim switch instead of the flight deck door switch, resulting in left rudder trim being applied.

The trim input exceeded the autopilot’s capacity to control the aircraft’s attitude, resulting in an unusual attitude developing. The FO’s recognition of the unusual attitude was delayed, and the subsequent recovery was insufficient, resulting in the aircraft’s entering a nosedive before being recovered to normal flight about 60 seconds after trim application commenced.

The inadvertent selection of the trim control was partially attributed to the FO having previously flown B737 aircraft with a different trim control/door switch arrangement. In particular, the investigation identified that the rudder trim switch on the occurrence aircraft was in approximately the same location as the flight deck door switch on the B737-500, the type from which the FO had recently transitioned. There were many recommendations arising from this investigation, including the following (JA16AN) to the Federal Aviation Administration (FAA) of the United States:

The aircraft designer and manufacturer shall study the need to reduce or eliminate the similarities between the rudder trim control and the switch for the door lock control of the Boeing 737 series aircraft, in terms of the shape, size and operability as mentioned in this report. In particular, it shall consider the effectiveness of changing the shape and size of the rudder trim control to the design adopted for the rudder trim control for Boeing models other than those of the Boeing 737 series, in which the switch has a cylindrical shape about 50mm in diameter without a brim, so that the difference of the size and shape can be recognized only with a touch.

Boeing human factors analysis of the Tokyo occurrence

Following the Tokyo occurrence, Boeing human factors subject matter experts (SME) conducted a comprehensive analysis of the 2 error types that led to that event. The first error type concerned variation in aisle stand layout across the operator’s fleet and related to the pilot’s transfer from an older B737 model with a different aisle stand layout. This variability in layout was found to have contributed to the inadvertent selection of the rudder trim instead of the door lock switch. To mitigate against that, the SMEs recommended consistency in aisle stand configurations across the various B737 fleet types.

The second error type was substitution, where once having operated the incorrect switch, the pilot continued to believe that the rudder trim knob was the door control knob. To address this error, the SMEs fitted different knob shapes to a simulator to determine if they would more clearly differentiate between the 2 switches. The study found that none of the alternative knob styles prevented confusion in all circumstances, and changing styles could introduce a further inconsistency risk through the period of adoption over the full fleet. The SMEs also considered alternative actions for those controls to further distinguish between them but noted that the 2 switches already had a distinct difference in activation methods.

Boeing’s analysis determined that switch location was more important than shape, and that the most important factor to minimise inadvertent activation was consistency in aisle stand configuration across an operator’s fleet type. While both switches had a similar feel and operation, a standard location and sufficient separation between these controls was recommended. The recommended switch locations were those consistent with the generic Boeing-delivered aircraft (Figure 1). Having the controls placed in these recommended locations:

• created a distinctive reach posture for both pilots
• provided sufficient separation in relation to reach direction from both seats
• provided adjacent tactile landmarks[6] to assist in distinguishing between the switches.

Boeing response to Tokyo occurrence

On 16 July 2012, in response to the Tokyo occurrence, Boeing transmitted a multi operator message (MOM-MOM-12-0489-01B) titled Information – Inadvertent Activation of Rudder Trim. The message was addressed to a broad scope of addressees, including all 737 customers, and had an Engineering and Flight Operations categorisation. It summarised the JTSB incident and alerted operators to the potential for confusion between the rudder trim control and the flight deck door switch on certain models of B737 aircraft. This was based on variability in switch locations on the aisle stand across the B737 fleet, and the similarity in the operation of the 2 controls. It recommended several actions to mitigate the potential for inadvertent rudder trim activation, including:

• ensuring flight crew awareness of this specific potential for error and the need for visual identification prior to operating a control
• ensuring that no aircraft in their fleet had the rudder trim control in the same location as the flight deck door switch on another aircraft of the same type.

Boeing 737-SL-27-238

Also in response to the Tokyo occurrence, Boeing released service letter 737-SL-27-238, titled Inadvertent Activation of Rudder Trim, dated 19 September 2012.[7] The purpose of the service letter was to notify operators of the potential for confusion of the rudder trim knob and the secure flight deck door knob located on the aisle stand. It contained a description of the Tokyo occurrence, Boeing’s actions in response to this occurrence, and recommendations to operators to prevent any future occurrences. The recommendations reflected those stated in the July 2012 multi operator message.

FAA SAIB NM-15-03

In November 2012, the FAA issued a Special Airworthiness Information Bulletin (SAIB) to advise all owners of Boeing transport category aircraft of an airworthiness concern regarding inadvertent actuation of flight deck controls. The SAIB summarised the Tokyo occurrence and identified the varying locations of the rudder trim control and flight deck door switch across various B737 models. It stated the potential for confusion when pilots transferred between similar model aircraft, but with variation in the switches’ location, and discussed the differences in the switch shapes and similarities in their operation. It referenced Boeing’s MOM and service letter published in response to the event.

The SAIB identified that the potential for error may not be applicable to many operators due to differences in their flight deck procedures to that of the Tokyo occurrence operator. One of those differences was where operators did not use the flight deck door switch to enable fight deck entry, but instead used alternate methods of entry.

The bulletin also provided a summary of Boeing’s human factors analysis on the switch mis‑selection and possible methods to mitigate it.

The SAIB concluded with recommended procedural changes for operators. Where operators did not adopt those procedural changes, the SAIB recommended they should undertake certain configuration changes in the aisle stand location of those controls and where operators did modify their procedures as recommended, they should still undertake the recommended configuration changes.

FAA response to JTSB recommendation

The FAA formally responded to JTSB recommendation JA16AN in May 2015. That response stated that the FAA determined that the risk associated with the Tokyo occurrence warranted the issue of an SAIB and a Continued Airworthiness Notification to the International Community (CANIC). Prior to their issue, the FAA had requested the JTSB review those documents. As publication of the SAIB and CANIC had been finalised, the FAA considered the JTSB recommendation JA16AN had been effectively addressed.

An update of 737-SL-27-238

With the introduction of the B737MAX, Boeing became aware that the issue addressed by 737‑SL-27-238 could also apply to the new model. In May 2017, Boeing issued service letter 737‑SL-27-238-A, a re-issue of the original service letter but modified to include the B737MAX aircraft. The substance of the original service letter remained unchanged. 

FOTB 737 21-03 Erroneous Use of Rudder Trim Control

In 2021, Boeing received a report concerning a B737-800 pilot who had mis-selected the rudder trim control and applied left rudder trim while attempting to use the flight deck door switch. The autopilot countered the resultant roll, but the authority limit was reached, after which the aircraft continued to roll. The aircraft was recovered, but not before a BANK ANGLE alert was triggered and the aircraft rolled to nearly 50° bank angle. The occurrence was not subject to an official state‑based investigation, however, the similarities with the Tokyo occurrence prompted Boeing to issue an FOTB on erroneous use of rudder trim.

The FOTB identified the similarities between the new 2021 event and the event reported in the July 2012 MOM and the May 2017 service letter. The FOTB identified that risk of these types of events was elevated when there was variability in the switch locations on the aisle stand across the airline fleet, and due to the similarity in the control operation. As a result, Boeing recommended that operators standardise aisle stand configuration across its B737 fleet, and conduct awareness training for flight crews about the prevention of unintended operation of flight deck controls. This included an emphasis on visual identification of controls and switches prior to operation.

Virgin response to Boeing alerts concerning inadvertent rudder trim activation

Virgin advised that the MOM and service letters had been reviewed by its engineering department, and that while there were some B737 aircraft fitted with a variation in aisle stand layout to the generic configuration, the various aircraft ages and types did not enable exact same aisle stand configurations. Further, the advice in those documents specifically focused on configurations where the rudder trim on one type was in the same location as the door lock on another, and this was not the case for the Virgin fleet. As such, Virgin complied with the advice stated in the MOM and service letters. Virgin did not provide any advice on how the MOM, service letters or FOTB was actioned by the flight operations department.

Safety analysis

In response to a request for entry into the flight deck, the pilot monitoring (PM) intended to activate the flight deck door lock switch. The operator’s policy and procedures manual required flight crew to positively identify any control or switch before manipulating them. The PM visually identified the flight deck door switch, but in reaching for it, did not visually confirm selection or manipulation of the correct switch, instead mis-selecting and activating the rudder trim switch.

A human-factors analysis of the mis-selection of the rudder trim control found that the error was consistent with an unintentional slip. The action occurred during a period of possible distraction when the PM was talking to the pilot flying (PF) and monitoring the aircraft as it approached cruise altitude. The PM’s action of looking away from the panel when selecting the switch was also an example of attention diversion. The distraction and attention diversion were both likely factors that could lead to an unintentional slip. Furthermore, the act of twisting the door switch was a substitution error, predicated by a prior intention to act, and was therefore a routine action which did not go as planned.

As it was routine to operate the door switch, the PM probably did not give sufficient attention to this task. This was further compounded by the physical similarities in the switches and their operation, and their co-location on the aisle stand panel. However, a Boeing human factors examination of possible mitigations to these factors in response to a similar previous occurrence found that changing the switch design was unlikely to mitigate the mis-selection risk, and that the current generic aisle stand configuration and an emphasis on confirmation of switch selection prior to manipulation was the most effective control measure. Finally, Boeing identified the risk of unintentional rudder trim application in an FOTB issued to operators 2 years prior to the occurrence. The FOTB specifically acted as an alert to flight crew of the risk of mis-selection of rudder trim in circumstances identical to those in this incident.

On the initial application of the rudder trim, both pilots felt the aircraft’s immediate yaw/roll response, but were unable to identify the likely cause. Over the following 5 seconds, while the captain maintained activation of the switch and waited for the door to open, the rudder trim progressively increased to the left, causing the rudder to correspondingly move to the left. The autopilot was initially able to compensate for the increasing left yaw input and induced left roll through application of increasing right wing down roll input. This right wing down input was replicated on the pilots’ control wheel.

After 5 seconds of trim input and increasing induced left roll, the autopilot reached its authority limit – that is, the autopilot had reached the maximum roll control input it could apply and maintain. Up to this point, the autopilot had managed to limit the induced roll to a bank angle of less than 5° to the left. However, on reaching the roll authority limit, the increasing rudder trim resulted in the aircraft’s bank angle to the left increasing. As the trim input continued for a further 3 seconds, the aircraft responded with a rapidly increasing rate of roll to the left.

The unexpected and increasing bank angle alerted both pilots to the developing aircraft upset. The PF initially responded by attempting to control the increasing left roll through the use of the mode control panel heading selections and the autopilot. As this had no apparent effect, and with the bank angle continuing to increase, the PF applied a large right wing down control input while almost simultaneously disengaged the autopilot and autothrottle. At about the same time the bank angle alert triggered. The PM responded with an ‘upset’ call, and the PF responded by executing the upset recovery procedure. The aircraft was quickly recovered to about straight and level flight.

Having recovered the aircraft to an approximate wings level attitude, the PF was required to hold about 35° of right wing down control wheel displacement to maintain that attitude. While this large roll input required to maintain a wings level attitude strongly indicated a yaw‑related issue, the crew continued to investigate the cause of the inflight upset unsuccessfully for a further minute. About 70 seconds after the initial misapplication of rudder trim, the PF requested the PM check the rudder trim. Shortly after, the rudder trim was returned to a neutral position. While large right wing down aileron input required to maintain a wings level attitude provided a strong indicator that the upset was linked to a yaw related issue, a combination of the very small displacement of the rudder pedals at the point of maximum trim application, and the PF’s limited experience on the aircraft, probably contributed to some of the delay in identifying the unintended rudder trim. 

Findings

From the evidence available, the following findings are made with respect to the inadvertent rudder trim activation resulting in an in-flight upset involving Boeing 737-8FE, VH-YQR, 143 km west of Ballina/Byron Gateway Airport, New South Wales on 6 September 2023.

Contributing factors

• While actioning a request for entry into the flight deck, the pilot monitoring mis-selected the rudder trim switch instead of the intended flight deck door switch and inadvertently applied rudder trim for about 8 seconds.
• The autopilot responded to the trim input and its consequential yaw and roll with application of opposing roll. The maximum roll that the autopilot could apply and maintain (the roll authority limit) was reached after 5 seconds of left rudder trim input, after which the continuing rudder trim input resulted in a rapidly increasing rate of roll and an inflight upset.
• During the period of the development and recovery from the upset, and despite the need to use a large right wing down aileron input to maintain an approximate wings level attitude, the flight crew were not able to promptly identify the significant left yaw as the primary initiator of the upset, which in turn delayed the restoration of balanced flight.

Safety actions

Virgin Australia Airlines advised that, following this occurrence, the flight deck door unlock procedure was reviewed and modified. The new procedure is designed to indicate that the crewmember requesting entry is at the door and ready to enter, thereby limiting the time required for the door unlock switch to be held in the unlock position. Other safety action included a briefing on the event for flight crews, and changes to the non-technical skills program.

Sources and submissions

Sources of information

The sources of information during the investigation included:

• the flight crew
• Virgin Australia Airlines
• Boeing
• recorded data from the aircraft.

References

Heckhausen, H and Beckmann, J (1990). Intentional Action and Action Slips. Psychological Review, 97(1), 36–48.

Reason, J (1990). Human Error. Cambridge University Press, New York.

Salvendy, G and Karwowski, W (2021). Handbook of Human Factors and Ergonomics. John Wiley & Sons Incorporated, New Jersey.

Wickens, CD, Helton, WS, Hollands, JG and Banbury, S (2022) Engineering psychology and human performance. Routledge, New York.

Submissions

Under section 26 of the Transport Safety Investigation Act 2003, the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. That section allows a person receiving a draft report to make submissions to the ATSB about the draft report. 

A draft of this report was provided to the following directly involved parties:

• the flight crew
• United States National Transportation Safety Board
• Boeing
• Civil Aviation Safety Authority
• Virgin Australia Airlines.

Submissions were received from:

• the flight crew
• Boeing
• Civil Aviation Safety Authority
• Virgin Australia Airlines.

The submissions were reviewed and, where considered appropriate, the text of the report was amended accordingly.

Purpose of safety investigations The objective of a safety investigation is to enhance transport safety. This is done through:  identifying safety issues and facilitating safety action to address those issues providing information about occurrences and their associated safety factors to facilitate learning within the transport industry. It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action. Terminology An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue. Publishing information Released in accordance with section 25 of the Transport Safety Investigation Act 2003 Published by: Australian Transport Safety Bureau © Commonwealth of Australia 2024 Ownership of intellectual property rights in this publication Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia. Creative Commons licence With the exception of the Commonwealth Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this report is licensed under a Creative Commons Attribution 4.0 International licence. The CC BY 4.0 licence enables you to distribute, remix, adapt, and build upon our material in any medium or format, so long as attribution is given to the Australian Transport Safety Bureau.  Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.

Investigation summary

What happened

On 16 July 2023, a Bell 206B‑1 helicopter, registered VH‑ZDI, was being operated on a private flight from a rural property near Tumbarumba to Khancoban, New South Wales, with the pilot and 3 passengers on board. Shortly after take‑off, the pilot brought the helicopter into a hover around 7 ft above the helipad located near a hangar. The pilot then initiated a hovering turn to the left and reported that they were able to complete around 90º of an intended 180º turn before they experienced a shudder, and the helicopter began to rotate to the right. While continuing to rotate to the right for around 2 full rotations, the pilot attempted several pedal control inputs and was unable to regain directional control. The pilot elected to lower the collective, reducing height, and later closed the throttle. As the helicopter descended, the left skid contacted the soft earth beside the pad and broke off. The helicopter rolled over. The occupants were uninjured, and the helicopter was substantially damaged.

What the ATSB found

The helicopter was hovering in ground effect near an obstacle, the hangar. The high all-up weight of the helicopter would have strengthened the recirculation of downwash from the main rotor blades generated by the proximity to the hangar. The hovering left turn, initiated by the pilot, brought the tail of the helicopter from its position away from the hangar, where recirculation would be less, closer to the hangar where recirculation would be greater. It was likely that the flow of air through the tail rotor was disturbed, resulting in a loss of tail rotor effectiveness, which manifested as a right yaw.

The ATSB found that a miscalculation of fuel led to the helicopter being operated about 15 kg above the maximum take-off weight. 

The ATSB also noted that the occupants were wearing 4- and 5-point restraints, and a helmet was worn by the pilot. The use of such items reduces the risk of injury to occupants in the event of an accident.

Safety message

Helicopter pilots should remain cognisant of the factors that may induce unanticipated yaw (a loss of tail rotor effectiveness), and that helicopter performance can be adversely affected by the proximity to obstacles, including terrain, vegetation, and buildings. If unanticipated yaw is encountered, prompt and correct pilot response is essential. 

This accident also illustrated the importance of operating within the weight and balance limitations prescribed in the flight manual. A weight and balance calculation tool, such as a mobile application, can be a useful way to check hand calculations, but it must be validated to ensure that it accurately reflects the flight manual limitations. 

The investigation

The occurrence

On 16 July 2023, a Commonwealth Aircraft Corporation Pty Ltd manufactured Bell 206B‑1 helicopter, registered VH‑ZDI, was being operated on a private flight from a rural property near Tumbarumba to Khancoban, New South Wales, with the pilot and 3 passengers on board. This flight was the third time the pilot had taken off from that helipad in VH‑ZDI, and the first time with more than 2 occupants. 

The pilot completed pre‑flight checks and did not identify any defects or outstanding maintenance issues with the helicopter. Following this, the pilot assisted the passengers to board the helicopter and secure their seatbelts. The pilot then briefed the front left seat passenger regarding the seatbelt mechanism and emergency locator transmitter activation. The front passenger had been in the pilot's other helicopter on previous occasions and the pilot stated that the passenger knew not to touch the anti-torque control pedals.[1] 

Following a normal engine start, the take‑off commenced at about 1300 local time. As per their normal procedure when departing from this location, the pilot brought the helicopter into a hover around 7 ft, in ground effect,[2] above the helipad facing the hangar. They then initiated a left turn, which initially progressed as the pilot expected. There was no evidence to suggest that the rate of yaw was rapid or deviated from normal. The pilot reported that they were able to complete around 90° of an intended 180° turn before they experienced a shudder, which could be felt through the flight controls and airframe. The helicopter then began to rotate to the right.[3] 

In response, the pilot reported that they first applied left, and then both right and left pedal inputs, in an attempt to control the right rotation. They did not recall if either of the left or right pedal stops were reached, nor did they notice any unusual resistance associated with the pedals. The pilot was unable to regain directional control and recalled that ‘nothing could stop’ the right yaw, when describing the pedal control inputs. After about 2 full rotations, the pilot lowered the collective,[4] reducing height, and closed the throttle just prior to the helicopter touching the ground.

Initially, the left skid contacted the soft earth beside the helipad and broke off. The helicopter then rolled over coming to rest on the left side. The pilot reported that they switched the fuel valve into the ‘off’ position as the helicopter contacted the ground. The front passenger was able to exit the helicopter with the assistance of the pilot. They then assisted the rear passengers to exit, whereupon all occupants moved away from the helicopter. The pilot collected fire extinguishers and discharged them into the engine exhaust. There were no injuries. The helicopter was substantially damaged.

The pilot initially reported the accident as a loss of tail rotor effectiveness,[5] but later stated that they believed that there had been a mechanical issue with the helicopter.

Figure 1: VH‑ZDI in final resting position

Source: McClaren Aviation, annotated by the ATSB

Context

Pilot information 

The pilot obtained a private pilot licence (helicopter) in 2018. Their flying experience totalled around 300 hours, with about 260 hours on Bell 206 variants. In the 90 days prior to the accident, the pilot had flown 20 hours, all on VH‑ZDI. The pilot’s latest flight review was on 15 March 2023. The pilot reported feeling fully awake immediately preceding the accident, so fatigue was not considered as a contributing factor to the accident.

Helicopter information

VH‑ZDI was a Bell 206B‑1 helicopter, manufactured in 1976, by the Commonwealth Aircraft Corporation Pty Ltd for the Australian Army, powered by a single-engine Allison Gas Turbines 250‑C20 engine. In 2020, a special certificate of airworthiness in the ‘limited category’[6] was issued for the helicopter. At the time of the accident, the total time-in-service was 9,856.5 hours.

The helicopter was maintained in accordance with the Australian Warbirds Association Limited[7] Bell 206B‑1 Kiowa maintenance program. The latest maintenance was performed 15.4 flight hours (20 days) prior to the accident, on 26 June 2023. An engine power assurance check[8] was performed at that time, but the conditions under which this check was conducted could not be validated by the ATSB. The maintainer conducting this check assessed that the minimum acceptable torque requirement was met. A previous engine power assurance check performed on 11 March 2022, 16 months prior to the accident, also indicated the engine was producing above the minimum acceptable torque. No defects were noted in the technical logs.

The helicopter was fitted with an anti-torque control pedal lock‑out kit on the left pedal assembly. The kit is designed to disconnect the passenger pedals without the use of tools, allowing pilots the ability to quickly isolate the pedals to prevent passenger interference with the tail rotor during flight.

Helipad information

The helipad (Figure 2) was a concrete pad, large enough to accommodate the helicopter. It adjoined a hangar, with the terrain sloping downwards to the west, away from it. The centre of the helipad was around 16 m from the hangar. The elevation of the pad was about 1,962 ft.

Figure 2: Helipad location

Source: Google Earth, annotated by the ATSB

The method used to conduct a take‑off from the helipad was to push the helicopter out of the hangar tail first (towards the west) and position it such that the tail was over the downhill slope, affording the helicopter the greatest possible clearance from the hangar. The pilot would initiate a hover while facing the hangar, rotate left 180º, before commencing forward flight. 

The pilot described experiencing helicopter-building interference from the hangar on other occasions. They stated that they had only experienced interference when they came into land and did not position the helicopter on the ground quickly. They likened the experience to rotor-head shake, bad turbulence, and bad air. 

The pilot also stated that, though there was a lot of wildlife around the property, they did not observe any at the time of the accident. Furthermore, they ensured that items were stowed away and clear of the helipad.

Meteorological information

The Bureau of Meteorology analysis of weather observations around the Tumbarumba area found that a large high‑pressure system commonly associated with light winds and clear skies was present. This was consistent with the pilot’s recollections that it was not a windy day, that the wind was barely registering on the windsock, and it was about 15 ºC. 

The Bureau of Meteorology did not have observations for Tumbarumba, the nearest airport. Instead, they provided observations for Wagga Wagga and Albury with the note that these airports were located on the same side of the ranges as Tumbarumba and experienced similar weather. Winds were very light, tending to a light to moderate north to north‑easterly and QNH[9] pressure was between 1026 and 1027 hPa. Visibility was greater than 10 km and no significant cloud was detected. Both airports recorded the temperature to be 15 ºC.

Wreckage examination

The ATSB’s examination of the site photographs provided by the pilot and the insurer found that the damage to the helicopter was consistent with a heavy landing and rollover event. The relative locations of the major components were consistent with power being supplied to the main rotor just prior to, or during, impact. The proximity to the ground and integrity of the occupant space contributed to a high probability of occupant survival.

ATSB investigators did not deploy to the site but inspected the wreckage once it was transported to a storage facility. No mechanical issues were identified during that inspection and there was no indication of pre-accident failure. The tail rotor and associated controls were inspected and showed no signs of failure, no restriction of normal operation, and no contact marks indicating a strike with a foreign object or animal. The exception to this was the inspection of the tail rotor control rigging, where system functionality was unable to be confirmed due to the airframe disruption. As a result, the possible contribution of a tail rotor control rigging error could not be eliminated. 

While the helicopter was fitted with a pedal lock‑out kit on the left pedal assembly, the kit was not configured such that the passenger was ‘locked-out’. The pilot stated that they had never used the pedal lock‑out kit and were unfamiliar with its use. The passenger side cyclic control[10] was not present and there was no cover. The pilot stated that they removed the cyclic and that there was no cyclic control stub cover available. The passenger side collective was present.

The insurer’s assessment reported that they were not able to confirm the pre‑event serviceability of the tail boom attachment bolts. The tail boom was attached to the fuselage by 4 bolts. Two of the bolts had fractured and laboratory examination conducted by the ATSB identified that the failure of the bolts was consistent with overstress. There was no evidence of pre‑existing flaws or fatigue. Overall, the insurer concluded that the tail boom exhibited damage consistent with impact from a main rotor blade on the left side.

Weight and balance

Limits

The flight manual included forward and aft centre of gravity limits and specified that the maximum take-off weight (MTOW) for the helicopter was 3,200 lbs (1,452 kg). A type‑specific weight and balance assessment was performed on VH-ZDI on 30 June 2020. The resulting load data sheet listed the MTOW as 1,452 kg, the forward centre of gravity limit as 2,672 mm and the aft limit as 2,901 mm (Figure 4). 

Calculations by the pilot

The pilot performed a weight and balance assessment prior to commencing the flight. In their handwritten calculations, the pilot included the weight and position of the 4 occupants and 430 lbs of JetA1 fuel. Calculation of the weight and centre of gravity required converting the fuel amount from the indicated units of lbs to kg, as all other amounts were measured in kgs. When converting the fuel from lbs to kg, the pilot mistakenly substituted volume, L, for mass, kg, and believed they had converted 430 lbs to 195 L (with the conversion factor of ÷2.2) (Figure 3). They subsequently converted 195 L to weight (with the conversion factor x0.8), arriving at 156 kg of fuel. This resulted in the pilot calculating the weight of the helicopter to be 1,428 kg (Figure 4). The pilot also listed the MTOW for the helicopter as 1,455 kg. This led the pilot to believe the helicopter was 27 kg under its MTOW. 

Figure 3: JetA1 fuel conversion chart

Source: Airservices Australia, the pilot, annotated by the ATSB

The pilot used a third‑party application (App), iBal Rotary, as a secondary check to ensure that the helicopter was appropriately loaded. The pilot had selected ‘Sample Bell 206B3 (Bell 206B3 Jet Ranger)’ from the available models and input the 4 occupant details and 156 kg for fuel. The pilot was aware that this model selection did not represent VH‑ZDI and, to compensate, included an additional centre aft passenger weighing 100 kg as an adjustment to account for the unrepresentative model selection. The App indicated that the weight and balance of the selected model, which did not reflect the limits specified in the flight manual, was within limits.

According to the developer of the App, a more representative model selection for VH‑ZDI was the ‘Bell OH‑58A/C’.[11] The weight and balance envelope for the App ‘Bell OH‑58A/C’ model was sourced from the Operator's Manual Army Model OH-58 A/C Helicopter (Department of the Army (United States), 1989). This differed to the weight and balance envelope specified in the VH-ZDI flight manual but more closely resembled the flight manual than the model selected by the pilot. When the 4 occupants and 156 kg of fuel was input into the App with ‘Bell OH‑58A/C’ selected, the App indicated that the loading was within MTOW, but that the forward centre of gravity limit was exceeded. When the 4 occupants and 195 kg (430 lbs)[12] of fuel was input into the App, the App indicated that the helicopter loading had exceeded the MTOW. 

Calculations by the ATSB

The ATSB performed a weight and balance calculation with the information provided by the pilot and determined the take‑off weight to be 1,467 kg with an associated moment arm of 2,715 mm (Figure 4). This exceeded the MTOW of the helicopter by 15 kg.

Figure 4: VH‑ZDI weight and balance limits and calculations

Source: Flight manual, load data sheet, and pilot, annotated by the ATSB

Operational information

Engine torque required and available

From the flight manual, the minimum engine torque required to hover for the accident conditions was about 61.9 psi and the minimum acceptable torque that the engine should produce under those conditions was about 68.6 psi. 

Factors affecting performance

According to the United States Federal Aviation Administration (2019) Helicopter Flying Handbook:

A helicopter’s performance is dependent on the power output of the engine and the lift produced by the rotors, whether it is the main rotor(s) or tail rotor. Any factor that affects engine and rotor efficiency affects performance. The three major factors that affect performance are density altitude,[13] weight, and wind.

An increase in density altitude can affect helicopter performance by reducing the hovering ceiling, operating margins, and rate-of-climb performance. The higher the gross weight, the greater the lift or rotor thrust required for hovering or climbing. Therefore, the margin between the engine power available and the power required to hover at higher weights and density altitudes may often be small for helicopters (Civil Aviation Authority of New of Zealand, 2020). The Helicopter Flying Handbook noted that, while more engine power was required during the hover than in any other phase of flight, if a hover could be maintained, a take-off could also be made.

Recirculation and helicopter-building interference

Recirculation is a type of interference between a helicopter and its surroundings (Royal Air Force (UK), 2010). According to the UK AP3456 Central Flying School (CFS) Manual of Flying, Volume 12 – Helicopters: 

Whenever a helicopter is hovering near the ground, some of the air passing through the disc is recirculated and it would appear that the recirculated air increases speed as it passes through the disc a second time (Figure 5). This local increase in induced flow near the tips gives rise to a loss of rotor thrust.

Recirculation will increase when any obstruction on the surface or near where the helicopter is hovering prevents the air from flowing evenly away. Hovering close to a building, wire link fencing or cliff face may cause severe recirculation (Figure 6).

Figure 5: Helicopter hovering near the ground with recirculated air

Source: Royal Air Force (UK) (2010), annotated by the ATSB

Figure 6: Recirculation near a building

Source: Royal Air Force (UK) (2010), annotated by the ATSB

The section of the rotor disc largely affected by recirculation was the side closer to the obstruction (right side of disc in Figure 6). A tail rotor positioned on the far side of the helicopter relative to the obstacle would experience less recirculated air than a tail rotor positioned on the near side. 

Łusiak et al. (2009) described wind tunnel testing of a model helicopter with surrounding elements (buildings). Their paper stated:

The phenomenon of interference between the helicopter and the surrounding elements appears with a visible intensity when the helicopter operates at a low speed in the near vicinity of objects with specific geometrical shapes, such as buildings or ship hulls.

All computational analyzes and experimental investigations which were performed in order to study the mutual helicopter-building interaction indicate that in the considered specific situations the phenomenon of aerodynamic interference can seriously disturb the flow around the helicopter and change the loading of some of its elements. Substantial changes in the value of the resulting loads can make the helicopter difficult to control.

Wagtendonk (2011) discussed recirculation within the context of confined area operations, which included the following points:

As rotor downwash strikes the surface it splits, and a large part diffuses horizontally. If obstructions such as buildings or trees interfere with the escaping airflow, it moves vertically up the obstruction and re-enters the disc from above, increasing the induced flow.

The greater the gross weight, the stronger the downwash and the greater the degree of recirculation.

The lower the hover height, the stronger the outbound flow and the greater the degree of recirculation.

The more solid the obstruction, the greater the recirculation. Hovering close to large buildings (such as hangars) creates more recirculation than hovering near trees.

The highest velocity of horizontal outflow escaping from beneath the helicopter occurs at a distance that is roughly 30 percent of the disc diameter beyond the disc tip. For example, with a 30-foot disc the highest velocity occurs about 10 feet away from the tips. Although the velocity beyond that distance decreases sharply, substantial horizontal velocity values can still be encountered.

Recirculation can occur when obstructions are reasonably far away from the disc tip, but in general, the shorter the distance, the greater the risk of recirculation.

Not always is the entire disc involved in recirculation. For instance, when hovering close to a building, only half the disc may be affected by recirculation and a roll or pitch movement may develop, depending on the aircraft’s heading. In all likelihood, however, the air hitting the building will surge out in all directions, disturbing the entire airflow through the disc, resulting in random roll, pitch and yaw.

Loss of tail rotor effectiveness

The emergency procedures section of the flight manual for VH‑ZDI identified loss of tail rotor effectiveness[14] as an anti‑torque system malfunction. As the main rotor rotated in the anti‑clockwise direction when viewed from above, in instances of anti-torque system malfunction the helicopter will most likely yaw to the right. 

The flight manual stated:

The prime contributing causes of LTE[15] are:

a. Tail Rotor Vortex Ring. This condition may be encountered with wind azimuths caused by crosswinds, left sidewards flight, or right pedal turns.

b. Weather Cock Stability. Wind azimuths aft of the beam will cause the helicopter to weather cock.

c. Main Rotor Vortex Interference. Certain wind azimuths will cause the tail rotor to ingest main rotor vortices.

d. Tail Rotor Precessional Flapping. High yaw rates will cause the tail rotor to precess. This, coupled with the pitch change characteristics of the tail rotor flapping hinge, will reduce tail rotor thrust.

e. High Gross Weight. High gross weights require increased torque and reduce tail rotor operating margins.

f. High Density Altitude. High DAs[16] require increased torque and reduce tail rotor operating efficiency.

g. Ground Vortex Interference. Interaction between the main rotor vortex and the ground can reduce tail rotor efficiency.

h. Limited Directional Control Margin. Right relative wind azimuths reduce left pedal travel margins.

i. Governor Droop. Governor droop leads to main rotor RPM droop. This requires increased torque to accelerate the rotor and also reduces tail rotor efficiency.

j. Low Airspeed. The aircraft is dynamically unstable in the yawing plane at low airspeed.

The best recovery technique detailed for ‘Loss of Tail Rotor Effectiveness’ was:

1. Pedal – Full left. 

2. Cyclic – Forward.

3. Collective – Reduce if altitude permits.

4. Adjust controls for normal flight as control is regained.

If yaw cannot be controlled and an uncontrolled landing is imminent:

5. Throttle – CLOSED.

6. Collective – Autorotate.

7. Pedal – Full left until yaw stops.

The United States Federal Aviation Administration has produced advisory circular 90-95 that related to loss of tail rotor effectiveness, which they also term ‘unanticipated yaw’. The recommended recovery techniques in the circular were:

a. If a sudden unanticipated right yaw occurs, the pilot should perform the following:

   (1) Apply full left pedal. Simultaneously, move cyclic forward to increase speed. If altitude permits, reduce power.

   (2) As recovery is effected, adjust controls for normal forward flight. 

b. Collective pitch reduction will aid in arresting the yaw rate but may cause an increase in the rate of descent. Any large, rapid increase in collective to prevent ground or obstacle contact may further increase the yaw rate and decrease rotor rpm.

c. The amount of collective reduction should be based on the height above obstructions or surface, gross weight of the aircraft, and the existing atmospheric conditions.

d. If the rotation cannot be stopped and ground contact is imminent, an autorotation may be the best course of action. The pilot should maintain full left pedal until rotation stops, then adjust to maintain heading.

Survival aspects

Seatbelts

The helicopter was fitted with 5‑point turn‑to‑open restraints in the front seats and 4‑point lift‑latch‑to‑open restraints in the rear seats. Zimmermann and Merritt (1989) stated that:

• The overall probability of survival in an accident depends to a large extent on the manner of the restraint.
• The use of upper and lower torso restraints to prevent such critical body parts as the head and chest from striking surrounding structure can significantly reduce the probability of serious or fatal injury under given accident conditions.
• Studies have shown the addition of a shoulder harness greatly reduced injuries from head impacts and maintain proper spinal alignment. The further addition of a lab belt tie down strap (crotch strap on a 5-point harness) may nearly double the tolerance to impact forces.

Helmets

The Flight Safety Foundation (2022) stated that the primary purpose of a helmet was to provide impact protection and thereby reduce the risk of head injury in the event of an accident. The helmet worn by the pilot was damaged (Figure 7), indicating that the helmet sustained an impact during the accident sequence. 

Figure 7: Top view of helmet worn by the pilot of VH‑ZDI showing damage

Source: Pilot, annotated by the ATSB 

Similar occurrences

A search of the ATSB’s occurrence database for helicopter incidents, serious incidents, or accidents with the occurrence category ‘loss of control’ or ‘control issues’ from 2013 onwards returned 151 results. Eight of these occurrences contained sufficient information to be identified as unanticipated yaw or loss of tail rotor effectiveness. None of the occurrences related to helicopter‑building interference. 

The 3 examples detailed below include an occurrence where the pilot was able to recover directional control, one that took place in a confined landing site with nearby obstacles, and an international event where helicopter-building interference was a probable factor. 

ATSB investigation AO-2015-091

On 20 July 2015, the pilot of a Bell 206L3 (LongRanger) helicopter, registered VH-BLV, conducted a charter flight from Essendon Airport to Falls Creek, Victoria, with 5 passengers on board. The pilot refuelled at a property near Lake Eildon and departed close to its MTOW. 

On approach to the helipad at Falls Creek, the pilot assessed that there was insufficient power available to continue to land and elected to abort the approach. The pilot pushed forward on the cyclic to increase the helicopter’s airspeed and conducted a left turn towards the valley whereupon the helicopter started to yaw rapidly to the right. The pilot applied full left pedal to counteract the yaw, but the helicopter continued to yaw. The helicopter turned through one and a half revolutions, as the pilot lowered the collective. Lowering the collective reduced the power demand of the power rotor system, thereby increasing the ability of the anti-torque pedals to stop the right yaw. The combination of lowering collective and applying forward cyclic to gain forward airspeed, allowed the pilot to regain control of the helicopter. The pilot then conducted a left turn towards the helipad and made an approach to the helipad from an easterly direction. The helicopter landed following the second approach without further incident.

ATSB investigation AO-2022-060

On 19 November 2022, the pilot of a Robinson Helicopter Company R44, registered VH-TKI, was conducting a private flight from a nearby property to a function centre at Forresters Beach, New South Wales with 2 passengers onboard. The proposed landing site was the carpark of the venue and was considered a confined area due to the proximity of roads, powerlines, and palm trees. 

During the approach, the pilot reported an uncommanded yaw to the right, which was unable to be recovered. The ATSB found that, during the approach to a confined area landing site, the helicopter experienced a loss of tail rotor effectiveness and accompanying right yaw. The pilot’s response was ineffective at recovering control. The position of the helicopter on approach to the confined area was such that it could not be established if control of the helicopter could have been recovered before colliding with powerlines and terrain. The occupants received minor injuries and the helicopter was substantially damaged. 

Federal Safety Investigation Authority (Austria) investigation reference: 2020-0.701.771

On 20 July 2018, a privately‑owned Airbus Helicopters AS350B, registered N36033, was destroyed while the pilot attempted to hover taxi closer to a fuelling station at Wolfsberg airfield in Austria (Aerossurance, 2020; Federal Safety Investigation Authority (Austria), 2020). The pilot, who did not hold a valid licence, sustained a minor leg injury. At the time the wind was 1 to 2 kt. 

After lifting into a 1 m hover there were excessive pitching movements forwards and backwards and the helicopter yawed around 90° to the right. The pilot reported feeling turbulence from the side of the fuelling station building, which was a 5.2 m x 5.2 m, flat‑roofed building, 3.2 m high. The Austrian Federal Safety Investigation Authority determined the probable cause was a loss of lateral control during hover in ground effect. The probable factors were:

• excessive control inputs
• flight crew induced oscillations about the helicopter longitudinal axis
• lack of corrective action to stop flight crew induced oscillations
• proximity of obstacles
• formation of ground effect air vortices in ground effect.

Safety analysis

Loss of tail rotor effectiveness

The ATSB considered several reasons to explain the unanticipated right yaw. Although the pilot described a shudder immediately prior to the right yaw, which could have indicated a mechanical issue, examination of the helicopter and maintenance documents did not reveal any anomalies. A wildlife strike or contact with a foreign object was considered but there was no indication of strikes on the rotors, a strike on the hangar, or animal remains to support this hypothesis. Inadvertent interference from the front seat passenger was also explored. This possibility was unlikely as the pilot did not feel any resistance when manipulating the anti-torque pedals.

While the helicopter was loaded above the MTOW, at the estimated density altitude for the time of the accident, the helicopter likely had sufficient power available to sustain a hover in-ground effect. Additionally, the left turn was not likely to be at a rapid yaw rate, and the weather conditions were calm.

During the hover, the helicopter was in a position close to an obstacle, the hangar, where helicopter-building interference was known to have occurred in the past. As described by Wagtendonk (2011), the obstacle would have prevented downwash from the main rotor escaping and the air would have recirculated. This recirculation would have been strengthened by the high all-up weight of the helicopter. 

The literature indicated that the side of the main rotor disc closest to the obstruction would be more affected than the side further from the obstruction. Furthermore, recirculation from obstacles, such as buildings, can disturb the airflow though the disc, which can result in random movements and controllability difficulties. The hovering left turn, initiated by the pilot, brought the tail of the helicopter from its position away from the hangar, where recirculation would be less, closer to the hanger where recirculation would be greater. This was a position where the air flow through the tail rotor was more likely to be disturbed. Disturbance to the flow through the tail rotor, to an extent that the anti-torque forces could no longer overcome, likely account for the unanticipated right yaw. The pilot had likened their previous experience to turbulence or ‘bad air’, which could potentially explain the shuddering. 

Helicopter-building interference is a variation on one of the contributors to a loss of tail rotor effectiveness described in the flight manual, specifically, ground vortex interference. Instead of the interference being generated from the proximity to the ground, it is generated by close proximity to a building. Therefore, with insufficient evidence to support other potential reasons for the unanticipated right yaw, it was likely that, as the left turn brought the tail rotor closer to the hangar, with recirculation strengthened by the high all-up weight, the flow of air through the tail rotor became disturbed. As a result, a loss of tail rotor effectiveness occurred, and the helicopter began to yaw right. 

Once the right yaw initiated, the pilot’s control input included both left and right pedals. This was not consistent with the recommended procedures in the flight manual and advisory circular 90-95 for loss of tail rotor effectiveness, which stated that full and sustained left pedal input was required. This did not give the pilot the best opportunity to regain directional control. Ultimately, the pilot reduced the throttle, but this did not prevent the helicopter from colliding with the terrain.

Maximum take-off weight (MTOW) exceedance

When manually calculating the weight and balance of the helicopter, the pilot inadvertently made an error when converting the fuel load from lbs to kg. Their calculation indicated that the helicopter weight was 27 kg under the MTOW. 

When the hand calculation appeared to be acceptable, the pilot used the third-party App for verification. The pilot was aware that the model selected in the App did not represent VH‑ZDI and added an unverified correction factor. Under these conditions, the App confirmed that the loading of the helicopter was within limits. Had the pilot selected the model that the App developer stated more closely reflected VH‑ZDI, the App would have shown that the centre of gravity was beyond the forward limit, even with the fuel conversion error. It is worth noting that third‑party applications are not a controlled source of information, and the flight manual and manufacturer’s documentation is the authoritative source of information. 

Applying the required conversion factor, the ATSB weight and balance calculation established that the helicopter was loaded in a way that exceeded the MTOW by around 15kg. Had the pilot not made the conversion error and instead identified that the MTOW was exceeded, it was unlikely that they would have proceeded with the planned flight.

Operation at higher helicopter weights can affect performance and controllability, and potentially exacerbate other conditions such as helicopter-building interference. It is not known what loading configuration would have been sufficiently conservative such that the helicopter-building interference would not have resulted in a loss of control for the conditions. Regardless, compliance with the limitations set out in the flight manual remains vital for safe helicopter operation. 

Survivability

The front occupants of the helicopter were wearing 5‑point turn‑to‑open restraints, while the rear occupants were wearing 4‑point lift‑latch‑to‑open restraints. The pilot was also wearing a helmet, on which only minor damage was observed. There was no comparative evidence, such as, one occupant with a seatbelt and one without to determine whether the severity of the accident was such that the occupants would have sustained greater injury if they were not wearing seatbelts. Nevertheless, the literature indicated that the use of upper and lower torso restraints and helmets reduces the risk of injury.

Findings

From the evidence available, the following findings are made with respect to the collision with terrain involving Bell 206B‑1, VH‑ZDI, 9.3 km south-south‑east of Tumbarumba, New South Wales, on 16 July 2023. 

Contributing factors

• After lift-off and initiating a hover turn to the left, while operating at a high all-up weight, it was likely that the helicopter’s tail rotor encountered helicopter-building interference from the hangar, which resulted in a loss of tail rotor effectiveness, and a subsequent collision with terrain.

Other factors that increased risk

• Errors when calculating the weight and balance for the flight likely resulted in the maximum take-off weight being exceeded by 15 kilograms.

Other findings

• The helmet worn by the pilot and the use of 4- and 5-point restraints reduced the risk of injury to the occupants.

Sources and submissions

Sources of information

The sources of information during the investigation included:

• the pilot  
• the flight instructor of the pilot
• maintenance organisations for VH‑ZDI
• the weight and balance application developer
• Bureau of Meteorology
• Civil Aviation Safety Authority.

References

Aerossurance. (2020). Helicopter Destroyed in Hover Taxi Accident. Aerossurance. Accessed 26 September 2024. https://aerossurance.com/helicopters/hover-taxi-accident/  

Australian Transport Safety Bureau. (2015). Loss of control involving a Bell 206L3, VH-BLV Falls Creek, Victoria, on 20 July 2015 [ATSB Transport Safety Report](Aviation Occurrence Investigation AO-2015-091). /publications/investigation_reports/2015/aair/ao-2015-091

Australian Transport Safety Bureau. (2023). Collision with terrain involving Robinson Helicopter Company R44, VH-TKI, Forresters Beach, New South Wales on 19 November 2022 [ATSB Transport Safety Investigation Report](Aviation Occurrence Investigation (Short) AO-2022-060). /publications/investigation_reports/2024/report/ao-2022-060

Civil Aviation Authority of New of Zealand. (2020). Helicopter Performance [Good Aviation Practice]. Accessed 9 January 2024. https://www.aviation.govt.nz/assets/publications/gaps/helicopter-performance.pdf

Civil Aviation Safety Authority. (2018). Limited category aircraft - operation [Advisory Circular](AC 132-01v1.1). File ref D17/105699. 

Department of the Army (United States). (1989). Operator's Manual Army Model OH-58 A/C Helicopter [Technical Manual](TM 55-1520-228-10).

Federal Safety Investigation Authority (Austria). (2020). Accident involving the helicopter type AEROSPATIALE AS350B on 20.07.2018 at approximately 06:33 UTC at Wolfsberg airfield, A-9400 Wolfsberg, Carinthia [Investigation report](Reference: 2020-0.701.771). 

Flight Safety Foundation. (2022). Basic Aviation Risk Standard Implementation Guidelines (Version 9). https://flightsafety.org/bars/the-bar-standards-and-manuals/

Łusiak, T., Dziubiński, A., & Szumański, K. (2009). Interference between helicopter and its surroundings, experimental and numerical analysis. Task Quarterly, 13(4), 379-392.          

Royal Air Force (UK). (2010). AP3456 The Central Flying School (CFS) Manual of Flying (Volume 12 - Helicopters). Revised November 2013.

Royal Australian Navy. (n.d.). Bell Kiowa 206B-1 [Webpage]. Accessed 7 February 2024. https://www.navy.gov.au/aircraft/bell-kiowa-206b-1

United States Federal Aviation Administration. (1995). Unanticipated right yaw in helicopters [Advisory Circular: 90-95]. Accessed 3 October 2023. https://www.faa.gov/regulations_policies/advisory_circulars/index.cfm/go/document.information/documentid/23136

United States Federal Aviation Administration. (2019). Helicopter Flying Handbook (FAA-H-8083-21B). Department of Transportation (U.S.). Accessed 9 February 2024. https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook

Vietnam Helicopter Museum. (28 March 2016). OH-58C Kiowa Helicopter [Webpage]. Accessed 7 February 2024. https://www.vietnamhelicopters.org/oh-58c-kiowa/

Wagtendonk, W. J. (2011). Principals of Helicopter Flight (Second revised ed.). Aviation Supplies & Academics, Inc.             

Zimmermann, R. E., & Merritt, N. A. (1989). Aircraft crash survival design guide: Volume I Design criteria and checklists [Final Report](AD-A218 434, TR 89-D-22A). Aviation Applied Technology Directorate. 

Submissions

Under section 26 of the Transport Safety Investigation Act 2003, the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. That section allows a person receiving a draft report to make submissions to the ATSB about the draft report. 

A draft of this report was provided to the following directly involved parties:

• the pilot
• the maintenance organisations for VH‑ZDI
• the weight and balance application developer
• Civil Aviation Safety Authority.

Submissions were received from the weight and balance application developer. The submission was reviewed and, where considered appropriate, the text of the report was amended accordingly.

Purpose of safety investigations The objective of a safety investigation is to enhance transport safety. This is done through:  identifying safety issues and facilitating safety action to address those issues providing information about occurrences and their associated safety factors to facilitate learning within the transport industry. It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action. Terminology An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue. Publishing information Released in accordance with section 25 of the Transport Safety Investigation Act 2003 Published by: Australian Transport Safety Bureau © Commonwealth of Australia 2025 Ownership of intellectual property rights in this publication Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia. Creative Commons licence With the exception of the Commonwealth Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this report is licensed under a Creative Commons Attribution 4.0 International licence. The CC BY 4.0 licence enables you to distribute, remix, adapt, and build upon our material in any medium or format, so long as attribution is given to the Australian Transport Safety Bureau.  Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.

Investigation summary

What happened

On 11 March 2022, at about 1050 local time, the pilot of a Bell Helicopter Company B206L-1, registered VH‑BHF and operated by Heli Surveys Pty Ltd, departed Jindabyne aerodrome, New South Wales, to conduct a weed survey task on behalf of the New South Wales National Parks and Wildlife Service (NPWS). On board were the pilot and 4 NPWS officers. At about 1112, at a low level and low speed over the Snowy River, control of the helicopter was lost. While attempting an emergency landing in the river, the helicopter collided with a large boulder. Three of the occupants received serious injuries and 2 received minor injuries. The helicopter was destroyed. 

What the ATSB found

The ATSB found that, to conduct the weed survey above the riverbank, the helicopter was flown at low-level, at a slow speed, and yawed to the right by about 45°. It was also noted that the helicopter was operating at a high gross weight and higher density altitude. In combination, these conditions were conducive to the onset of a loss of tail rotor effectiveness. As such, it was likely that a loss of tail rotor effectiveness occurred at an insufficient height to recover and avoid a collision with terrain. Following the collision into the river, the carriage of dedicated emergency locator transmitting devices allowed for a timely response for retrieving the occupants.

Further, one of those on board was not required for the survey task, which unnecessarily exposed them to the risks associated with low-level flight. While the client’s operating procedures referred to ‘essential personnel’, they did not provide a definition or specify the roles and responsibilities of these personnel. 

The ATSB also identified that the operator’s risk assessment for low-level operations did not contain the hazard and control measures to avoid the likelihood of loss of tail rotor effectiveness. Further, there was no requirement for its pilots to conduct pre-flight risk reviews to ensure that operations could be conducted without unacceptable safety risk. 

What has been done as a result

Heli Surveys conducted a review of its risk management processes and made changes to its operational conduct. Its changes focused on identifying flight‑related hazards that included loss of tail rotor effectiveness and compiling mitigation controls in a dedicated risk assessment. Other changes included the introduction of a ‘Hazardous Flight Conditions’ course for pilots and a requirement for flight crews to ensure that only essential crew were to be on board its helicopters.

The NPWS revised its aviation safety policy and developed an aviation safety management system to enhance safety and manage risk across its aviation activities and operations. To define essential personnel, the NPWS committed to developing detailed task profiles to ensure that the roles and responsibilities of all personnel were clearly defined and committed to the development of task‑specific risk profiles to manage risks associated with its aerial work activities.

Safety message

Survey flights, particularly when performed in alpine environments, are generally conducted at low level and slow speeds. This creates a high-risk operating environment that requires effective risk management. Risk management should include an overarching pre‑operational risk assessment to identify the hazards and risks common to that type of operation. This assessment can then be used to inform the management of risk for specific taskings including a pilot’s pre-flight risk review, to ensure the operation can be conducted safely. 

This accident further highlighted the benefits of carrying multiple position transmitting devices. This not only eliminates potential doubt associated with transmissions generated from inadvertent beacon activation but can accelerate an emergency response.

The occurrence

On 11 March 2022, at about 1050 local time, the pilot of a Bell Helicopter Company B206L-1 helicopter, registered VH‑BHF and operated by Heli Surveys Pty Ltd, departed Jindabyne aerodrome, New South Wales, to conduct a low-level English Broom weed[1] survey task on behalf of the New South Wales National Parks and Wildlife Service (NPWS) (Figure 1). On board were the pilot and 4 NPWS officers.[2] 

Following departure, the flight tracked north along the western side of Lake Jindabyne and at about 1055, the pilot turned north-west and tracked upstream along the Snowy River before turning south-west towards Island Bend. At about 1102, the helicopter passed overhead Island Bend where a clump of the weed was located. This local infestation provided an opportunity for the NPWS officers to familiarise themselves with spotting the target weed in the local environment, to assist with identification during the survey.

From Island Bend, the flight continued south-west, following the course of the river. At 1110:35, the helicopter approached Guthega (Munyang) hydro‑electric power station where the pilot commenced a left turn, to pass to the east of the power station.

Figure 1: VH-BHF flight path from Jindabyne aerodrome to Guthega power station with inset showing location relative to capital cities

Source: Google Earth and TracPlus data, annotated by the ATSB

At 1110:47, and now south of the power station, the pilot commenced a right, high orbit to remain clear of power lines in the area and return towards the river course. 

By 1111:17, the helicopter was heading downstream above the southern riverbank and established in a descent towards the river in preparation for commencing the weed survey (Figure 2).

Corroborating reports from the occupants of the helicopter, which included the pilot, indicated that due to the seating position of the NPWS officers (3 seated on the left side of the helicopter), the later part of the descent was conducted with the nose of the helicopter yawed to the right about 45°. The right yaw was in response to the officers’ request to provide the best view of the riverbanks for them to identify and map the locations of the English Broom weed. The officers reported that they asked the pilot to fly lower and sideways to enhance their view. The pilot reported to the ATSB that, prior to setting up the right yaw position, the helicopter’s speed was about 30 kt and they noted they had sufficient power with no abnormal engine indications.

As the helicopter descended past Pipers Creek, the pilot reported that their vision of trees and other obstacles was obscured by the helicopter’s instrument console. To improve their vision for the final descent to the river, the pilot indicated that they ‘touched’ the left anti-torque pedal[3] to straighten the helicopter ‘a bit’, upon which the helicopter started an uncommanded yaw[4] to the right. 

In interview with the ATSB, the pilot stated that they believed they had full and free movement of the anti-torque pedals until the uncommanded yaw to the right started. After the yaw started, they felt that the helicopter did not respond to their pedal inputs, but they could not recall exactly what inputs they made. The pilot did not recollect any shock loading of the tail rotor, such as from a bird or tree strike. The officers reported that, when the uncommanded right yaw started, they thought it was a pilot‑initiated turn and that they were clear of trees and there were no physical knocks or signs of a failure before the yaw commenced. 

After the first turn, when the helicopter was facing downstream, the pilot attempted to gain forward speed, but the helicopter continued to yaw right, and the yaw rate started to accelerate. At 1111:58, when about 200 m past Pipers Creek, the pilot reported realising their only landing option was in the river and, to do so, they rolled the throttle to idle, which stopped the yawing motion. The helicopter entered an autorotation[5] with the pilot aiming for a spot in the river. The pilot attempted to cushion the landing but did not see a large boulder in the water at their aim point. 

At 1112:04, the helicopter collided with terrain. Three occupants received serious injuries, and 2 sustained minor injuries. The helicopter was destroyed. 

Figure 2: Approach to Guthega power station, orbit to the south, descent and collision with terrain

Source: Google Earth and TracPlus data, annotated by the ATSB 

At the time of the accident, the operator had another helicopter in the local area conducting sling‑work operations. At around 1130, the pilot of that helicopter, who was also the head of flying operations, received a report[6] of an alert notification from the emergency locator transmitter on VH‑BHF, and a subsequent report of a personal locator beacon activation. Aided by their onboard resources, the pilot identified the last recorded position of VH-BHF that was transmitted by its satellite‑based tracking system (TracPlus) and immediately ceased the sling-work operation and departed for that recorded position. While enroute, the pilot notified emergency services and directed their ground‑based resources in the local area to the expected helicopter location. 

The pilot located VH-BHF at about 1138 and confirmed the accurate position with emergency services. While surveying the scene from overhead, they were joined by another of the operator’s helicopters, and that pilot was able to unload an air crew person at the accident site. The air crew person was equipped with a first aid kit and provided a communications link between the ground and the overhead helicopters. At about 1210, the operator’s ground-based staff arrived to provide assistance and reported that emergency services had started to arrive. Following initial treatment, 3 of the injured persons were airlifted to hospital while the remaining 2 were able to walk from the site to awaiting ambulances.

Context

Personnel information

Pilot

Qualifications and experience

The pilot held a valid class 1 aviation medical certificate and a Commercial Pilot’s Licence (Helicopter) with single‑engine helicopter and low‑level rating, and a gas turbine endorsement. The operator’s pilot record sheet, dated 2 November 2021, indicated the pilot had accrued 900 hours turbine experience from a total of 2,065 flying hours experience. The pilot had also logged 530 hours aerial work and low flying, and 20 hours mountain flying. In the 28 days prior to the accident, the pilot had accrued 47.1 hours flight time, and 98.7 hours in the previous 90 days. In total, the pilot had 145 hours experience on the Bell 206L-1 helicopter, which included 9.3 hours in the previous 90 days. 

Operator training

The pilot joined the operator, Heli Surveys, in early November 2021. On 21, 22 and 23 October 2021 they completed 6 pre-employment check flights on the AS350 helicopter with a contracted training and checking organisation. The syllabus for the checks included low flying within the normal procedures and tail rotor malfunction, autorotation, fire, jammed controls and system failures within the emergency procedures.

The pilot reported that a loss of tail rotor effectiveness (LTE) (refer to section titled Loss of tail rotor effectiveness) would have been covered in their training history at some stage but could not recall any specific occasion, and that they had never experienced it before in flight. The operator’s head of flying operations (HOFO) reported that they conducted a flight with the pilot before they were released to line and was impressed with their attention and focus on control of the helicopter during take-off and landing. The HOFO did not specifically discuss LTE during their flight with the pilot but did discuss mountain and survey operations. They further reported that they considered LTE a component of the low-level flying conducted in the pilot’s pre‑employment check flights.

National Parks and Wildlife Service officers 

The National Parks and Wildlife Service (NPWS) team on board consisted of:

• A task coordinator who had the lead role in terms of liaising with the pilot and the other officers and was logging the location of the English Broom weed on a hand-held electronic device.
• Two officers designated as primary observers (spotters). Their role was to look for the weed, and when a plant was identified, advise the coordinator. One of these observers was logging the position of the weed on a hand-held electronic device.
• Another NPWS officer had joined the group given their employment as the area ranger. The survey task had provided the opportunity for the officer to familiarise themselves with the area from the air and observe the conduct of the weed survey task. While the officer did not have a specific function to perform for the survey, they assisted the team in locating the English Broom weed.

Helicopter information

General

VH-BHF was a Bell Helicopter Company B206L-1 powered by a Rolls-Royce model 250‑C30P gas turbine engine driving a 2‑blade main and tail rotor system. It was manufactured in the United States in 1979 and assigned serial number 45164. The helicopter was issued with an Australian Certificate of Airworthiness on 7 April 1987 and first registered in Australia on the same date. Including the pilot, the helicopter provided seating for 7 occupants. At the time of the accident, the helicopter had accumulated about 11,849 hours, total time in service.

Recent maintenance history

At the last 100-hour periodic inspection on 27 November 2021, a maintenance release was issued, permitting night visual flight rules[7] operations. The maintenance release showed that an engine hot start defect had been recorded in December 2021. Rectifications for that included the replacement of the engine turbine assembly, and post‑repair power assurance checks that were certified as completed on 14 February 2022, deeming the engine serviceable. The maintenance release also showed that:

• other than items that would be addressed during a daily inspection, no maintenance was due
• there were no defects that required rectification before the next flight
• the helicopter had been flown for about 22 hours from when the maintenance release was issued prior to the accident. 

Modifications

The helicopter was fitted with Van Horn Aviation 2062200-101/-301 tail rotor blades with a United States Federal Aviation Administration (FAA) approved rotorcraft flight manual supplement (206L1‑FMS‑901). The supplement stated that the tail rotor blade design increased the stall margin, thereby improving high altitude performance:

Satisfactory stability and control has been demonstrated in relative winds of 30 MPH (26 knots) sideward and rearward at all loading conditions… 

The helicopter was also fitted with main rotor yoke part number 206-011-149-101 allowing flight operations up to a gross weight limit of 1,882 kg (4,150 lb), up from 1,837 kg (4,050 lb) as stated on the type certificate data sheet.

Weight and balance 

The ATSB completed weight and balance calculations for the helicopter, considering the pilot and 4 NPWS officers on board. Including fuel, baggage and cargo, the helicopter all‑up weight at take‑off was determined to be about 1,842 kg, 40 kg below its gross weight limit of 1,882 kg, and within its centre of gravity limits. Accounting for fuel burn-off, the helicopter’s all-up weight at the time of the accident was about 1,799 kg, 83 kg below its gross weight limit.

Meteorological information

The Bureau of Meteorology grid point wind and temperature forecast (relevant to the accident) for 1100 on 11 March 2022 was 5 kt of wind from the west (280°) and a temperature of 8°C at 5,000 ft. The graphical area forecast, valid from 1000, was for visibility greater than 10 km with scattered[8] stratus cloud between 2,000 ft and 3,500 ft until 1100.

The nearest aerodrome with an automatic weather information service was Cooma, New South Wales, located 50 km east of the accident site at an elevation of 3,106 ft. The recorded conditions at Cooma at 1100 were a wind of 9 kt from 030°, visibility greater than 10 km, no cloud detected, a temperature of 13°C and QNH[9] at 1021.

The pilot reported fine weather conditions with light winds from the south-west of no more than 5 kt when in the vicinity of the power station, dropping to nearly nil wind conditions once below treetop height on descent towards the river. The NPWS officers reported that the weather was calm. One of the first responders provided a similar report of light and variable winds, as they noted that the wind conditions allowed each rescue helicopter to assume a different heading while hovering as the injured persons were winched on board. 

A similar report regarding local weather conditions was received from the operator who maintained an airborne presence during the initial discovery of the wreckage and throughout the rescue operation. They described the conditions on the day as very good with visibility greater than 10 km and wind speed predominantly below 5 kt. They added that there was a very light wind flowing in the downstream direction of the river at the accident site.

Recorded data

A TracPlus™ RockAIR tracking device was recovered from the helicopter following the accident. The device recorded global positioning system tracking information at a frequency of 1 Hz on a removable micro-SD card. ATSB analysis of the recorded data for the last 60 seconds of the flight is shown in Figure 3 for illustrative purposes. 

For a period of about 32 seconds before the helicopter started to yaw, the recorded data indicated that its groundspeed was below 25 kt and further decreased below 20 kt about 5 seconds before the yaw began. About 3 seconds after the yaw commenced, and from a height of about 200 ft above ground level, the helicopter’s rate of descent (vertical speed) increased and reached a peak of about 2,500 ft/min, consistent with the pilot rolling off the throttle and entering an autorotational descent. The data indicated that the yaw lasted for about 5 seconds and was arrested within about 3 seconds of the start of the descent. When the yaw stopped, the helicopter’s height was about 65–100 ft above ground level. 

Figure 3: Ground positioning system flight tracking data over the last 60 seconds of recording

Graphical representation of flight data showing helicopter forward and vertical speeds, altitude, height above terrain and helicopter track with descriptive comments added. Source: TracPlus data, accessed and annotated by the ATSB

Wreckage and impact information

The accident site was located less than 600 m downstream from the Guthega power station (Figure 2) and 20 km north-west of Jindabyne, New South Wales. The helicopter landed on top of a large boulder in the shallows of the Snowy River and came to rest on a heading of 310°, with the fuselage canted significantly to the right (Figure 4).

The helicopter struck the boulder at a point forward of the external cargo hook fuselage mount and slightly aft of the forward skid gear cross tube. The impact with the boulder structurally damaged the helicopter, breaking the forward cockpit section from the cabin area, and resulted in the tailboom partially fracturing near its fuselage attachment point.

The tailboom fracturing and subsequent deflection likely resulted in a tail rotor ground strike and loss of a portion of a tail rotor blade, which was not recovered from the site. Apart from the missing section of tail rotor blade, the rest of the helicopter was present at the accident site. No evidence of a bird or in-flight tail rotor strike was identified and there was no post‑impact fire.

The location of the helicopter in the riverbed and the surrounding environment precluded a complete examination of the wreckage at the accident site. The operator reported receiving advice that anticipated water inflows at Guthega Dam would result in increased water levels downstream of the dam from water exiting the uncontrolled spillway. In response, the wreckage was removed from the accident site at the earliest opportunity, airlifted from the riverbed and relocated to a secure site in Cooma for detailed examination.

Figure 4: VH-BHF following collision with terrain against large boulder in the Snowy River, New South Wales

Source: ATSB

The ATSB’s site examination did not reveal any pre-existing defects that may have affected the operation of the helicopter or its systems. The detailed examination of the flight control systems in Cooma did not identify any pre-existing defects that may have affected the control of the helicopter. 

Where evidence of structural fractures and breaks were identified, the failures were found to be fresh and were attributed to being either collision‑related, or as the result of torsional overload forces. Of note was the torsional overload of the tail rotor driveshaft at the tail rotor gear box location. This indicated that the driveshaft was driving the tail rotor when the tail rotor experienced a sudden stoppage (Figure 5).

The engine presented as intact, securely mounted, and with controls functional but with restricted movement due to fuselage damage. The compressor and turbine were found to spin freely. No defects were identified with the supply, delivery and quality of the fuel that was available to the engine. 

Figure 5: Tail rotor drive shaft showing torsional overload

 Source: ATSB

Survival aspects

Seating layout 

The seating configuration of the helicopter consisted of 2 cockpit seats and, in the cabin section, a centre row of 2 aft-facing seats and a rear row of 3 forward‑facing seats. For the accident flight, the pilot was in the front right seat with an NPWS officer (coordinator/recorder) in the front left seat, another officer (area ranger – observer) in the centre row left seat (facing rearwards), and the 2 remaining officers in the left (observer/recorder) and right (observer) seats of the rear row (Figure 6). Each seat was equipped with a 4-point restraint harness.

Figure 6: VH-BHF cockpit and cabin seating layout and NPWS officers’ functional positions

Bell 206 LongRanger III seating layout adopted for illustrative purposes only. Source: FlyFlapper.com annotated by the ATSB

Injuries 

The pilot, task coordinator, and observer in the rear‑facing cabin seat sustained serious injuries. The 2 observers in the rear row received minor injuries.

Evacuation

While airborne above the accident site, the HOFO reported they contacted the power station and advised them of the accident downstream of their location and for consideration of the possible impact on power generation commitments. They were advised that power generation would be postponed, however, water levels downstream of Guthega Dam were dependent on natural inflows and outflows from the dam.

At interview, 2 of the NPWS officers advised that they were aware that the water level would likely rise in response to power generation activity. As a precaution, after assisting the injured with evacuating from the helicopter, they were immediately moved to higher ground.

Survival equipment

The NPWS aviation standard operating procedure for low-level flying specified that, when engaged in such activities, helicopters were to carry an emergency locator transmitter (ELT) and be fitted with a tracking system that could be tracked by the agency. As such, the helicopter was equipped with an ELT, and a survival pack that included a personal locator beacon (PLB), a first aid kit and a satellite phone. A TracPlus RockAIR device was also mounted on the instrument console, which provided real-time location tracking of the helicopter through GPS technology. The tracking device was designed to transmit an alert if a sudden impact of 16g or more for a period greater than 10 milliseconds was detected. 

ELT and PLB emergency radio beacons are used to provide a location fix on a person, aircraft or other vehicle (ATSB, 2013). ELTs are usually fixed in an aircraft and are designed to activate automatically during an impact, typically by a g-force[10] activated switch but can also be wired to be manually activated by a cockpit-located switch usually mounted within reach of the pilot or a front‑seat passenger. PLBs are designed for personal use and may be carried on the person or carried as part of a survival kit. They are manually activated and may be used as an alternative to a fixed ELT, provided certain requirements are met.

In the event of an accident followed by beacon activation, the aircraft wreckage and its occupants can be located quickly by search and rescue authorities. Finding the aircraft wreckage quickly not only increases the chance of survival of the occupants but also reduces the risk to pilots of search and rescue aircraft who commonly need to operate in marginal weather conditions and over mountainous terrain (ATSB, 2013).

The collision resulted in both the ELT and tracking device activating. The collision alerts were received by the operator (HOFO) and were followed by a third report of a PLB that was manually activated by one of the NPWS officers. This allowed the HOFO to promptly identify the last known position of VH-BHF and commence an emergency response. The operator reported that the multiple transmissions from independent sources provided the surety that a distress situation existed. 

Operational information

Helicopter performance

The out-of-ground effect performance chart in the B206L-1 rotorcraft flight manual indicated the helicopter had the performance required to hover out-of-ground effect at the elevation and temperature conditions for the accident. The accident site was located at an altitude of 4,308 ft. Accounting for temperature and QNH, the density altitude for the flight just prior to the accident was calculated to be about 4,500 ft. 

The recorded data for the flight indicated that the groundspeed had dropped below 20 kt before the loss of control, and accounting for density altitude influence, this equated to a calibrated[11] airspeed of about 1–2 kt below the groundspeed in nil wind. The height and airspeed of the helicopter at this time placed it inside the avoid area of the height-velocity diagram[12] (Figure 7 – left). The helicopter’s weight and density altitude also placed the operation outside of the weight-altitude limit for the height-velocity diagram (Figure 7 – right). 

Consequently, the helicopter was operating in a region of the flight envelope where there was no assurance that a safe autorotation could be made without damage and injuries to occupants. At interview, the operator advised that flight operations in the avoid area was common practice, and necessary to effectively and accurately conduct a weed survey task.

Figure 7: B206L-1 flight manual performance charts showing operational caution zones and VH-BHF relative position in preparation for survey task

Source: Bell Helicopter Company, annotated by the ATSB

Aerial work operations

Heli Surveys

Heli Surveys Pty Ltd was approved by the Civil Aviation Safety Authority (CASA) to conduct various flight operations including Civil Aviation Safety Regulation (CASR) Part 138 aerial work operations. Its aerial work operations were varied and included roles associated with feral animal control and survey flights of pest animals, weeds and power lines. 

Part 138 aerial work operations

CASR Part 138 and the Part 138 (Aerial Work Operations) Manual of Standards (MOS) addressed the certification, operational and safety risk management requirements for operators engaged in aerial work operations (CASA, 2021e). At the time of the accident, aerial work encompassed the core activities of external load operations, dispensing operations or task specialist operations.[13] Advisory circular AC 138-01 v1.0 Part 138 core concepts defined task specialist operations as:

carrying out a specialised activity using an aircraft in flight and includes training for such an activity. An example of a task specialist operation is a low level weed survey or pipeline inspection.

Additional guidance for aerial work operations applicable at the time of the accident was provided in advisory circular AC 138-05 v1.1 Aerial work risk management (July 2021b) and the Part 138 Acceptable means of compliance and guidance material – Aerial work operations v2.2 (December 2021f). 

Conducting the survey flight

At interview, the HOFO described the accident task as an ad hoc type survey in which the helicopter would be flown up-valley and then down-valley to view both sides of the river and that the airspeed, direction and height was not prescribed. The HOFO expressed the view that the optimum profile for survey flights was a height of 300 ft and airspeed of 55 kt. However, if adopting that profile, it would make it impractical to identify English Broom weed in surveys of the Snowy River. 

The HOFO reported that, from experience, they did not consider that it was unusual when the client presented with 4 NPWS officers for the conduct of the survey flight. In terms of managing client requests, all pilots are provided with a ‘stop work authority’ and can therefore decline a client request if they perceive a safety of flight issue. 

The NPWS officers indicated that, on the morning of the accident flight, they discussed their English Broom weed survey plan while waiting for the pilot and helicopter to return from a prior task. After the pilot arrived, they completed the operator’s online induction and a safety brief with the pilot and then briefed the pilot on their plan for the weed survey. 

None of the officers had previously met the pilot who they understood was new to the company and had not previously done the English Broom weed survey task with them. They reported that the pilot was operating in a cautious manner and appeared to be safety‑conscious, advising them all to speak-up if they identified any hazards during the flight. On departure, the pilot made a radio call to their NPWS contact for flight‑following purposes, and they conducted a hazard identification for wires during the flight upstream to the Guthega Power Station.

Persons permitted on board during aerial work operations

For aerial work operations conducted under Part 138, CASA advisory circular 138-01 specified that persons who were permitted on board must be categorised as either:

• crew members (including flight crew, air crew and task specialists)
• passengers that meet the requirement to be aerial work passengers. 

The advisory circular further defined an air crew member, task specialist and aerial work passenger as:

Air crew member

An air crew member…includes crew members who carry out a function during the flight relating to the safety of the aircraft.

Task specialist

A task specialist … includes crew members who carry out a function for the flight relating to the aerial work operation (as distinct from a safety related role).

Examples of a task specialist would include a camera operator that operates an external camera pod, or an aerial shooter used in an animal culling operation. 

A task specialist will require training to be inducted into the operation and to ensure they are competent in carrying out their assigned function as a member of the operator's crew.

Aerial work passenger

…are persons who are closely associated with the purpose of the aerial work operation. Their presence in the aircraft must not be for mere convenience or enjoyment. 

Examples of such persons would include: Personnel involved in carrying out or supporting a mustering activity carried on a positioning flight before or after the mustering operation, such as ground based personnel to assist with refuelling or for opening and closing of gates etc. and yarding of stock for the mustering operation…

In most circumstances aerial work passengers do not require training before their carriage on an aerial work operation or a positioning flight, but they will in all cases (except for some notable situations, such as a person being rescued) require a safety briefing prior to the flight...

On the accident day, as the helicopter was being used to conduct a low-level weed survey activity, it met the definition of a task specialist operation. In terms of the roles as defined above, the pilot was the only flight crew member and there were no air crew members. The 3 NPWS officers with the roles of task coordinator and primary observers would be classed as task specialists. While the area ranger assisted with the task, they reported that they were on the flight as an opportunity for familiarisation of the survey area.

Operational hazards

CASA flight crew licencing uses a competency-based training and assessment system for pilots. Various competencies are required to be demonstrated by pilots during both initial and recurrent licence testing. The competencies vary by aircraft type and licence type. 

For pilots to achieve their helicopter rating, they are required to demonstrate that they have the skills and underpinning knowledge to manage abnormal and emergency situations in helicopters (CASA, 2021c). The range of situations include, but are not limited to: 

• key hazards – underpinning knowledge of their causal factors, contributing operational situations, avoidance and recognition of symptoms and recovery techniques that include:
  • vortex ring state[14]
  • loss of tail rotor effectiveness (LTE) (refer to the section titled Loss of tail rotor effectiveness)
  • overpitching[15] or low rotor revolutions per minute (RRPM) – rotor stall
  • recirculation[16]
• the impact of high gross weight and high-density altitude on key hazards
• techniques for how to avoid a potentially hazardous situation whilst in flight.

These competencies were consistent with the list of hazards detailed in the jointly developed CASA and Civil Aviation Authority of New Zealand helicopter flight instructor manual, issue 3 (CASA, 2012). The instructor manual differentiated hazards from emergencies, which are the technical failures particular to the helicopter model and addressed in the flight manual emergency procedures section.

To be licensed for low-level helicopter operations, pilots must demonstrate skills to safely conduct low‑level operations include managing variable terrain and weather, surface conditions, loose objects and personnel. The required underpinning knowledge related to critical operational conditions that included retreating blade stall,[17] vortex ring state, over pitching and loss of anti‑torque or tail rotor effectiveness (CASA, 2021c). 

The ATSB reviewed the emergencies and hazards chapter of the FAA Helicopter Flying Handbook (2019) and found key operational hazards presented were the same as those that CASA required pilots to demonstrate. The FAA handbook provided a thorough description of each of the key hazards, which included techniques for avoidance and recovery. The FAA handbook also reported the following about LTE events:

Certain flight activities lend themselves to being more at high risk to LTE than others. For example, power line and pipeline patrol sectors, low-speed aerial filming/photography as well as in the Police and Helicopter Emergency Medical Services (EMS) environments can find themselves in low and slow situations over geographical areas where the exact wind speed and direction are hard to determine. 

Loss of tail rotor effectiveness

Introduction

Loss of tail rotor effectiveness (LTE) or unanticipated yaw is a phenomenon that can occur in single main rotor, tail rotor-equipped helicopters. It is a condition that occurs when the air flow through a tail rotor is changed in some way, by altering the angle or speed at which the air passes through the rotating blades of the tail rotor disc (FAA, 2019). If uncorrected, LTE can result in loss of control of the helicopter and serious to fatal occupant injuries. In 1995, the FAA published advisory circular 90-95 Unanticipated right yaw in helicopters, which described a loss of tail rotor effectiveness as:

…a critical, low-speed aerodynamic flight characteristic which can result in an uncommanded rapid yaw rate which does not subside of its own accord and, if not corrected, can result in the loss of aircraft control. 

Any manoeuvre which requires the pilot to operate in a high-power, low-airspeed environment with a left crosswind or tailwind creates an environment where unanticipated right yaw may occur.

LTE is not related to a maintenance malfunction and may occur in varying degrees in all single main rotor helicopters at airspeeds less than 30 knots.

Single-rotor helicopters manufactured in the US, such as the Bell 206, have main rotors that rotate anticlockwise when viewed from above. When powered, their rotation produces a torque reaction or tendency of the helicopter to turn in the opposite direction, which is a right yawing motion from the pilot’s view. The tail rotor thrust provides the anti‑torque control. An effective tail rotor relies on a stable and relatively undisturbed airflow in order to provide a steady and constant anti-torque reaction (FAA, 2019). 

The FAA AC described 3 wind conditions conducive to the onset of LTE. One of these conditions refers to the relative wind[18] azimuth of 285° to 315°, which can produce ‘main rotor disc vortex interference’ with the tail rotor (Figure 8) and is described as: 

As the main rotor vortex passes the tail rotor, the tail rotor angle of attack is reduced. The reduction in the angle of attack causes a reduction in thrust and a right yaw acceleration begins. The thrust reduction will occur suddenly and, if uncorrected, will develop into an uncontrollable rapid rotation about the [main rotor] mast.

The relative wind from the critical quadrant may present when the nose of the helicopter is pointing forward (Figure 8), or the condition is generated when the helicopter is flown with the nose sufficiently yawed to the right.

Figure 8: Main rotor disc vortex interference with tail rotor

Source: FAA Helicopter Flying Handbook (FAA, 2019), annotated by the ATSB

Factors affecting loss of tail rotor effectiveness

Other than main rotor blade action affecting the quality of the airflow about the tail rotor disc and impacting its ability to provide useful thrust, additional factors are also considered when discussing LTE. According to the FAA Helicopter Flying Handbook (2019):

The design of main and tail rotor blades and the tailboom assembly can affect the characteristics and susceptibility of LTE but will not nullify the phenomenon entirely. 

FAA AC 90-95 also identifies other factors that influence the severity of the onset of LTE including:

Gross Weight and Density Altitude. An increase in either of these factors will decrease the power margin between the maximum power available and the power required to hover. The pilot should conduct low-level, low-airspeed manoeuvres with minimum weight.

Recovery technique

The Bell 206L-1 rotorcraft flight manual revision 14 did not have an emergency procedure for LTE but did have a procedure for a complete loss of thrust under the heading tail rotor control failure, which was a mechanical failure. Following the procedure for a complete loss of thrust, pilots were to reduce the throttle to idle and immediately enter an autorotation while maintaining a minimum airspeed of 52 kt during the descent. 

The FAA AC 90-95 recommended recovery technique from LTE was:

a. If a sudden unanticipated right yaw occurs, the pilot should perform the following: 

(1) Apply full left pedal. Simultaneously, move cyclic[19] forward to increase speed. If altitude permits, reduce power. 

(2) As recovery is effected, adjust controls for normal forward flight.

b. Collective[20] pitch reduction will aid in arresting the yaw rate but may cause an increase in the rate of descent. Any large, rapid increase in collective to prevent ground or obstacle contact may further increase the yaw rate and decrease rotor rpm. 

c. The amount of collective reduction should be based on the height above obstructions or surface, gross weight of the aircraft, and the existing atmospheric conditions. 

d. If the rotation cannot be stopped and ground contact is imminent, an autorotation may be the best course of action. The pilot should maintain full left pedal until rotation stops, then adjust to maintain heading.

Heli Surveys operations manual

The Heli Surveys Operations Manual volume 10 – Specialist operations, prescribed the operator’s general low flying requirements. Paragraph 0.7.3, under Conduct of flight during low flying stated the following:

Pilots shall be aware of recovery techniques and avoid flight configurations which could include:

• Vortex ring/ settling with power. 

• Tail rotor vortex ring or loss of tail rotor effectiveness. 

• Downwind operations outside the aircraft performance envelope. 

• Loss of close visual cues to indicate actual aircraft relative movement and out of wind operations (particularly over water), leading to possible unanticipated control difficulties.

The operations manual did not include any avoidance or recovery procedures for LTE nor any reference material to address this condition. 

Safety risk management 

Aerial work risk management

Pre-operational risk assessment

CASR Part 138 required an operator conducting aerial work to undertake risk assessments of its operations. The Part 138 MOS and corresponding advisory circular (AC 138-05 v1.1) detailed a layered approach to risk assessments. One of the key requirements was that an operator should undertake an overarching assessment (pre‑operational risk assessment) to consider and evaluate the risks associated with its proposed operations, in this case, low-level helicopter survey. This assessment recognised the underlying principles of CASR Part 138, where the risks and hazards associated with a type of aerial work operation are common to that type of operation. The MOS indicated that the matters to be considered for such an assessment included:

• the operation and its particular characteristics

• the location of the operation and its particular characteristics

• the aircraft to be used in the operation, its particular characteristics, and its performance

• the qualifications and experience of the crew members to be used in the operation

• the hazards, external to the aircraft, that may be met in the course of the operation.

The operator is required to gather data for inclusion in the pre-operational risk assessment using a range of sources. Acknowledging that certain risk factors may be common to all operators, may be particular to the aircraft type operated or may be unique to the operator; potential sources include, but are not limited to (CASA, 2021b):

• CASA ‘sector risk profiles’ for the varying types of operations
• ATSB incident and accident reports
• industry association safety reports
• manufacturers' safety bulletins and advisory notices
• input from experienced pilots and other operators.

Once the pre-operational risk assessment has been populated, it should be updated over time to include lessons learnt from previous operations. It should also form part of the operator’s operations manual. 

Flight risk management plan

The results of the pre-operational risk assessment were to be considered when preparing the flight risk management plan, which was specific to an individual flight or task within the type of operation. The plan should outline the specific mitigators or risk controls that were to be used during the flights. 

Pre-flight risk review

The next step was for the pilot, on behalf of the operator, to conduct a pre-flight risk review, with reference to the pre‑operational risk assessment, flight risk management plan, and the most recent data for the operation. The review was to be completed prior to the commencement of the operation and was to consider the conditions and circumstances that existed at the site or area at the time of the proposed activities. This ensured that the operation could be conducted without unacceptable safety risk. 

Operator risk management

As per CASR Part 138, Heli Surveys was required to undertake risk assessment and mitigation processes and include those processes in its suite of operational documents. The Heli Surveys Operations Manual described that the operator would address its risk management obligations via the use of Safe Work Method Statements (SWMS). 

The Heli Surveys Safety Management Systems Manual further detailed how risk was identified, controlled and documented. Their safety risk management process started with hazard identification, which included internal sources and external sources. A hazard was defined in their SWMS as ‘what could result in harm’ and was used to describe both the hazard and associated risk. 

Internal sources for hazard identification included, but were not limited to:

• safety assessments of systems and operations
• voluntary and mandatory safety reports
• inspections and audits.

Its list of external sources included, but was not limited to:

• accident and incident reports
• safety information bulletins, safety alerts and other safety publications from CASA, Airservices Australia, the ATSB and other authorities worldwide.

The operator had prepared SWMSs to comply with the CASR Part 138 requirements which was equivalent to a pre-operational risk assessment. As the accident flight was a low‑level survey operation in the Snowy Mountains, the 2 SWMS relevant to the flight were Low level surveys and aerial photography (henceforth referred to as Low-level surveys) and Alpine operations.

The SWMS documents provided the means to record the specific tasking event, the equipment and approvals that were relevant, and any specific checks or personal protective equipment required to perform the task. A risk matrix was also included. The risk matrix described the likelihood and consequence of each identified hazard and provided the means to assess the initial and residual risk level following the implementation of suitable risk controls. 

The ATSB reviewed the SWMSs that were developed by the operator. A summary of the internal and external hazards that were identified by the operator are below (Table 1).

Table 1: Summary of hazards related to Safe Work Method Statements for low-level survey tasking and alpine operations

The heavy landing hazard associated with the alpine operations SWMS was assessed by the ATSB to be related to the CASA flight crew licensing competency requirement to manage the hazard associated with overpitching. The SWMS provided some control measures, such as a power check, landing into wind and monitoring environmental conditions between a landing and take-off. 

With the exception of the relationship between overpitching and the operator’s heavy landing hazard in its alpine operations SWMS, the ATSB did not find references to hazards associated with abnormal situations and emergencies specific to the operator’s unique activities in its SWMS. Of note, there was no reference to LTE and vortex ring state, and the impact of flight regimes and operations at high gross weights and density altitudes that may affect such hazards.

The operator reported that pilots were required to have read and understood the suite of SWMS documents, which were provided during their induction process and at scheduled intervals thereafter. However, there was no requirement for pilots to conduct a pre-flight risk review for low-level survey operations and reference the relevant SWMS when conducting pre-flight tasks in preparation for the activity. As such, the pilot had not conducted a review prior to the accident flight.   

Client risk management

The NPWS (the client) had contracted Heli Surveys to conduct the weed survey operation. Its Aviation Safety Policy and related documents were provided to the ATSB. The policy identified a range of aviation operations that utilised rotary wing aircraft. 

The policy adopted a risk management approach to aviation operations and safety. Key elements of the policy were the development and observance of aviation‑related standard operating procedures and the use of a job safety analysis (JSA).[21] The JSA assessed the risks associated with each task, which was equivalent to a flight risk management plan.

Regarding vegetation‑related activities that necessitated low-level flight operations, the NPWS provided several task-related JSA documents that identified specific hazards. The documents also detailed the control measures to be implemented to manage the associated risks. The JSA documents that were provided related to low-level flying in general, low-level flying when undertaking Scotch (English) Broom survey and aerial application (spraying) activities. 

When engaging in those activities, a key control measure specified in the JSA advised that only ‘essential personnel’ were to be on board the operating helicopter. The NPWS reported that the suite of documents supporting aviation operations did not provide a definition of essential personnel nor was there a procedure on record that detailed the roles and responsibilities of NPWS personnel reflected in the JSA control measure.

Related occurrences

Loss of tail rotor effectiveness

Between 2013 and 2022, the ATSB received 16 notifications where the reporter advised of an LTE or unanticipated yaw event. Of the 16 notifications, 12 were investigated by the ATSB. Most of these resulted in nil to minor injuries to those involved and one serious injury and one fatality. Some of these investigations are described below. 

ATSB investigation AO-2013-016

On 19 January 2013, a Bell 206B3 helicopter was being operated on an aerial filming task over hilly terrain on the north-eastern outskirts of Perth, Western Australia. After hovering and manoeuvring at about 500 ft above ground level to allow the camera operator to record footage of a truck accident, the pilot conducted a right orbit to complete filming and depart the area. The pilot had initiated the turn when the nose of the helicopter moved left, then suddenly and rapidly to the right as the helicopter yawed and developed a rotation of about 5 revolutions.

The ATSB found that, when the pilot turned to the right to commence the orbit, the helicopter was exposed to a crosswind from the left while at an airspeed around the 30 kt threshold value for susceptibility to LTE, precipitating an unanticipated right yaw and temporary loss of control. The pilot regained sufficient control for a forced landing.

ATSB investigation AO-2015-091

On 20 July 2015, the pilot of a Bell 206L3 (LongRanger) helicopter, registered VH-BLV, conducted a charter flight from Essendon Airport to Falls Creek, Victoria, with 5 passengers on board. The helicopter took off from Essendon close to its maximum take‑off weight.

When at 700 ft above ground level and tracking from the north-west, the pilot conducted a shallow approach towards the helipad at Falls Creek. As the helicopter descended to about 50 ft above ground level, the pilot found that significantly more power was required to conduct the approach than anticipated. The pilot assessed that there was insufficient power available to continue to land and elected to abort the approach. The pilot pushed forward on the cyclic to increase the helicopter’s airspeed and conducted a left turn. 

As the helicopter turned left, it started to yaw rapidly towards the right. The pilot applied full left anti-torque pedal to counteract the yaw, but the helicopter continued to yaw. The helicopter turned through one and a half revolutions, as the pilot lowered the collective. Lowering the collective reduced the power demand of the power rotor system, thereby increasing the ability of the anti‑torque pedals to stop the right yaw. The combination of lowering collective and applying forward cyclic to gain forward airspeed, allowed the pilot to regain control of the helicopter. The pilot then conducted a left turn towards the helipad and made an approach to the helipad from an easterly direction. The helicopter landed following the second approach without further incident. 

The ATSB’s report highlighted the importance for pilots to understand and avoid conditions that are conducive to unanticipated yaw or LTE and noted that pilots can reduce their exposure to LTE by maintaining awareness of the wind and its effect on the helicopter. Further, if a pilot encounters unanticipated yaw, quick application of the correct response is essential to recover control of the helicopter.

Carriage of additional personnel

ATSB investigation AO-2019-008

On 28 January 2019, the crew of a Sikorsky S-64E Skycrane helicopter was conducting firebombing activities when it collided with water at Woods Creek Dam, Victoria. The collision occurred following an approach to the dam to fill an external tank with water. The helicopter was crewed by 2 pilots, and a maintenance crew chief was also on board. Following the collision, all the occupants were able to exit the helicopter and swim to shore. One crewmember was seriously injured and 2 were uninjured. The helicopter was substantially damaged.

The ATSB found that the helicopter was placed in a steep flare, which contributed to the helicopter entering vortex ring state when on approach to the dam.  

It was also noted that the operator’s operations manual stated that only flight crew and crew essential to the operation could be carried aboard the aircraft during firefighting operations. The operation could be conducted without the crew chief, and not all company crew chiefs were on board their aircraft during firefighting operations. While the crew chief had significant system and task knowledge, they were not required to be on board the helicopter.

On this occasion, their presence on board subjected them to the significant hazards associated with underwater egress. More generally, the carriage of additional personnel during specialised operations like firefighting exposes them to unnecessary risk. 

ATSB investigation AO-2019-025

On 21 May 2019, while engaged in a planned cull of feral animals in Kakadu National Park, Northern Territory, a crew of 3 were using a Bell 206B3 JetRanger helicopter for aerial platform shooting. While the helicopter was operating at about 50 ft above the ground, the engine decelerated to idle, resulting in an immediate loss of power, and subsequent collision with terrain. The 3 occupants (pilot, shooter and spotter) were seriously injured. 

The investigation identified that it was normal practice across industry that an aerial culling task was performed with just 2 persons on board the helicopter, the pilot and a shooter. Experienced aerial shooters interviewed after the accident expressed a preference for carrying just the pilot and shooter on board to reduce risk to crew, carry more fuel to improve endurance and to complete more work. In 2016, the aerial culling task was redesigned for 3 crew, including a spotter. There was no formal risk analysis of the inclusion of the spotter position, or consideration of the potential benefits of improved data collection when weighed against operational difficulties in recording data, reduced efficiencies in operation, and increased exposure of employees to risk.

The investigation identified that, given the increased complexity and risk in low-level operations, the number of crew should be kept to a minimum. That is, only personnel essential for conducting the task should be carried. 

Safety analysis

Introduction

On the morning of 11 March 2022, a Bell B206L-1 helicopter, registered VH-BHF, departed Jindabyne aerodrome, New South Wales, to conduct a weed survey task on behalf of the National Parks and Wildlife Service (NPWS). On board were the pilot and 4 NPWS officers. While descending towards the riverbed in the vicinity of the Guthega power station, the helicopter started an uncommanded yaw to the right. The pilot was able to stop the yaw but was unable to arrest the descent before the helicopter collided with terrain. The helicopter was destroyed. Three occupants received serious injuries, and the remaining 2 occupants received minor injuries.

The following analysis will discuss the uncommanded yaw, and the carriage of persons on the flight. It will also consider the risk management practices of both the operator and its client and discuss the emergency response following notification of the accident.

Helicopter position

The weed survey task was a low-level, low-speed flight activity. On the accident flight, in addition to the pilot seated in the front right seat, there was an NPWS officer in the front left seat and 3 NPWS officers in the cabin area with 2 seated on the left of the helicopter. With 3 of the NPWS officers seated on the left, the pilot was asked if the helicopter could be flown sideways to provide the best view of the target vegetation for those officers. In response, the pilot yawed the helicopter about 45° to the right of their track. Forward flight with the helicopter yawed 45° to the right, in calm wind conditions, produced a relative wind opposite to the motion of the helicopter, from an angle of about 315°.

Weight and balance data indicated that with the 5 occupants on board, the helicopter was operating within 100 kg of its maximum all-up weight. It was also operating at a density altitude of about 4,500 ft. As weight and density altitude increase, the margin between the power available and power required for the flight is reduced. Further, the flight data identified that the groundspeed of the helicopter was below 25 kt and further reduced to less than 20 kt for several seconds prior to the uncommanded right yaw. As there was little wind, the airspeed was close to the recorded groundspeed. 

As described by the United States Federal Aviation Administration in its Helicopter Flying Handbook and advisory circular 90-95, there are certain conditions that can change the air flow through a tail rotor, subsequently resulting in a loss of tail rotor effectiveness (LTE). In this case, the combination of a low speed and right yaw placed the helicopter inside the region of main rotor disc vortex interference with the tail rotor, a condition conducive to the onset of LTE. The severity of the onset of LTE was further influenced by the high gross weight and density altitude.

Loss of tail rotor effectiveness 

The pilot’s description of flying the helicopter with a significant amount of right yaw at about 30 kt was consistent with the recorded data at the start of their run from the Guthega power station. However, the speed slowly decayed below 20 kt just prior to an uncommanded right yaw when the pilot applied some left anti-torque pedal to straighten the helicopter and improve their vision on their approach to the river below. After the helicopter started yawing to the right, the pilot identified a forced landing site in the river and rolled the throttle back to idle, which stopped the yawing motion. The cessation of the yawing motion when the engine power was reduced indicated the yaw was being driven by the reaction to the engine torque applied to the main gearbox and there was insufficient anti-torque to prevent it. 

The ATSB determined that there was no evidence of a pre-existing mechanical issue, and the helicopter had the performance capability to operate at the altitude of the survey area. However, the helicopter was positioned in the region of main rotor disc vortex interference with the tail rotor just prior to the loss of control. As such, the ATSB concluded that the uncommanded right yaw was likely an LTE event. 

At the time of the event, the helicopter was operating at about 150 ft above ground level in the avoid area of the height-velocity diagram, in addition to which, it was also outside the weight-density altitude limits for the height‑velocity diagram. Therefore, there was no assurance a safe forced landing with minimal damage and injuries could be achieved from the height that the autorotation was commenced.

Operator’s risk management

As a Part 138 operator, Heli Surveys was required to adopt a layered approach to risk management. This approach included conducting a pre-operational risk assessment, which considered all the generic risks and hazards common to the type of operation, in this case, low‑level survey. Heli Surveys achieved this requirement through the Safe Work Method Statements (SWMS).

To inform the pre-operational risk assessment, a range of internal and external data sources could be used that considered the risks common to all low-level survey operators, particular to the aircraft type operated, or unique to the operator. For example, for low-level helicopter operations this may include hazards such as a high-density altitude, retreating blade stall, LTE, vortex ring state and over pitching. Therefore, it was foreseeable that hazards influenced by the particular operating environment would be included in the operator’s SWMS for both Low-level surveys and Alpine operations.

The ATSB reviewed the SWMS accounting for the circumstances of the accident. The SWMS incorporated heavy landings, adverse weather events, collisions with obstacles and hazards associated with the carriage of passengers and task specialists. In consideration of the operation and activities, which included the carriage of passengers and task specialists, the hazards identified by the operator appeared to be relevant. However, their SWMS did not address LTE, although this was identified in its operations manual as a condition specific to low flying and is a known hazard as discussed by the Civil Aviation Safety Authority and the United States Federal Aviation Administration.

The English Broom weed survey operation was conducted at low level and low speed, which were conditions conducive to the onset of LTE. Therefore, and in establishing the context for the operation, LTE was relevant. However, while the risk of LTE was not considered in the SWMS, the accident pilot was familiar with LTE and indicated that it had been covered in their training at some point. As a result, the ATSB was unable to determine if having LTE identified in the SWMS would have influenced the accident outcome. That said, the absence of this consideration did not allow for formal mitigation strategies to be implemented, nor provide assurance that the risk level associated with LTE was as low as reasonably practical. Consequently, there was a reliance on the underpinning knowledge and operational experience of the individual pilot to manage the risk of LTE. 

In addition, as a requirement for Part 138 operators, the pre-operational risk assessment, or in this case the SWMS, was to inform the pre-flight risk review. This review was to be performed by a pilot, on behalf of the operator, before a flight commenced. The operator reported that such a review was not conducted for its low-level survey operations nor was one performed by the accident pilot. The merits of this process would have provided the operator an opportunity to validate the SWMS against the proposed operation and allow pilots to determine that the operation could be conducted without unacceptable safety risk. 

Documenting and detailing known hazards and the associated risk controls in a dedicated SWMS, reviewed pre-flight, would complement a pilot’s underpinning knowledge. In turn, this would raise immediate awareness of the possibility of encountering hazards such as LTE when conducting a low-level survey task. Further, the pre-flight risk review would provide the means for all the participants involved to consider these critical operational conditions and associated controls. This would complement the safety briefing provided by the pilot in conjunction with the NPWS officers as they prepared for the accident flight.

Helicopter occupants

There were 5 occupants on the helicopter, including the pilot. It was very likely that the weed survey could have been completed with just the NPWS coordinator in the front left seat and 2 officers in the left and right rear forward‑facing seats. As such, they would meet the criteria of a task specialist as described under Part 138. If not required as a task specialist, and excluding the pilot, all others on board would be regarded as aerial work passengers and would not be permitted. As such, it was likely that the additional NPWS officer on board (the area ranger) was not fulfilling the role of a task specialist. The additional person’s presence appeared to be motivated by opportunity, and while it was acknowledged that they could contribute as a survey team member, their involvement was not essential to a successful task outcome.  

Given the nature of the task and the operating conditions under which it was being conducted, the inclusion of personnel who were not essential to fulfilling the task outcomes exposed them to the risks of low-level helicopter flight and, in the event of an accident or incident, potential injury. On this occasion, the occupant who did not have a specific role to perform, for either the spotting or logging activity, was seriously injured in the accident when operating at low level with limited landing options available due to the surrounding terrain. 

Client’s risk management

The client (NPWS) arranged for the survey flight to be undertaken, and its officers presented at Jindabyne to board the helicopter on the appointed day. Risk assessments covering low-level flying operations and weed survey tasks in the form of a job safety analysis were on record, and a key risk control measure advised that only essential personnel were to be on board. However, no definition of essential personnel was available to potentially limit the number of persons that would be exposed to the identified risks. Defining essential personnel would also support informed distinctions between those who would appropriately fulfil roles as task specialists and those who were aerial work passengers. 

Further, the procedure and roles of the persons conducting the survey were not documented. This likely allowed a degree of discretion to be applied by the participants, which resulted in others participating alongside task specialist(s) whose presence may, on occasion, be unnecessary. For example, for this accident one of the NPWS officers who did not have a specific role received serious injuries.

The client was also engaged in other activities such as aerial spraying and culling, both of which likely involved helicopter operations at low level. Having a definition of essential personnel and documenting their respective roles and responsibilities as task specialists would provide the necessary information for determining who should be involved. This would potentially confine the numbers to the minimum required to conduct the task thereby minimising risk exposure. 

Accident notification

The helicopter was equipped with a fixed emergency locator transmitter and an electronic flight tracking device (TracPlus), which provided active monitoring of the helicopter’s position. Additionally, a personal locator beacon and a satellite phone were carried on board as part of the operator’s survival kit. 

Within a very short time of the accident occurring there were reports of the helicopter's fixed emergency locator transmitter activating, the TracPlus unit transmitting the helicopter’s last recorded position and manual activation of a personal locator beacon. The multiple reports removed any doubt of a spurious transmission from any of the units and, as a result, the operator and emergency services were able to respond with minimal delay.

The timely alerts also provided the means for the power station to be alerted to the presence of injured persons on the riverbank who required urgent medical assistance. Their recovery would likely have been impacted by an increase in water level and provided the opportunity for decisions to be made regarding water discharge into the river via the power station. 

The extraction of the damaged helicopter from the Snowy River was also influenced following advice of water storage buildup and possible uncontrolled discharge from the Guthega Dam spillway. The early notification likely provided sufficient time to plan for and safely airlift the helicopter wreckage from the river for detailed examination and removed a potential environmental issue.

Findings

From the evidence available, the following findings are made with respect to the collision with terrain involving Bell 206L-1, VH-BHF, 20 km north-west of Jindabyne, New South Wales, on 11 March 2022. 

Contributing factors

• The sideways movement of the helicopter during the weed survey operation, combined with the high-density altitude, high gross weight, and low airspeed, were conditions conducive to the onset of a loss of tail rotor effectiveness.
• It was likely that a loss of tail rotor effectiveness occurred at a height that was insufficient for the pilot to recover before the helicopter impacted the ground.
• The carriage of an additional person on board the helicopter who was not essential to the tasking, exposed them to risks associated with low flying operations.
• The New South Wales National Parks and Wildlife Service operating procedures referred to, but did not define ‘essential personnel’, or specify their roles and responsibilities as task specialists when performing aerial work activities. (Safety issue)

Other factors that increased risk

• The Heli Surveys safe work method statements for low-level survey and alpine operations did not identify the operational factors that could affect the control of the helicopter. There was also no requirement for its pilots to conduct a pre‑flight risk review for low-level survey operations. Combined, this limited the operator’s ability to manage the possibility of loss of tail rotor effectiveness and ensure that the risks associated with low-level survey operations were as low as reasonably practicable. (Safety issue)

Other findings

• The activation of the on-board emergency locator transmitter and a flight monitoring device, and manual activation of a personal locator beacon, resulted in an immediate emergency response.

Safety issues and actions

Operator's risk assessment

Safety issue number: AO-2022-012-SI-01

Safety issue description: The Heli Surveys Safe Work Method Statements for low-level survey and alpine operations did not identify the operational factors that could affect the control of the helicopter. There was also no requirement for its pilots to conduct a pre-flight risk review for low-level survey operations. Combined, this limited the operator’s ability to manage the possibility of loss of tail rotor effectiveness and ensure that the risks associated with low‑level survey operations were as low as reasonably practicable.

Client’s risk assessment

Safety issue number: AO-2022-012-SI-02

Safety issue description: The New South Wales National Parks and Wildlife Service operating procedures referred to, but did not define, ‘essential personnel’, or specify their roles and responsibilities as task specialists when performing aerial work activities.

Safety action not associated with an identified safety issue

Additional safety action taken by Heli Surveys

In addition to the safety action detailed above, Heli Surveys has revised its risk register detailing both flight-based and ground‑based threats in its operations and associated risk controls. It has also introduced a ‘Hazardous Flight Conditions’ ground-based course that was proactively developed in response to this accident. The intent of the course was to refamiliarize pilots with such conditions (for example, loss of tail rotor effectiveness) to ensure currency and assist with informed decision‑making and is to be completed every 12 months. The flying aspects discussed in the course will be covered in operator proficiency checks. 

Additionally, Heli Surveys has defined ‘essential crew’ in its operations manual. It has also added a requirement that, prior to flight, the pilot in command is to confirm that when undertaking Part 138 operations, all persons on board are deemed essential and each person has a relevant and specific task.        

Glossary

Sources and submissions

Sources of information

The sources of information during the investigation included:

• the pilot
• New South Wales National Parks and Wildlife Service officers
• Heli Surveys Pty Ltd
• New South Wales National Parks and Wildlife Service
• Bureau of Meteorology
• Civil Aviation Safety Authority
• New South Wales Police Force
• recorded data – TracPlus unit. 

References

ATSB. (2013). A review of the effectiveness of emergency locator transmitters in aviation accidents (AR-2012-128). Australian Transport Safety Bureau, Canberra, ACT, Australia. 

CASA. (2012). Helicopter Flight Instructor Manual, Issue 3. Civil Aviation Safety Authority, Canberra, ACT, Australia.

CASA. (2021a). Advisory Circular: Part 138 core concepts (AC 138-01 V1.0). Civil Aviation Safety Authority, Canberra, ACT, Australia.

CASA. (2021b). Advisory Circular: Aerial work risk management (AC 138-05 V1.1). Civil Aviation Safety Authority, Canberra, ACT, Australia.

CASA. (2021c). Part 61 Manual of Standards Instrument 2014. Civil Aviation Safety Authority, Canberra, ACT, Australia.

CASA. (2021d). Part 91 (General Operating and Flight Rules) Manual of Standards 2020. Civil Aviation Safety Authority, Canberra, ACT, Australia.

CASA. (2021e). Part 138 (Aerial Work Operations) Manual of Standards 2020. Civil Aviation Safety Authority, Canberra, ACT, Australia.

CASA. (2021f). Acceptable means of compliance and guidance material, (Aerial work operations - Part 138 of CASR). Civil Aviation Safety Authority, Canberra, ACT, Australia.

CASA. (2023). Multi-part Advisory Circular: AC 91-30, AC 121-12, AC 133-03 and AC 135-14 V1.0, Emergency locator transmitters. Civil Aviation Safety Authority, Canberra, ACT, Australia.

FAA. (1995). Advisory Circular: Unanticipated right yaw in helicopters (AC 90-95). U.S. Department of Transportation, Federal Aviation Administration, Washington, D.C., USA. 

FAA. (2019). Helicopter Flying Handbook (FAA-H-8083-21B). U.S. Department of Transportation, Federal Aviation Administration, Oklahoma City, OK, USA.

NSW Government. (2023). Scotch broom, www.environment.nsw.gov.au accessed July 2024.

NTSB. (2017). Safety Alert SA-062: Loss of tail rotor effectiveness in helicopters. National Transportation Safety Board, Washington, D.C. USA. 

Weeds Australia. (2019). Broom, English Broom, Scotch Broom, Common Broom, Scottish Broom, Spanish Broom, www.weeds.org.au accessed July 2024.

Submissions

Under section 26 of the Transport Safety Investigation Act 2003, the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. That section allows a person receiving a draft report to make submissions to the ATSB about the draft report. 

A draft of this report was provided to the following directly involved parties:

• pilot of the accident flight
• Heli Surveys Pty Ltd
• National Parks and Wildlife Service officers
• National Parks and Wildlife Service
• Civil Aviation Safety Authority
• Transportation Safety Board of Canada.

Submissions to the report were received from the following parties:

• Civil Aviation Safety Authority
• Heli Surveys Pty Ltd
• National Parks and Wildlife Service
• National Parks and Wildlife Service officers.

The submissions were reviewed and where appropriate, the text of the report was amended accordingly.