Investigation summary

What happened

On 1 July 2024, a Cessna 310R, registered VH-ZMB, was returning to Alice Springs from Willowra aircraft landing area, Northern Territory (NT), with only the pilot on board. 

During an instrument approach in instrument meteorological conditions, the pilot reported receiving false indications from the attitude indicator and directional gyroscope. The aircraft deviated from the published approach path and tracked perpendicular to the approach track, below minimum sector altitude (MSA). 

The pilot notified air traffic control at Alice Springs tower of the situation, before obtaining a clearance to track from their present position back to the initial instrument landing system approach point, for a subsequent attempt at landing.

A second approach was then flown, followed by a successful landing at Alice Springs Airport.

What the ATSB found

The ATSB found that the pilot, whilst established on the ILS approach to Alice Springs, likely experienced spatial disorientation that led to an undesired flight path, below the MSA.

In their state of distress, the pilot did not broadcast a PAN PAN call notifying air traffic control of their situation. Further, air traffic control did not issue a safety alert, which would have alerted the pilot that they were in unsafe proximity to terrain and needed to climb immediately. This was also influenced by the pilot not broadcasting a PAN PAN, but could have been made independently.

Once the pilot was outside of the required tolerances for the instrument approach and below the MSA, the pilot did not conduct a missed approach, remaining below minimum sector altitude for an extended period.  

Other factors that increased the risks identified in this investigation include post‑occurrence fault finding that found the artificial horizon exhibited deviations outside the manufacturer’s required tolerances. Additionally, the pilot’s choice to not make use of the autopilot for the approach may have increased their workload and the subsequent risk of spatial disorientation during the instrument approach procedure.

What has been done as a result

The operator has since introduced an automation policy for the use of autopilot in instrument meteorological conditions and in high workload single-pilot operations. 

Safety message

Pilots should not hesitate to report an urgent condition when encountering situations that may not be immediately perilous but significantly increase risk. Broadcasting a PAN PAN call when there is uncertainty about the safety status of the aircraft will alert ATC to the need for immediate assistance.

Air traffic control has a duty of care to provide safety alerts to pilots on becoming aware that an unsafe situation such as proximity to terrain has, or may, occur. 

Once an aircraft is no longer on an established approach path and doubt exists as to its lateral position and location, a missed approach should be conducted, including an immediate climb to achieve a safe altitude, clear of terrain.

The investigation

The occurrence

On the morning of 1 July 2024, a Cessna 310R, registered VH-ZMB, conducted a passenger transport flight[1] to Willowra aircraft landing area, NT, and was repositioning[2] to Alice Springs, NT, with only the pilot onboard. 

At approximately 1020 local time, the pilot commenced an instrument landing system (ILS) approach[3] for runway 12 at Alice Springs Airport. This approach was manually flown (not utilising the autopilot system), in instrument meteorological conditions.[4] 

At 1022:05, the aircraft was established at 4,250 ft above mean sea level (AMSL) on the approach into Alice Springs Airport on the published ILS approach profile. About 25 seconds later, the pilot recalled receiving erroneous instrument indications from the artificial horizon (AH). At 1022:30, flight data showed the aircraft departing the ILS to the left, and tracking at a perpindicular direction from the approach path with unusual aircraft bank angles (AOB) (Figure 1). 

Figure 1: VH-ZMB flight path

Source: ATC recordings and recorded flight data, overlaid on Google Earth and annotated by the ATSB

At 1022:47, the pilot contacted air traffic control (ATC), using their callsign twice. The pilot reported an issue with the instruments and requested clearance to commence a second approach. The ATC controller observed, and ATC recordings indicate, a level of stress in the voice of the pilot at this time. 

ATC subsequetly cleared the pilot to climb to 5,500 ft and to track directly to the initial approach fix for the ILS (position LISZT).

At 1023:36, nearly a minute after obtaining a clearance from air traffic control and over a minute from leaving the ILS profile, the pilot commenced a sustained climb. 

Recorded flight data indicated that during this time, the aircraft was below the minimum sector altitude of 4,300 ft and tracking towards rising terrain. The aircraft came within its closest proximity to terrain as it passed the ridgeline at about 810 ft above ground level.

At 1024:30 the pilot acheived an altitude of 4,300 ft enroute to position for a second approach.

A subsequent ILS approach was then flown into Alice Springs, followed by a successful landing.

Table 1: Sequence of events

Source: ATC recordings and recorded positional data tabulated by the ATSB 

Context

Pilot qualifications and experience

The pilot held a commercial pilot licence (aeroplane) and a valid class 1 aviation medical certificate. The pilot reported a total flying time of 386 hours with about 66 of those being on the Cessna 310. The pilot obtained a multi-engine aeroplane instrument rating in February 2024. The pilot reported accruing 27.3 total hours of instrument flight time with 5.7 hours being accrued in the last 90 days. The pilot had been employed with the operator since April 2024 and had completed their Operator Proficiency Check – IFR [5], on 17 May 2024. 

Aircraft

The Cessna 310R is a twin-engine, low-wing, 6-seat, unpressurised aircraft equipped with retractable landing gear. The aircraft was manufactured in 1976 and had greater than 16,600 hours recorded on the maintenance release. VH-ZMB was fitted with Garmin 430W avionics, coupled with a traditional avionics suite (Figure 2). 

Figure 2: Photo of cockpit instruments from perspective of left (pilot) seat

Source: Operator annotated by the ATSB

The pilot reported that placement of the standby artificial horizon on the far right‑hand side of the instrument panel (Figure 2) precludes the pilot from observing angles of bank (especially to the left). However, the pilot also reported utilising the standby AH as the primary means of spatial orientation, both during the occurrence and post‑occurrence to fly the second approach and identified that recovery to a safe altitude was ultimately slowed by the significant workload of stabilising the aircraft on a limited instrument panel.

Weather conditions

Weather conditions in the Alice Springs terminal area at the time of the occurrence were identified as a moderate south-easterly wind of 10 kt, with greater than 10 km of visibility. The cloud was reported as scattered (between 3–4 oktas[6]) at 900 ft, broken (between 5–7 oktas) at 1,300 ft and overcast (8 oktas) at 2,400 ft above ground level. The pilot reported the approach was conducted in instrument meteorological conditions and recalled being in stratiform cloud [7] from 7,000 ft to 2,500 ft AMSL.

Instrument landing system approach

The Alice Springs ILS runway 12 initial approach fix is a waypoint designated as LISZT which is about 15 NM (27.8 km) from the end of runway 12. The approach descent commences at 11.5 NM (21.2 km) from the runway 12 threshold, on a standard 3° descent profile. The missed approach procedure is to track 116° magnetic and climb to 5,500 ft AMSL. 

Minimum sector altitude 

Minimum sector altitude (MSA) is the lowest altitude which will provide a minimum clearance of 1,000 ft above all objects located within a specified area. This specified area is contained within a circle, or a sector of a circle of 25 NM (46.3 km) or 10 NM (18.5 km) radius centred on a significant point.

In the case for Alice Springs, the significant point being used as the datum reference point is the Alice Springs VHF Omni Directional Range (VOR) station[8]. 

The 10 NM MSA in the area around Alice Springs Airport is 4,300 ft AMSL.

Missed approach procedures

The missed approach procedure plays a pivotal role in instrument approach safety. It provides a standardised procedure for managing an aborted approach and landing attempt, ensuring appropriate terrain clearance to safely conduct flight operations in diverse environmental conditions. 

Section 15.11 of the Part 91[9] Manual of Standards contains specific circumstances where a missed approach must be conducted. 

A summary of these circumstances is as follows:

• during the final segment of an instrument approach, where the aircraft is not maintained within the applicable navigation tolerance for the aid in use
• when the required visual reference is not established at or before reaching the missed approach point from which the missed approach procedure commences
• when a landing cannot be made from a runway approach, unless a circling approach can be conducted in weather conditions equal to or better than those specified for circling
• when visual reference is lost while circling to land from an instrument approach.

Procedures outlined in the Aeronautical Information Publication (AIP) state that a missed approach must be conducted under certain conditions if the aircraft is below MSA. These conditions include, but are not limited to: 

• issues arising with the radio aid,

• visual reference not being established, and

• a landing cannot be effected from the runway approach. 

Operational procedures require that during a missed approach manoeuvre, an immediate climb is carried out to achieve an altitude that will remove the aircraft’s exposure to the risks of collision with terrain. 

Instrument approach procedures

An instrument approach or instrument approach procedure (IAP) is a series of predetermined manoeuvres for the orderly transfer of an aircraft operating under instrument flight rules from the beginning of the initial approach fix to a landing, or to a point from which a landing may be made visually.

An IAP enables a descent below the MSA, positioning the aircraft to safely approach and land.

Operations below MSA increase the risk of collision with terrain or obstacles which are an immediate threat. Maintaining the published instrument approach path assures the pilot of obstacle clearance below the MSA. Outside of these areas, while below the MSA and in instrument meteorological conditions (IMC), separation from terrain and obstacles cannot be guaranteed and the pilot must conduct a missed approach procedure. 

Recorded data

Automatic dependant surveillance broadcast (ADS-B) Exchange and Flightradar24 data was collected by the ATSB and was supplemented with OzRunways data provided by the pilot.

ATSB analysis combined the ADS-B flight data and the OzRunways aircraft track data to ascertain the track position and orientation of the aircraft during the occurrence.

At 1022:30 the aircraft deviated significantly to the left of the approach path resulting in the aircraft no longer being established on the ILS approach. The aircraft was below the MSA at 3,900 ft and continued to descend to the lowest point of 3,320 ft.

About 35 seconds later the aircraft crossed a ridgeline, further reducing the vertical separation with terrain to 810 ft above ground level.­­­­

Recorded data indicated (Figure 3) that 30–40 seconds after speaking to ATC and approximately 70 seconds after leaving the ILS approach profile, the pilot commenced a sustained climb and began tracking to the initial approach fix of LISZT. During this time the aircraft was operated below the MSA. 

Figure 3: Aircraft vertical profile

Source: ADS-B Exchange, Flightradar24 and OzRunways data analysed and annotated by the ATSB

Recorded data identified a significant left turn, greater than 60° AOB, with a subsequent bank to the right of greater than 40° AOB and a further left correction (Figure 4). These occurred while the aircraft was still descending. The descent was arrested, at an altitude of about 3,320 ft. With minimal climb observed for about 30 seconds before approaching rising terrain, the aircraft then passed over the ridgeline at a height of approximately 810 ft (Figure 3). (Note: Graphical figures contain smoothed data profiles that may not precisely reflect the exact data point at an exact period).

Figure 4: VH-ZMB bank angles

Source: ADS-B Exchange, Flight Radar 24 and OzRunways data analysed and annotated by ATSB

Vacuum‑powered gyroscopic instrumentation (artificial horizon)

The Cessna 310R is fitted with gyroscopic instruments[10] including an artificial horizon (AH), heading indicators and turn coordinators (turn and bank).

The vacuum system instruments on the Cessna 310R consist of 2 directional gyros, 2 AH gyros and the suction gauge. 

The artificial horizon is the main instrument pilots use to fly through IMC. This instrument is considered a master instrument because it presents pitch and bank attitude information directly against an artificial horizon. It is a critical instrument to allow pilots to fly through non-visual and low-visibility conditions. It indicates the aircraft's orientation relative to the earth, expressed in pitch, roll, and yaw.[11]

Figure 5: Generic example of an artificial horizon

Source: Wikipedia

The gyroscopic instruments are powered by the vacuum system, consisting of a vacuum pump on each engine, pressure relief valve for each pump, a common vacuum manifold, vacuum air filter and suction gauge. Air pressure is used to rotate vanes to spin the instrument gyroscopes thus utilising gyroscopic forces as a mechanism that keeps the instrument level with respect to the direction of gravity. The AH gyro is mounted in a double gimbal, which allows the aircraft to pitch and roll as the gyro stays vertically upright.

The pilot reported that during the approach they noticed that the suction gauge was indicating ‘low pressure’. A partial blockage or issue in the pilot suction line, immediately after the air filter, could affect the pilot (left-side) AH and the suction gauge, with nil effect on the copilot (right-side) gauges.

However, post‑occurrence maintenance inspections and ground runs could not identify any abnormalities in the vacuum system.

Figure 6: Example of the suction gauge

Source: ATSB

Post occurrence maintenance testing of the artificial horizon identified a gradual drift in pitch, up to 7°, and up to 4° drift in the roll axis over a period of 20 minutes. The AH deviations were gradual, inconsistent and outside the manufacturer’s required tolerances. 

Flight automation and operator policy

Flight automation, such as an approved autopilot, utilises different control systems and technologies that reduce the requirements of human interaction.

An autopilot system can reduce the pilot’s workload. This is achieved by the automation taking over routine tasks such as maintaining altitude, heading, and airspeed. Subsequently allowing the pilot mental capacity to focus on other critical aspects of the flight, such as monitoring systems, flight path, weather conditions and communicating with air traffic control. This is particularly useful in times of a high workload environment. 

The operator’s policy did not detail requirements on when it was appropriate or required to use the autopilot.

PAN PAN call

A ‘PAN PAN’ transmission is used to describe an urgent situation, but one that does not require immediate assistance. Examples of such situations include instrument malfunctions, deviation from route or entering controlled airspace without a clearance. 

When an air traffic controller receives a PAN PAN call from an aircraft, the controller will declare an alert phase.[12] The Safety bulletin What happens when I declare an emergency, released by Airservices Australia, stated that ATC may provide a range of support services including: 

• passing information appropriate to the situation, but not overloading the pilot

• allocating a priority status

• allocating a discrete frequency (where available) to reduce distractions

• notifying the Joint Rescue Coordination Centre (JRCC), appropriate aerodrome or other agency

• asking other aircraft in the vicinity to provide assistance.

An aircraft is in an urgency condition the moment that the pilot becomes doubtful about position, fuel endurance, weather, or any other condition that could adversely affect flight safety. The time for a pilot to request assistance is when an urgent situation may or has just occurred.

No ‘PAN PAN call was made by the pilot during the occurrence.

Air traffic control safety alert

A safety alert issued by air traffic control is instructions prefixed by the phrase 'SAFETY ALERT'. The AIP outlines a safety alert as:

ATC will issue a Safety Alert to aircraft, in all classes of airspace, when they become aware that an aircraft is in a situation that is considered to place it in an unsafe proximity to:

1. terrain;

2. obstruction;

3. active restricted areas; or

4. other aircraft. 

A safety alert should trigger an appropriate response from the pilot to address and resolve the undesirable state.

When the pilot made contact with ATC, the controller reported observing a level of stress in the pilot’s voice and noticed the pilot tracking perpendicular to the approach path for the runway 12 ILS, below the MSA, towards rising terrain. 

No safety alert was made by air traffic control during the occurrence.

Spatial disorientation 

Spatial disorientation (SD) occurs when a pilot has a false perception of the motion or orientation of the aircraft with respect to the Earth (Ledegang & Groen, 2018), subsequently incorrectly interpreting the aircraft attitude, altitude or airspeed.

The ATSB publication Accidents involving Visual Flight Rules pilots in Instrument Meteorological Conditions (AR-2011-050) explains the basis of SD.

In order to correctly sense the orientation of the body relative to its environment, a pilot relies on a number of sensory systems in order to establish or maintain orientation: 
» the visual system 
» the vestibular system, which obtains its information from the balance organs in the inner ear 
» the somatic sensory system which uses the nerves in the skin and proprioceptive senses in our muscles and joints to sense gravity and other pressures on the body. 

The visual system is by far the most important of the three systems, providing some 80 per cent of the raw orientation information. 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.

The ATSB research report, An overview of spatial disorientation as a factor in aviation accidents and incidents (B2007/0063)identified that spatial disorientation is a very common problem and estimates that the chance of a pilot experiencing SD during their career is in the order of 90 to 100%. This report also detailed several international studies showing that SD accounts for some 6 to 32% of major accidents, and some 15 to 26% of fatal accidents. The report also identified that the true prevalence of SD events is almost certainly underestimated. 

The FAA Advisory Circular, Pilot’s spatial disorientation (FAA AC60-4A) 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 a loss of an applicable visual reference.

Safety analysis

This analysis will explore the factors that involved aircraft directional changes, resulting in the aircraft deviating from the published ILS approach. The deviations occurred whilst the pilot was manually flying, in instrument meteorological conditions. The consequence of this deviation led to extended flight below the minimum sector altitude with increased pilot workload prior to recovering to a safe altitude. 

The pilot reported being concerned with the aircraft’s location in relation to terrain, however, believed the aircraft was under control, attributing the unusual attitudes indicated on the artificial horizon to an instrument error rather than the aircrafts attitude.

The pilot reported that, at the time, false indications by the vacuum instruments were incorrectly indicating a turn to the right, which the pilot believed to be a consequence of erroneous instrument indications. The pilot recalled that their initial response to correct this was a turn to the left and believed that the vacuum instruments were still incorrectly indicating a level of bank even though the aircraft was level. 

ATSB analysis of the recorded data reviewed the aircraft pitch and bank angles, descent and climb profiles, and aircraft tracks and timings confirmed that the instrument indications (at this time) correlated with high levels of bank and the aircrafts track. 

Furthermore, although the pilot remembered observing a low vacuum pressure indication, a post‑incident system inspection indicated no identified problems with vacuum pumps or the check valves.

As such, it is almost certain that there was no instrument malfunction to the extent believed by the pilot. Rather, with no visual cues due to the IMC, the pilot likely became spatially disorientated and interpreted the real instrument indications as false as they mismatched the pilot’s sensed orientation.

As is common in spatial disorientation, the pilot likely followed their sense of direction rather than the (perceived faulty) instruments, leading to directional changes of up to 90° from the approach track as well as left and right angles of bank up to 65°, whilst continuing to descend. 

However, post‑occurrence maintenance fault‑finding of the artificial horizon did identify a gradual drift in pitch, (up to 7°) and roll, (up to 4°) over a period of 20 minutes. While this may have indicated a degree of unserviceability of the instrument, this was not consistent with the reported sudden and absolute failure reported by the pilot. 

Prior to the deviations on approach in IMC, the pilot descended below the minimum sector altitude. However, after deviating from the approach and no longer meeting approach tolerances, the pilot did not conduct a missed approach as quickly as practicable to achieve an altitude that would remove the aircraft’s exposure to the risks of collision with terrain. 

Subsequently, the pilot was below MSA and no longer offered the protection of being on the approach. This situation was further exacerbated by the aircraft being in unusual attitudes and tracking perpendicular to the approach path, without intent. If this high-risk situation had been identified either by ATC issuing a safety alert or the pilot issuing a PAN PAN call, (a PAN PAN call should have triggered a safety alert to climb), a climb could have been expedited and the risk of proximity to terrain removed sooner than was the case. 

Pilots should not hesitate to report an urgent condition when encountering situations that may not be immediately perilous but significantly increase risk. 

ATC recordings indicated that the pilot notified ATC that they had incorrect artificial horizon information and had lost glidepath guidance. The pilot used their callsign twice, (which can often precede a distress call), and other verbal cues were also identified by the controller to indicate the pilot was under a level of stress. Being below the MSA and off the ILS, with indications of stress, was an opportunity for the controller to issue a safety alert to the pilot to climb immediately.

In an urgent situation such as this where the safety of the aircraft was uncertain, the broadcast of a PAN PAN call would have been appropriate. Had a PAN PAN call been broadcast, ATC would have almost certainly issued a safety alert. This would have required the pilot to conduct an immediate climb, removing their subsequent risk exposure to collision with terrain.

Flight data and recordings indicated that the aircraft was below MSA, from leaving the approach profile to commencing a sustained climb to a safe altitude, for greater than one minute. Additionally, the time elapsed from notifying ATC (below MSA), to commencing a sustained climb to a safe altitude, was greater than 30 seconds. During this time, in IMC, the aircraft came within 810 ft of terrain.

Instrument flight can be considered one of the more challenging operational environments to which a pilot can be exposed. Single-pilot operations have the potential to increase pilot workload (ALPA 2019). 

Manually flying a single pilot approach in IMC increases the workload of any pilot. In this occurrence, the suspected loss of a primary instrument during an instrument flight rules approach, departing the ILS approach, experiencing unusual aircraft attitudes in IMC, and subsequently conducting a second approach all increased the workload of the pilot. Use of the autopilot system has the potential to significantly reduce the workload on pilots during this approach. This is achieved by the autopilot taking over routine tasks such as maintaining altitude, heading and airspeed. Thus, allowing the pilot to focus on other critical aspects of the flight. Whilst compliant with operator procedures at the time, use of the autopilot may have reduced the risk of spatial disorientation of the pilot on approach. The pilot reported that the autopilot could not be engaged post the occurrence, when positioning for the second approach.

Use of automation can afford the pilot spare mental capacity to recognise and address navigational deviations and tolerances. Thus, aiding the pilot to respond to the operational demands of the flight in a correct and timely manner.

Findings

From the evidence available, the following findings are made with respect to the flight below minimum sector altitude involving Cessna 310R, VH-ZMB, 14 km west-north-west of Alice Springs Airport, Northern Territory, on 1 July 2024.

Contributing factors

• At about 8 NM from Alice Springs whilst established on the ILS approach in instrument meteorological conditions, the pilot likely experienced spatial disorientation that led to directional changes of up to 90° from the approach track as well as left and right angles of bank up to 65°, whilst continuing to descend.
• The pilot did not maintain track or glidepath and deviated from instrument landing system below the minimum sector altitude. Once outside of the required tolerances, the pilot did not conduct a missed approach, which increased the risk of collision with terrain.
• Air traffic control did not issue a safety alert. This would have alerted the pilot that they were in unsafe proximity to terrain and needed to climb immediately.
• The pilot did not broadcast a PAN PAN call notifying air traffic control and other traffic of their situation, leading to the pilot remaining below minimum sector altitude for an extended period without air traffic control instruction to climb.

Other factors that increased risk

• Post occurrence fault‑finding of the artificial horizon, identified gradual and inconsistent deviations outside the manufacturer’s required tolerances.
• The pilot did not utilise the autopilot for the approach even though they were in a high workload environment. The appropriate use of autopilot can reduce workload and subsequent risk of spatial disorientation such as during an instrument approach.

Safety actions

Safety action by Avcharter

The operator has since introduced an automation policy for the use of autopilot in conditions applicable to instrument meteorological conditions (IMC) and in high workload single-pilot environments. 

Sources and submissions

Sources of information

The sources of information during the investigation included:

• the pilot of the flight
• the head of flight operations for the operator
• Civil Aviation Safety Authority
• the aircraft manufacturer
• the maintenance organisation for VH-ZMB
• independent avionics specialists
• Airservices Australia
• recorded data from the GPS unit on the aircraft.

References

Airservices Australia. (2024) Aeronautical Information Publication Australia. 

Airline Pilots Association International (ALPA) (2019) The dangers of single pilot operations Retrieved from 

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.

Bailey, R. E., Kramer, L. J., Kennedy, K. D., Stephens, C. L., & Etherington, T. J. (2017, September). An assessment of reduced crew and single pilot operations in commercial transport aircraft operations. InN2017 IEEE/AIAA 36th Digital Avionics Systems Conference (DASC) (pp. 1‑15). IEEE.

Barnum, F., & Bonner, R. (1971). Epidemiology of USAF spatial disorientation aircraft accidents, 1 Jan 1958-31 Dec 1968. Aerospace Med, 42, 896-898.

Braithwaite, M., Durnford, S., & Crowley, J. (1998b). Spatial disorientation in US Army rotary-wing operations. Aviation Space Environ Med, 69, 1031-1037.

Cessna 1976, Pilot’s Operating Handbook, Cessna 310 Skyhawk, model C310R 

Cheung, B., Money, K., Wright, H., & Bateman, W. (1995). Spatial disorientation implicated accidents in the Canadian forces, 1982-92. Aviation Space Environ Med, 66, 579-585.

Civil Aviation Safety Authority. (2020) Part 91 (General Operating and Flight Rules) Manual of Standards.

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

Gillingham, K., & Previc, F. (1996). Spatial orientation in flight. In R. DeHart (Ed.), Fundamentals of aerospace medicine (2nd ed., pp. 309-397.). Baltimore: Williams & Wilkins

Hixson, W., Niven, J., & Spezia, E. (1972). Major orientation error accidents in regular Army UH-1 aircraft during FY 1969. Accident factors report namrl1169. Pensacola, FL: Naval Aerospace Medical Research Laboratory.

Knapp, C., & Johnson, R. (1996). F-16 class a mishaps in the U.S. Air Force, 1975- 93. Aviat Space Environ Med, 67, 777-783

Ledegang, W. D., & Groen, E. L. (2018). Spatial disorientation influences on pilots’ visual scanning and flight performance. Aerospace medicine and human performance, 89(10), 873-882.

Lyons, T., Ercoline, W., O’Toole, K., & Grayson, K. (2006). Aircraft and related factors in crashes involving spatial disorientation: 15 years of U.S. Air Force data. Aviat Space Environ Med, 77, 720-723.

Moser, R., Jr. (1969). Spatial disorientation as a factor in accidents in an operational command. Aerospace Med, 40, 174-176.

Singh, B., & Navathe, P. (1994). Indian Air Force and world spatial disorientation accidents: A comparison. Aviation Space Environ Med, 65, 254-256.

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 peon 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 of the flight
• The operator
• Air traffic controller
• Airservices Australia
• Civil Aviation Safety Authority

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.

Executive summary

What happened

On 26 June 2024, a De Havilland Canada DHC-8-402 (Dash 8), was preparing to operate a QantasLink passenger flight from Horn Island to Cairns, Queensland. The flight crew identified that the take-off would be performance-limited due to the runway length at Horn Island and the high passenger and cargo weights. The crew determined that a flap setting of 15° and the bleed air system switched off was required for take-off. 

During pre-flight preparation, the first officer (FO) was the pilot flying and inadvertently selected a flap setting of 5° instead of the required flap setting of 15°. The crew completed the after start checks and after start checklists in accordance with the standard operating procedures, however neither the first officer, nor the captain as pilot monitoring, detected the incorrect flap setting. 

During the initial take-off run, the Dash 8 accelerated normally, however the crew noted that the aircraft’s rotation was slow and that the aircraft performance differed from their usual experience during take-off. The first officer’s application of continued back pressure to the controls during the take-off run resulted in the aircraft successfully becoming airborne slightly after the expected rotation speed. The first officer identified that the flaps were configured at a setting of 5° and immediately advised the captain. The captain instructed the first officer to continue to fly the aircraft. The first officer slightly lowered the nose of the aircraft to increase airspeed. The aircraft accelerated in response to this action and a positive rate of climb was maintained. The flight continued to Cairns without further incident.

What the ATSB found

During preparation for take-off from Horn Island, the FO inadvertently selected the flap lever to 5° instead of the required setting of 15°. This was likely due to habitual behaviour as the flap setting of 5° was the most common take-off flap setting for other sectors in the network and was the flap setting required on the 2 sectors flown prior to the incident. Standard pre‑flight checks and crosschecks were conducted, however the flight crew failed to identify the incorrect flap setting before take-off at Horn Island. This was likely due to automatic behaviour by the crew registering flap 5° to be the usual setting at take-off.

What has been done as a result

Following the occurrence, the operator implemented the following organisational and operational, changes:

• Review of standard operations procedures as necessary to reduce the likelihood of erroneous flap selection and misidentification.
• Review of relevant checklists to strengthen crosschecking in accordance with the computed take-off performance data.
• Training for crew focusing on standard operating procedures and compliance. 

Safety message

The preparation, taxi and take-off phases of flight involve high workload and demand heightened attention to ensure correct settings are selected as unintentional slips can easily occur without. To minimise the risk of slips going unnoticed, flight crews must carefully verify and methodically complete checks and checklists. Maintaining focus and staying mindful of potential deviations to usual settings is essential during periods of high workload for safe operations.

The investigation

The occurrence

On 26 June 2024, a De Havilland Canada DHC-8 402 (Dash 8) registered VH-QOI was being prepared to operate a QantasLink passenger flight from Horn Island, Queensland, following an arrival from Cairns. The aircraft arrived at Horn Island airport about 1015 local time and was scheduled to return to Cairns at 1055 with the same flight crew. The crew consisted of the first officer (FO), assigned as the pilot flying, the captain in the role of pilot monitoring and 2 cabin crew.

The flight crew followed standard pre-departure procedures in preparation for the flight. The FO loaded the flight plan into the flight management system (FMS)[1] and the captain confirmed the FMS was programmed in accordance with the flight plan. During the confirmation, the captain had to adjust performance data calculations that had previously been entered. This was prompted by a revised forecast for adverse weather conditions for arrival at Cairns and a full passenger load. The captain reported repeatedly checking the automatic weather information service due to changes in the prevailing wind and efforts to avoid offloading any passengers due to weight restrictions. The captain stated that they made several calls to load control and frequently checked the automated weather information service to calculate the headwind component for the final performance data which allowed for the uplift of all the passengers and their baggage.

The main runway length for runway 08 at Horn Island is considered performance limiting for Dash 8 operations at a length of 1,389 m, with higher terrain near the departure end presenting a potential obstacle. The captain was cognisant of the aircraft performance limitations as the aircraft was carrying a full load. The calculation of thrust requirements, and the corresponding FMS entries, were made prior to the engine start. The FO reported that each flight crew programmed their own Aerodata apps[2] on their iPads to calculate the airspeeds required for take-off, based on the weather information and the aircraft weight. The final weight of the aircraft was 27,800 kg and was close to the maximum regulated take‑off weight (RTOW). The Aerodata results produced a requirement to turn off the bleed air system to provide the increased thrust necessary for take-off with a flap[3] setting of 15°. The FO recorded this information on the take-off and landing distance card (TOLD)[4] card.

After the passengers were boarded, the flight crew conducted the before start procedure and then commenced taxiing to runway 08. The crew then conducted the after start checks and checklist, during which time the FO inadvertently selected flap 5° instead of 15°. The checklist is completed as a ‘challenge and response’ format. The FO calls the check which requires a verbal confirmation from the captain. One of the items on the list specifically called for is the flap setting. The FO called for ‘flaps’ and recalled that they checked that flap 5 was indicating for the flap lever and the digital indications. However, the captain did not recall the response given for this challenge during interview.

For the take-off run, the crew reported no issues with acceleration as the aircraft increased speed along the runway until the captain called for ‘V₁’[5] and ‘rotate’. The FO described the rotation as slow but reported rationalising the heavy weight of the aircraft as the reason and continued to apply more pressure to the control column until the aircraft rotated about 5 kt after the planned rotation speed.

The flight crew noticed that aircraft performance was not what they would normally expect at the start of the climb. The captain checked the speeds and trim which were identified as being set correctly. At about the same time, the FO checked the flap setting and noticed that the flaps were configured to 5° and alerted the captain immediately. 

By this time, the main landing gear was up and the captain instructed the FO to continue to fly the aircraft. The FO momentarily lowered the nose of the aircraft by a small amount to assist with increasing the airspeed. The aircraft accelerated and continued to Cairns without further incident.

Context

Aircraft information

VH-QOI was manufactured in Canada in 2008 and was powered by 2 Pratt & Whitney Canada PW150A (turboprop) engines. 

Flight crew information

The captain held an air transport pilot licence (aeroplane) and had been flying for about 30 years with a total aeronautical time of about 8,450 hours, of which 5,800 hours were on the Dash 8. 

The FO had been flying for 12 years and held a commercial pilot licence (aeroplane) with about 3,870 hours, of which 346 were on the Dash 8. 

Both pilots had valid class one medical certificates.

Meteorological conditions

The meteorological aerodrome report provided the weather observations for Horn Island as warm and humid, with moderate south-east winds to 13 kt, good visibility and broken to overcast clouds. The valid terminal forecast for Cairns airport was issued at 0902 local time and identified a temporary forecast for showers of moderate rain and scattered cloud at 3,000 ft with periods of 30–60 minutes where showers of rain were forecast, potentially reducing visibility to 7,000 m. 

Airport information

Horn Island airport is located at the northern end of Cape York Peninsula and is one of the primary airports for local transportation. The main runway orientation is 08/26 and is 1,389 m in length and has an elevation of approximately 13 m above mean sea level. Some higher terrain and trees are located at the of end of runway 08 and are a potential obstacle.

Recorded information

Flight data from the aircraft’s quick access recorder (QAR) was provided to the ATSB and contained about 70 seconds of data, recorded at one-second resolution, commencing at the beginning of the take-off on runway 08 at Horn Island (Figure 1). The data indicated that the Dash 8 had the flaps set to 5° prior to taxi and were retracted as the aircraft was flying through 1,406 ft above mean sea level.

Figure 1: QAR derived flight data depicting aircraft position and flap setting 

Source: QAR data and Google Earth, annotated by the ATSB (all heights indicated as above mean sea level). Note: Dash 8 figures not to scale 

Take-off distance

In the event of an engine failure during take-off, pilots must actively monitor several key speed parameters in order to ensure the aircraft can either continue or reject the take-off and stop on the runway if necessary. These are collectively referred to as ‘V speeds’. 

• Should an engine failure occur before V₁ the aircraft should be able to stop within the distance of the remaining runway.
• V₂ is the speed that ensures that the aircraft can continue climbing safely with one inoperative engine in an emergency.
• V_r is the speed at which the rotation of the aircraft is initiated to take-off attitude. This speed cannot be less than V₁

The QAR data determined that the mismatch in actual versus intended take-off flap configuration meant that the calculated take-off reference speeds and aircraft performance differed. The Dash 8 aircraft scheduled speeds for flap settings are stipulated in the operator’s aircraft performance manual (APM) (Figure 2).

Flap settings for take‑off

Pilots use different flap settings to increase or decrease performance during take-off and landings. The flap setting deployed for the take-off phase contributes to the performance of the aircraft on the take-off roll and the initial airborne segment. The Dash 8 flap setting requirements for take-off are stated in the APM and are based on the performance data, such as the prevailing weather conditions and aircraft take-off weight. 

The crew advised that the performance calculations for the Dash 8 at Horn Island on the day required a flap setting of 15° for take-off to account for the lift required due to the limiting factor of the short runway. This setting was unusual for the crew as they stated that the last two sectors flown, the flap setting of 5° had been used and that, in their experience, flaps of 5° was the normal setting for take-off at most of the airports in the airlines network.

Figure 2: APM specified take-off performance for flap settings 5 and 15 degrees

Source: APM provided by operator, annotated by the ATSB

Bleed air system 

The bleed air system plays a role in multiple aircraft functions and operates through a network of ducts, valves, and regulators using high pressure air bled from the compressor section of the engine and auxiliary power unit. Turning off the bleed air system provides extra engine performance and maximises take-off power, which is beneficial for optimal performance on short runways with a high payload. The requirement to turn off the bleed air was not a routine occurrence, the FO reported they were very conscious of remembering ‘bleeds off’ for the take-off and so had written themselves a reminder note.

Procedures for pre-departure checks

At each critical phase of aircraft operation, pilots refer to checklists to guide them through specific items to configure the aircraft for the next planned phase of the flight. These enhance safety by providing an opportunity to confirm that the safety‑critical aspects of the aircraft configuration are correctly set. Consequently, any omissions or mistakes are more likely to be identified and rectified by the flight crew. Checklist deviations occur relatively frequently, compared to other forms of procedural deviation. A common form of checklist deviation includes responding without checking (Dismukes and others, 2010).

The company flight crew operating manual[6] (FCOM) states that many checklists are preceded by ‘checks’ which are memory items completed prior to actioning the checklist. For example, in the case of the after start checks, the captain will ask for the after start checks and the crew conduct their check flows, then the FO reads the after start checklist and the captain responds to the checklist. The standard operating procedures (SOPs) are outlined in the FCOM which documents 4 procedures that could potentially identify an incorrect flap setting:

• departure briefing
• take-off data crosscheck
• after start checks
• after start checklist.

Departure briefing

Crew departure briefings are intended to bring awareness of operational information and conditions that are not covered in the SOPs. The flap setting of 15° is an uncommon flap setting on the network and although there was no mandatory requirement to brief the take‑off flap setting, it did provide an opportunity to acknowledge the flap setting of 15° as an operational consideration or as a threat. Neither the FO nor the captain recalled if the flap setting was included as a threat in the content of the briefing. Calling out the flap setting as a threat would have provided a prompt to the crew which may have made the incorrect flap setting easier to detect. The crew stated that as they had previously flown out of Horn Island, the take-off flap setting of 15° was not considered abnormal unless a pilot had not previously flown from the aerodrome. The crew recalled that the ‘bleeds off’ procedure was briefed.

Take-off data crosscheck

The take-off data crosscheck required the captain to verify the take-off weight, runway distance intersection, and the flap setting from their Aerodata app while the FO was required to confirm the aircraft weight using the load sheet. The captain then ensured the correct take-off speeds were displayed on their primary flight display (PFD), which the FO cross referenced with their own PFD. The captain and the FO reported completing this step. Following this, the captain read the take-off speeds from the Aerodata and their PFD and the FO was required to verbally reiterate the speeds read from their own PFD. 

After start checks 

Among other items, the after start checks required the FO to set the flaps for take-off and that to complete this task, the Aerodata or TOLD card should be referenced to set and confirm the flap setting. The FO reported setting the flaps to 5° but could not remember referring to the Aerodata or the TOLD card to do this and recalled they may have had the load sheet or something else displayed on their iPad. 

After start checklists

Directly following the after start checks, the after start checklist was a ‘challenge and response’ format which prompted for verification of the take-off data and flap settings among several other items. The captain and the FO reported they had no recollection of the response provided for the flap-setting challenge. On completion of this checklist, the captain and the FO had not detected that the flap setting was 5° which contrasted with the Aerodata display that showed a required flap setting of 15°. 

Operator’s report 

The findings from the operator’s internal safety investigation report stated that the flap setting of 5° increased the risk for tailstrike and reduced the margin for obstacle clearance. The aircraft would have been 2,112 kg overweight for take-off configured with a flap setting of 5° (Figure 3). The report determined that the required take-off distance with flap setting of 5° was 1,680 m which was greater than the available runway distance. However, the report concluded that as the aircraft was configured with flap 15° speeds set for take-off, the decision to reject the take-off would have been made with reference to the bugged[7] V₁ speed of 116 kt and the accelerated stop distance required (ASDR)[8] would have been adequate. Therefore, the operator calculated that in the event of a rejected take-off, at or below the V₁ that was set, the aircraft would have had sufficient deceleration to come to a stop before the runway threshold.

Figure 3: Aerodata performance calculations for VH-QOI for a flap settings of 15° and 5°

Source: Reproduction provided by operator, annotated by the ATSB

Fatigue

Fatigue is defined as a decrease in performance as a function of time on task (Salas and others, 2010). Fatigue can have a range of adverse influences on human performance and can lead to slips or lapses associated with attention, problem-solving, memory, vigilance and decision‑making. ATSB has identified that people experiencing fatigue are not able to accurately evaluate their own fatigue level or their ability to perform tasks, rather they tend to overestimate their abilities (ATSB, 2019). As such, fatigue is considered an important safety concern and an ATSB SafetyWatch priority.

The flight crew’s previous 72-hour roster allowed for regulated sleep opportunities. The flight crew commenced duty from Townsville at 0600 where they had stayed overnight. Their duty roster included three sectors for the day:

• Townville–Cairns
• Cairns–Horn Island
• Horn Island–Cairns. 

Both flight crew members had presented for duty that morning stating they were fit to fly, although both reported to the ATSB that the environmental conditions at their accommodation in Townsville were not conducive for sleep. The captain noted that their sleep was broken due to noise and air‑conditioning issues and self-reported being a little tired at the commencement of duty as their previous night’s sleep prior to the incident was poor. The FO described their alertness at the beginning of duty as very lively and their sleep quality as good but also reported sleep disruption due to unreliable air-conditioning in the room. The ATSB concluded that it was very unlikely that either crew were experiencing fatigue to a level that could influence performance.

Safety analysis

Introduction

During a scheduled sector from Horn Island to Cairns, a reduced aircraft performance during take‑off and the initial climb did not align with the flight crew’s expectations. The first officer (FO), as the pilot flying, discovered that the flap setting was incorrectly configured at 5° instead of 15° and advised the captain. The aircraft was established in a positive climb and continued the flight without further incident. 

This analysis will discuss factors that contributed to the incorrect aircraft configuration and lack of detection during the pre-flight crosschecks. 

Crew actions

The inadvertent selection of the flaps to 5° instead of 15° by the FO was likely influenced by habitual behaviour driven by prior experience. A flap setting of 5° had been used for the previous two sectors flown by the flight crew and was, in their experience, the most common setting for take-off at most of the airports in the operator’s network. It is likely therefore, that the FO developed an unconscious habit for a flap setting of 5°, and the setting of 5° was a result of a slip. A slip is a form of human error defined to be the performance of an action that was not intended (Reason, 1990). Slips often occur during the largely automatic performance of routine tasks, usually in familiar surroundings, and characteristically involve an incorrect implementation of an intention (Mylopoulos, 2022). If other factors are also present, such as distraction, then the chance of such errors occurring increases. This type of slip cannot be eliminated by training alone, however, improvements in system design can reduce the likelihood of occurrence and provide a more error tolerant environment, which is why checks of the flap setting are built into the pre-flight processes (the after start checklist). 

The flight crew reported adhering to the procedures for the pre-departure checks which provided 4 occasions to potentially identify or rectify the incorrect flap setting. Flight crews are particularly vulnerable to checklist errors during the pre-flight sequence in the time‑compressed phase of pre‑departure (Loukopoulos, and others, 2001). Under normal pre‑flight procedures workload is usually at a high level and for this flight the extra calculations for weight, weather and performance limitations elevated the crew workload. When workload is high, automaticity can be amplified as the mind seeks to conserve energy by relying on well learned routines (Bermúdez and others, 2021). This can be detrimental when a situation requires mindful engagement. The flight crew likely conducted the after start checklist with a high degree of automaticity, rather than consciously verifying the flap setting against the Aerodata or TOLD card. In this case there was a breakdown of this normal layer of defence, which emphasises how safety measures can be degraded if crews are not consciously focused on the task. It was possible that the crew had diminished sensitivity for error detection due to completing the checklist without carefully ensuring the confirmation of each of the steps.  

As a result, the crew performed the take‑off with the flaps inadvertently at 5° instead of 15°. The incorrect configuration reduced the intended performance envelope of the aircraft. The FO’s continued back pressure to the controls during the take-off run resulted in the aircraft successfully becoming airborne slightly after the expected rotation speed. The FO’s actions to reduce pitch during the take‑off increased the airspeed and the aircraft maintained a positive climb.

Findings

From the evidence available, the following findings are made with respect to the incorrect configuration involving Bombardier DHC-8, VH-QOI, at Horn Island Airport, Queensland on 26 June 2024.

Contributing factors

• The FO mistakenly set flaps to 5° for take-off at Horn Island when pre-flight planning identified flaps 15° should have been set.
• Standard pre-flight checks and crosschecks were conducted, however the flight crew failed to identify the incorrect flap setting before take-off at Horn Island. This was likely due to automatic behaviour by the crew registering flap 5° to be the usual setting at take-off.
• The incorrect configuration of the aircraft reduced the climb performance and potential obstacle clearance on departure from Horn Island.

Safety actions

Safety action by QantasLink 

In response to this occurrence the ATSB was advised by QantasLink that the following actions had been undertaken:

• Review of standard operating procedures to strengthen procedures as necessary to reduce the likelihood of erroneous flap selection and misidentification.
• Review of relevant checklists to strengthen crosschecking in accordance with the computed take-off performance data.
• Training for crew focusing on standard operating procedures and compliance. 

Sources and submissions

Sources of information

The sources of information during the investigation included:

• interviews with the captain and first officer
• QantasLink internal safety report
• recorded data from the aircraft QAR. 

References

ATSB (2019) Fatigue experiences and culture in Australian commercial air transport 

Bermúdez, J. P., & Felletti, F. (2021). Introduction: Habitual Action, Automaticity, and Control. Topoi, 40(3), 587-595.

Dismukes, R., & Berman, B. (2010). Checklists and monitoring in the cockpit: Why crucial defenses sometimes fail (No. ARC-E-DAA-TN1902).

Loukopoulos, L. D., Dismukes, R. K., & Barshi, I. (2001). Cockpit interruptions and distractions: A line observation study. In Proceedings of the 11th international symposium on aviation psychology (p.1-6). Columbus: Ohio State University Press.

Mylopoulos, M. (2022). Oops! I did it again: the psychology of everyday action slips. Topics in Cognitive Science, 14(2), 282-294.

Reason, J. (1990). Human error. Cambridge University Press.

Salas, E., & Maurino, D. (Eds.). (2010). Human factors in aviation. Academic Press.

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:

• flight crew consisting of captain and first officer
• QantasLink
• CASA

Submissions were received from:

• QantasLink
• flight crew – captain and first officer
• CASA

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 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.