An ATSB investigation into the collision with terrain involving a Bell 206 helicopter shows how rapidly emergency situations can develop.
The accident occurred 75 km north-east of Emerald Airport, Queensland, when the helicopter was conducting power line inspections. The pilot had positioned the helicopter in a hover 30 feet above the ground and 40 metres from a transmission tower so that it could be photographed by a crew member.
After a short period of hovering, the pilot heard a ‘very loud bang’ through the airframe, which was also felt through the controls. The helicopter began to shake violently and bounce vertically. The pilot also reported seeing tiny pieces of debris falling in front of the helicopter.
In response, the pilot immediately lowered the collective intending to land in a clear area below, however the helicopter did not respond to collective or cyclic control inputs. Instead, the helicopter began to pitch upward and drift backwards before yawing to the right. The yaw could not be controlled with the tail rotor pedals, so the pilot moved the throttle to the idle position.
Despite attempts to remain level during the landing, the left skid contacted the ground and the helicopter rolled over. The pilot and crewmember were uninjured.
While recognising that on this occasion the pilot had limited control authority, the safety message from this investigation is regular practice and briefings of emergency actions will increase the likelihood of a correct response during an emergency situation.
An examination of the helicopter identified that the mast, both pitch links, and the swashplate were fractured. The fractured components and trunnion bearings (connecting the pitch links to the rotor blades) were sent to Bell Helicopter’s Engineering Laboratories for detailed examination. Bell Helicopter concluded that all of the fractures, including that resulting in the mast collar separation, were a result of overstress.
There were no findings made during the inspection that identified the probable factors contributing to this occurrence.
This investigation highlights how rapidly an emergency situation can develop. While recognising that on this occasion the pilot had limited control authority, the safety message from this investigation is regular practice and briefings of emergency actions will increase the likelihood of a correct response during an emergency situation.
An Australian Transport Safety Bureau (ATSB) investigation into a dual hydraulic failure involving a Boeing 757 aircraft highlights that non-mandatory service bulletins can still have an impact on an aircraft’s reliability.
Soon after departing Auckland International Airport on 5 February 2018, while passing through 6,000 ft, the aircraft’s left hydraulic system quantity warning message displayed. The crew actioned the Quick Reference Handbook (QRH) checklist and obtained clearance from air traffic control (ATC) to level out in a holding pattern north-west of the airport
Shortly after commencing the holding pattern, the crew received further warning messages regarding both the left and right hydraulic systems. Following the QRH checklists, the crew shut down both system, declared a PAN call to ATC requesting an immediate return to Auckland. The aircraft landed without incident.
The ATSB investigation identified that the dual hydraulic failure was the result of sequential failures in the hydraulic system. The initial failure was a ruptured left main landing gear flex hose, which resulted in the loss of the left hydraulic system pressure. Subsequently, the power transfer unit (PTU) pressure switch did not operate as intended, following depletion of the hydraulic fluid from the left system. This resulted in overheating and failure of the right hydraulic system.
Although some service bulletins are not deemed to be safety-critical, they can still have an impact on aircraft reliability.
The pressure switch was the subject of a 2010 non-mandatory service bulletin that, if implemented, would likely have prevented the failure of the right system.
The safety message from this investigation is that although some service bulletins are not deemed to be safety-critical, they can still have an impact on aircraft reliability. While operators are required to implement all airworthiness directives, the ATSB recommends that operators familiarise themselves with non-mandatory service bulletins and consider the potential impacts of implementation.
Following the occurrence, the operator conducted preventative maintenance on the aircraft, replacing all of the main landing gear flex hoses as well as updating the PTU pressure switch to confirm to the published service bulletin. In addition, the operator revised the aircraft maintenance plan to introduce improved inspections and a time-limited life on the flex hoses.
Boeing recommends that operators perform periodic inspection of hose assemblies, paying particular attention to the high-risk areas of the landing gear area, the engine/pylon area and the tail/empennage area.
Recent practice of emergency procedures helped a pilot to conduct a successful forced landing of a scenic flight in remote and difficult terrain.
On 14 May 2018, a Gippsland Aeronautics GA-8 Airvan departed Bellburn Airstrip, Western Australia on a scenic charter flight with the pilot and seven passengers on board. About 12 minutes after departure, as the pilot commenced a planned climb, he felt the aircraft’s performance become a bit ‘sluggish’, with lower climb and fuel flow rates than expected.
Various checks and troubleshooting did not yield any improvement in performance, and engine power slowly started to decrease. The pilot determined that a forced landing was required, and advised the passengers, briefing them on the process for evacuating the aircraft after landing.
During the descent, at approximately 150 ft above ground level, the pilot secured the engine and turned off the aircraft’s electrical system. During the landing roll, the Airvan’s wings hit some small trees and the aircraft came to rest in a ditch, tearing off the nose landing gear. The pilot and three passengers sustained only minor injuries.
The pilot’s handling of the forced landing contributed positively to the survivability of this accident in difficult terrain.
The ATSB investigation found that the air intake pipe to engine cylinder number six had probably detached in flight, leading to the loss of engine performance. There was insufficient evidence to determine why the intake pipe detached from the engine.
The pilot advised the investigation that recent practice forced landings in the Airvan had helped him to feel more comfortable with the emergency. In particular, the recent practice gave him a good appreciation for the aircraft’s glide ratio, which helped when selecting a suitable landing site.
The ATSB’s safety message from this investigation is the value of frequent emergency procedures training. The pilot’s handling of the forced landing contributed positively to the survivability of this accident.
Although flight reviews are required every two years as a minimum, the ATSB reminds pilots and operators of the benefits of more frequent practice of emergency procedures.
On 23 October 2017, a Cessna 210 light aircraft with two pilots on board, one operating as pilot-in-command and the other as pilot-in-command-under-supervision, departed Darwin Airport for Elcho Island, 530 km to the east of Darwin.
Despite making track diversions to avoid severe thunderstorms, the aircraft entered an area of strong convective activity with rapidly developing rain cells. This resulted in the aircraft experiencing severe turbulence and possibly reduced visibility.
The aircraft entered an uncontrolled descent, with witnesses reporting seeing the aircraft descending rapidly in a relatively flat attitude with a section of each wing missing. A combination of airspeed, turbulence and control inputs probably led to excessive loading on the aircraft’s wings, which separated from the fuselage in-flight.
The aircraft subsequently collided with terrain, fatally injuring both pilots.
In many cases, deviations of 10 nautical miles may not be enough for an aircraft to remain safely clear of the turbulent and powerful forces associated with storms.
“There are several safety messages from this investigation including the recognition and avoidance of tropical weather conditions that present significant hazards to flight,” ATSB Executive Director Transport Safety Nat Nagy said.
“Particularly challenging is identifying an appropriate distance to keep from thunderstorms, which comes through experience in operating in the tropics. In many cases, deviations of 10 nautical miles may not be enough for an aircraft to remain safely clear of the turbulent and powerful forces associated with storms.”
The ATSB’s investigation found that the operator’s risk mitigation of pairing a supervisory pilot with a trainee did not adequately address the weather-related risks because neither pilot had experience flying in the build-up to the wet season.
“Smaller operators employing pilots with limited exposure to local conditions can better manage related risks by pairing new pilots with ones who are experienced in flying in the Top End,” Mr Nagy said.
“The ATSB also encourages pilots to use all available resources to avoid adverse weather, including forecasts and requesting air traffic control assistance. Awareness of the weather avoidance actions of other pilots in the area can also be useful.”
Further, there is considerable value in ongoing education and guidance for all pilots in recognising and responding to deteriorating weather conditions during flight.
The standard industry practice of re-using self-locking nuts on Robinson helicopters may inadvertently result in the omission to replace MS21042L or NAS1291-series nuts with D210-series corrosion resistant (CRES) nuts on critical fasteners.
Safety advisory notice
The Australian Transport Safety Bureau advises all maintenance personnel for Robinson helicopters to ensure that before re-using a self-locking nut, that the correct part number is fitted, and that the D210-series corrosion-resistant nuts are used for reassembly of critical fasteners in accordance with the Robinson Helicopter Company instructions for continued airworthiness.
What happened
On 2 August 2017, a Robinson R22 Beta helicopter impacted terrain about 7 km north-north-west of Cloncurry Airport, Queensland. The pilot, who was the only person on board, was fatally injured and the helicopter was destroyed. The accident flight was the first commercial flight post the helicopter’s second 2,200-hour overhaul.
Finding of increased risk
During the review of on-site accident photographs to verify the flight control joints, the ATSB noted an anomaly with the helicopter’s bellcrank (part number A958-1) in the cyclic control system. The fastener, which attached the horizontal push-pull tube (part number A121-1) to the bellcrank, was missing. The bellcrank’s remaining fasteners were all attached. The bellcrank and missing fastener bolt were recovered, but the reason for the separation of the nut from the bolt has not yet been determined.
The missing fastener was part of the longitudinal cyclic pitch control system, which controls the fore-aft tilt of the main rotor disc (Figure 1).
Figure 1: Robinson R22 helicopter cyclic control system
Robinson R22 helicopter cyclic control system schematic: The horizontal push-pull tube, bellcrank, vertical push-pull tubes and jackshaft are highlighted in yellow. The location of the missing fastener is highlighted in red. Source: Robinson Helicopter Company, modified by ATSB
The examination of the bellcrank found that the bolt-holes for the missing fastener exhibited heat damage different to that of the bolt-holes for the remaining fasteners (Figure 2). In addition, the deformation of the bellcrank prevented the installation of a bolt where the fastener was missing, but did not prevent the removal of the remaining fasteners. This indicated that the bolt was not in-situ at the time of the post-impact fire.
Figure 2: Comparison of the bellcrank bolt-holes
Left: bolt-hole for the missing fastener. Right: bolt-hole for a removed fastener. Source: ATSB
Re-use of self-locking nuts
Cracking from hydrogen embrittlement of the type of nuts fitted to Robinson helicopters has been previously identified.[1] In October 2014, the Robinson Helicopter Company (RHC) published service letters for the R22 (SL‑64), R44 (SL-50) and R66 (SL-09) helicopters on the subject of D210 Corrosion-Resistant (CRES) Nuts. The service letters stated that, whenever maintenance that involves the disassembly and reassembly of a critical fastener is performed, the MS21042L or NAS1291 nut should be replaced with a D210-series nut.[2] The R22 maintenance manual was amended in October 2014 to incorporate what was stated in SL-64. For specific instances of cracked nuts, RHC have published service bulletins for their replacement within a compliance period.[3]
The R22-series maintenance manual included the following information under section 1.300 Fastener Torque Requirements:
D. Critical Fastener: A critical fastener is one which, if removed or lost, would jeopardize safe operation of the helicopter. This includes joints in the primary control system, and non-fail-safe structural joints in the airframe, landing gear, and drive system.
CAUTION: D210-series nuts, which supersede MS21042L-series and NAS1291-series nuts, are required on critical fasteners.
In the course of interviewing personnel employed by the maintenance organisation, the ATSB noted a low level of awareness of the need to replace MS21042L/NAS1291-series nuts with the D210-series nuts when critical fasteners were reassembled. In accordance with the R22 maintenance manual, critical fasteners include a self-locking nut in their assembly. It is a standard practice within sectors of the aviation industry to re-use self-locking nuts provided the nut cannot be turned onto the bolt thread by hand and the published torque value for the fastener is achieved.
During the course of the investigation the ATSB spoke with another maintenance organisation, who reported they employ the same practice of re-using self-locking nuts, and the helicopter manufacturer confirmed this was an acceptable practice. The United States National Transportation Safety Board reported on this practice as accepted by the manufacturers of light helicopters in their aircraft accident report AAR-13/01.[4] They noted that guidance on the re-use of self-locking nuts was provided by Eurocopter (now Airbus Helicopters), Sikorsky, Bell and the United States Federal Aviation Administration.
In December 2018, the ATSB received the accident helicopter’s jackshaft, which had the fasteners attached. The jackshaft was one of a number of parts within the flight control system that was disassembled and sent for non-destructive inspection during the 2,200-hour overhaul. The bellcrank was not subject to non-destructive inspection and therefore not required to be disassembled. In late January 2019, the ATSB completed semi-quantitative chemical analysis of the nuts fitted to the jackshaft and found they were consistent with a carbon/alloy steel, and therefore not consistent with D210-series stainless steel corrosion-resistant nuts. The nuts fitted to the jackshaft had similar markings to the nuts fitted to the bellcrank, which were consistent with MS21042L/NAS1291-series nuts.
At the time of the reassembly of the accident helicopter, the current R22 Illustrated Parts Catalog detailed the part number D210-4 for the nuts fitted to the jackshaft, and RHC confirmed there was no alternate part number to the D210-series nuts.
In consideration of the evidence, the ATSB concluded that the industry practice of re-use of self‑locking nuts on Robinson helicopters may result in the omission to install D210-series nuts when critical fasteners are reassembled.
The ATSB advises that this finding of increased risk applies to all approved maintenance organisations for Robinson helicopters and at present has not been identified as a contributing factor to this accident.
Qantas has incorporated more complex stall warning recovery events in recurrent lesson plans for its Boeing 747 flight crews.
The improvements follow an ATSB investigation into an in-flight upset involving a Boeing 747-438 110 km south-east of Hong Kong International Airport on 7 April 2017.
While descending toward Hong Kong, air traffic control instructed the flight crew to hold at a waypoint. When entering the holding pattern, the aircraft’s aerodynamic stall warning stick shaker activated a number of times and the aircraft experienced multiple oscillations of pitch angle and vertical acceleration.
During the upset, some passengers and cabin crewmembers struck the cabin ceiling and furnishings, sustaining minor injuries.
The ATSB found that while planning for the descent, the flight crew overwrote the flight management computer-provided hold speed. After receiving a higher than expected hold level, the flight crew did not identify the need to re-evaluate the hold speed. This was likely because they were not aware of a need to do so, nor were they aware that there was a higher hold speed requirement above flight level 200.
Prior to entering the hold, the speed reduced below both the selected and minimum manoeuvring speeds. The crew did not identify the low speed as their focus was on other operational matters. The ATSB also found that due to a desire to remain within the holding pattern, and a concern regarding the pitch-up moment of a large engine power increase, the pilot flying attempted to arrest the rate of descent prior to completing the approach to stall actions.
In addition, the pilot monitoring did not identify and call out the incomplete actions. This led to further stall warning stick shaker activations and pilot induced oscillations, which resulted in minor injuries to four cabin crewmembers and two passengers.
The ATSB found the flight crew had limited training and guidance for stall warning recovery techniques at high altitude or with engine power above idle. Inconsistencies were also found in flight crew training of the awareness of the need to re-evaluate holding speed when there are changes in altitude, especially above flight level 200.
“Balancing competing attention or decision demands can interrupt trained flight crew responses leading to procedures not being completed in full, particularly so if flight crews are not receiving comprehensive and regular training in the application of these skills,” ATSB Executive Director Transport Safety Mr Nat Nagy said.
“Comprehensive theory and practical training can ensure that flight crews have a complete understanding of aircraft systems and maintain effective manual handling skills. This training should provide the knowledge to correctly configure the aircraft’s automatic flight systems and manual handling skills to respond adequately to in-flight upsets.”
Subsequent to the incident, Qantas provided retraining for all Boeing 747 flight crews in stall warning recovery scenarios and amended ground school lesson plans to ensure flight crews were adequately prepared to recover from stall warning activations at high altitudes or with engine power above idle.
Qantas also amended flight crew training manuals relating to hold speed selection and updated ground school lesson plans and information to ensure standardised training and holding pattern training.
In addition, Qantas proactively applied these measures across its Boeing 737 and 787 fleets.
A reminder that a failure to follow procedure, such as functional checks, can result in unintended consequences.
A lock-out pin not removed after maintenance meant the crew of an Airbus A320 landed at Sydney with reverse thrust inadvertently deactivated. An ATSB investigation found a maintenance team had deviated from procedures, probably due to operational pressure.
Prior to the incident, the aircraft had undergone a three-day maintenance check at a facility in Brisbane. Two days in, the engineers identified that the horizontal stabiliser actuator required replacement, which added a half day of work to the schedule. To recover the lost time, the team was organised to begin work at 0400 the following day. Then the aircraft departure time was brought forward, and the engineers were instructed to complete the maintenance by the end of first shift. Many of the engineers worked through their lunch breaks to ensure they could complete the maintenance on time.
While working under the compressed schedule, engineers deviated from the written procedures, and the incorrect lockout pin was installed and then not removed later. As a result, the aircraft was returned to service with the thrust reverser system inadvertently deactivated.
There was no damage to the aircraft, or injuries as result of the incident.
The safety message from this investigation is that functional checks are the last line of defence in maintenance work. Failure to follow procedures, such as functional checks, can result in unintended consequences. Additionally, it is imperative that aircraft maintenance engineers feel empowered to stop a process when they observe procedural violations or foresee that an error is likely to occur.
A livestock carrier's contact with a wharf shows the importance of sharing knowledge. In this case, the harbour master and the pilot were unaware of limitations with fender heights.
The ATSB’s investigation into the contact with wharf by livestock carrier Angus Express at the port of Broome, Western Australia, highlights the challenging situations that marine pilots may encounter, and why it is important that local knowledge is captured and shared.
On 20 April 2018, the Angus Express had just completed a voyage from Singapore, and was expected to load cattle. As the vessel came into berth, the weight of the ship against the forward Yokohama fender forced it to pass under the fender posts, which resulted in the ship’s bow moving towards the wharf. Shortly after, an overhanging scupper protrusion made contact with the fender post.
The pilot thought that the ship had pivoted on the Yokohama fender and instructed the tug to push forward again, resulting in the forward Yokohama fender once again passing underneath the fender posts. The ship’s bow again moved towards the wharf, and the scupper protrusion again made contact with the fender post. The harbour master subsequently attended and found minor damage to the ship’s superstructure.
Marine pilots, in particular, may encounter operational parameters outside normal limits.
The ATSB investigation found that those fender posts were shorter than others in the port, and at certain tide heights, could be forced down by a ship. A former pilot at the port advised that they had been aware of this, but that knowledge had never been documented or shared.
As a result of this occurrence, the Kimberley Ports Authority has implemented guidelines for berthing and being alongside when using Yokohama-style fenders at times of limiting low water levels. Also, harbour masters and pilots are to conduct risk assessments for circumstances such as tug shortages, vessel manoeuvring issues and tidal levels.
The safety message from the accident is the importance of capturing and sharing lessons learned from experience. Marine pilots, in particular, may encounter operational parameters outside normal limits. Capturing the lessons through proper reporting process and sharing information through training and awareness can help to reduce risk and avoid future occurrences.
The challenges surrounding marine pilotage is one of the ATSB’s SafetyWatch priorities. Visit our website for important safety messages and advice.
The forced landing of a Cessna 172 into scrubland south-west of Kalgoorlie Airport, Western Australia, following fuel exhaustion again highlights the need for continued pilot education on the risks and controls associated with fuel management.
The Cessna 172M, registration VH-TUX, had been tasked to conduct fire-spotting operations in the Lake Johnstone area near Kalgoorlie on 5 January 2018. During the flight, the engine speed began to steadily reduce toward idle. The pilot switched between fuel tanks and adjusted the mixture and throttle settings. This resulted in the engine speed momentarily increasing before returning to idle.
Preparing for a forced landing on a dirt road, the pilot realised the aircraft’s glide range would be insufficient and identified an area of less dense scrub and landed the aircraft with minimal damage.
The pilot’s flying instructor experience instructing student pilots on the procedures for an engine restart and practice forced landings likely aided in managing workload during the emergency and led to the successful forced landing.
The investigation found the pilot’s in-flight fuel management resulted in insufficient endurance to safely conduct the planned flight and the aircraft exhausting its useable fuel supply.
Fuel starvation and exhaustion events continue to be reported to the ATSB
The aircraft had recently been refitted with wings sourced from another Cessna 172M. The replacement wings were fitted with smaller capacity tanks of 144 litres, compared to the previous 182 litres. Despite a number of opportunities to identify the change to the aircraft’s endurance, the pilot, who had flown VH-TUX on numerous occasions prior to its fitment of new wings, did not detect the reduced fuel capacity.
During the accident fight, the pilot observed a steady decrease in the indications on the fuel gauge, but the pilot discounted the accuracy of the indications. The pilot’s in-flight fuel management was likely based on the expectation of the aircraft’s endurance, rather than crosschecking the expected fuel burn against the fuel burn achieved during flight at the 30-minute intervals required under the operator’s standard operating procedures.
Further, the pilot’s pre-flight planning was inconsistent with both the regulatory requirements for flight planning and preparation, and the operator’s own electronic flight bag administration and in-flight fuel management procedures.
Fuel starvation and exhaustion events continue to be reported to the ATSB. The ATSB’s safety message from this investigation the importance for pilots to continue to educate themselves on the risks and controls associated with fuel management.
A Cessna 172 was flying from Queensland to New South Wales when it entered an area of reduced visibility, including low cloud, fog and drizzle. The aircraft diverted off the initial track and was last seen disappearing into cloud heading inland. A short time later, the aircraft collided with terrain and the pilot was fatally injured.
The ATSB found that the decision to depart on the flight had placed the pilot at risk of encountering conditions of reduced visibility. On entering those conditions, the pilot likely became spatially disoriented, resulting in a loss of control and a collision with terrain. The ATSB also found that the pilot was likely under some degree of self-imposed pressure to continue with the flight, despite the inclement weather conditions.
Weather-related accidents remain one of the most significant causes of fatal accidents in general aviation.
“The ATSB’s safety messages from this investigation are clear: visual flight rules pilots should conduct thorough pre-flight planning to avoid the possibility of flying into bad weather. They should also make alternate plans in case weather deteriorates, and make timely decisions about diverting or turning back,” said ATSB Executive Director Transport Safety Mr Nat Nagy.
“If visual flight rules pilots do find themselves in deteriorating weather and become disoriented or lost, they should seek whatever help is available including contacting air traffic control. This simple action has averted potential disaster in many instances.”
Weather-related accidents remain one of the most significant causes of fatal accidents in general aviation. Inflight decision-making is one of the ATSB’s SafetyWatch priorities, particularly regarding pilots flying with reduced visual reference.
Among the advice that SafetyWatch provides, the use of ‘personal minimums’ checklists is key. VFR pilots should use a checklist to help control and manage flight risks by identifying risk factors that include marginal weather conditions and only fly in environments that do not exceed their capabilities.
This report highlights an ongoing safety issue in aviation. Check out our SafetyWatch page, Inflight decision making, for more information on this important subject.
