Landing gear/indication

Safety summary

What happened

On 4 January 2017, a Boeing 737-8FE, registered VH-VUH (VUH) and operated by Virgin Australia Airlines Pty Ltd (Virgin) was holding on B3 taxiway at Brisbane Airport, Queensland, when the crew heard a loud noise from what they thought was a burst tyre on the left main landing gear wheel. The crew attempted to return the aircraft to the gate, but were held short of the gate when an attending engineer observed that the number one main wheel assembly (left hand outboard wheel) had failed.

What the ATSB found

The ATSB found that the number one main wheel ruptured due to tie bolt assemblies that had loosened while in service. This allowed the two wheel halves to move relative to each other, resulting in a fatigue crack and eventual wheel rupture. The loosening was most likely due to the presence of anti-seize compound between the wheel halves, which affected the clamping forces.

The ATSB also found that while tie bolt assemblies on this wheel-type (single-web) were more prone to in-service loosening than dual-web wheels, there were no mandated inspections suitable for detecting such loosening. There were also no mandated risk controls to prevent loosening or subsequent rupture.

What's been done as a result

Virgin advised that following this occurrence, regular inspections were implemented to identify and prevent the loosening of tie bolt assemblies.

The wheel manufacturer updated the wheel’s component maintenance manual with more detailed instructions for applying anti-seize compound.

Boeing has advised the 737 NG fleet of the issue and suggested two possible courses of action. These were based on two optional service bulletins that the manufacturer had in place prior to the occurrence:

• incorporation of a new inner half-wheel that allows for safe deflation if tie bolt assemblies loosen (Service Bulletin C20626-32-014)
• the addition of lockwire to the tie bolt nuts, to prevent loosening in the first place (Service Bulletin C20626-32-016).

Safety Message

This incident highlights the importance of compliance with all aspects of manufacturers’ maintenance procedures, including the appropriate application of anti‑seize. This is especially important if, as in this case, there is no simple means of detecting the effect that such excess product can have on fastener security.

The occurrence

What happened

On 4 January 2017, a Boeing 737-8FE, registered VH-VUH (VUH) and operated by Virgin Australia Pty Ltd (Virgin), was scheduled to fly from Brisbane, Queensland to Melbourne, Victoria. Prior to take off, at approximately 1710 Eastern Standard Time,^[1] VUH was holding on B3 taxiway when one of the main wheels experienced what was believed to be a burst tyre. The flight crew began to taxi the aircraft back to the gate. An attending engineer instructed the flight crew to stop the aircraft before reaching the gate, as they noticed the number one main wheel assembly (left‑hand outboard wheel) had failed (Figure 1).

Figure 1: Damage to the number one main wheel assembly on VUH

Source: Virgin Australia

The flight was cancelled and passengers disembarked, however, the aircraft could not be jacked and towed via the axle due to the damaged wheel. Instead, wing jacks were sourced and a double wheel change performed on the tarmac. The aircraft was then towed to a maintenance facility for examination.

An inspection at the maintenance facility found that the inner half of the wheel assembly had fractured (see the section titled Wheel design). The tie bolt assemblies that connected the two halves of the wheel were all present with mating faces intact, however some of the bolts were found to be loose.

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1. Eastern Standard Time (EST): Coordinated Universal Time (UTC) + 10 hours.

Context

Wheel design

This particular wheel-type was designed for use on Boeing 737 NG aircraft. At the time of publication, the wheel manufacturer, Safran Landing Systems, estimated that approximately 2,000 aircraft were using the wheel globally. The wheel assembly used an asymmetric single-web^[2] design, with both axle bearings contained within the outer half-wheel (Figure 2). Eighteen tie bolt assemblies connected wheel halves together, with the inner half-wheel attached to the axle only via the tie bolts and outer half-wheel. This asymmetric design allowed for larger brake assemblies to fit within the wheel, unlike dual-web designs where each half-wheel contains an axle bearing.

As a result of the asymmetry, the tie bolts were exposed to a more cyclic load profile (compared with a more symmetrical design). The forces in the bolt could potentially oscillate with the rotation of the wheel while in service.

In this incident, the failed wheel was one of the outboard wheels on the aircraft and a hubcap normally covered the tie bolt assemblies.

Figure 2: Cross section of the main wheel showing the fracture initiation point

Source: Manufacturer, modified by the ATSB

The fracture location was coincident with a region of high stress during service. As seen in Figure 2, the cross section of the inner half-wheel reduced in thickness from left to right. The fracture occurred at the point where its cross section was thinnest, and near a change in geometry (tie bolt hole).

ATSB examination

The wheel halves from the ruptured assembly were initially sent to the ATSB for examination, where the following was found:

• Twelve of the 18 tie bolt assemblies could be twisted by hand.
• Several of the nuts had noticeably backed off the thread, with only three of the remaining six nuts above the minimum torque of 163 Nm.
• The breakaway torque^[3] of each nut was tested, and four of the 18 nuts were below the acceptable value of 3.61 Nm.
• The tie bolt assemblies were visually inspected and found to be the correct part in every instance, with no evidence of damage.
• A region of the fracture surface near one of the tie bolt holes exhibited fatigue crack progression (beach) marks radiating away from several initiation points (Figures 3 and 4). The remainder of the fracture was consistent with shear overstress. There was no visual indication of material defect at the fatigue crack initiation sites.

Figure 3: Outboard fracture surface on the inner half-wheel, with a fatigue region

Source: ATSB

Figure 4: Fracture morphology of one of the fatigue cracks through the fractured inner half-wheel

Source: ATSB

Manufacturer’s examination

The wheel manufacturer conducted a further examination of the failed main wheel, including a detailed analysis of the fracture morphology. They concluded that the wheel failed as a result of the observed fatigue cracks propagating into a mixed fatigue/ductile rupture mode. A small crack was also detected within the adjacent tie bolt hole, although this did not appear to have contributed to the wheel failure.

The manufacturer verified that the material composition and dimensions of the wheel were within specifications. Their report did not speculate on possible causes or factors contributing to the initiation or propagation of the observed fatigue cracking. However, they subsequently indicated that there was excessive ‘interposition product’ (anti-seize compound in this case) at the mating face of the occurrence wheel halves and that:

The presence of interposition product could lead to loss of bolt tension and then wheel rupture.

Specifically, it was believed that this interposition product prevented the wheel halves from abutting squarely against each other during assembly. Relative movement of the halves during service then resulted in loosening of the tie bolt assemblies and ultimately failure of the wheel. The manufacturer advised that the same mechanism was believed to have caused wheel failure in this type of wheel in two other instances (see the section titled Previous occurrences).

In March 2018, the wheel manufacturer visited the two different maintenance facilities responsible for Virgin’s 737 NG fleet to examine how the wheels were being maintained. Both facilities were following the tightening technique recommended by the manufacturer. However, at one facility the manufacturer reported seeing silicone grease on the mating faces of some of the wheel halves. This facility did not perform any maintenance on the occurrence wheel, and a subsequent visit by the manufacturer in September 2019 found that the grease was being applied appropriately.

Wheel history and maintenance

The occurrence wheel had undergone three tyre changes since its initial assembly in January 2016. It had operated for 173 cycles (47 days) since its last tyre change. According to one of the maintenance facilities responsible for the work, tie bolt assemblies generally lasted about three to four tyre changes before the lock nuts failed their breakaway torque tests and needed to be replaced. A tyre generally lasted approximately 40 days in service before needing replacement.

When reassembling the wheel after a tyre change, the tie bolt assemblies were tightened using a two-stage process, starting with an automatic dual spindle wrench. This device automatically torqued diametrically opposing nuts to a pre-set value of 70 Nm. In the second stage, the maintenance facility would use a single electronic torque/angle wrench for the final tightening. This involved rotating the nuts 100 degrees, as prescribed in the component maintenance manual (CMM). The wrench then indicated with a coloured light whether each nut was in the acceptable torque range.

This second stage was a relatively new tightening method introduced in 2014 via an optional service bulletin (SB C20626-32-007), to ‘improve wheel tie bolt clamping forces… and improve prevention against corrosion’. The service bulletin also recommended a new type of anti-seize compound, which was adopted by the maintenance facility prior to the occurrence. Following the occurrence, this new tightening method was made mandatory.

Prior to the occurrence, the manufacturer had updated the CMM to explicitly prohibit the application of any product to the mating faces of the wheel halves. Illustrations were included to demonstrate the proper application of anti-seize compound. A warning message regarding lubricant on the mating faces was also engraved on the wheels themselves. Since the occurrence, one of the maintenance facilities reported a change in nut design had resulted in fewer nuts failing their breakaway torque check.

Eddy current inspections^[4] of the outer and inner wheel halves were mandated every tyre change. However, the CMM indicated that these were localised to the bead seat area of the wheel. These inspections would therefore not identify the fatigue cracks observed in this occurrence, which were at a different location. General inspections of Virgin wheels were performed every six tyre changes. These inspections included a visual inspection of each wheel half and dye penetrant inspection. It is not clear whether these tests would have revealed cracking such as that found in this occurrence. Regardless, the wheel ruptured after three tyre changes, so the inspection had not yet been performed.

Operator inspections for loose tie bolts

Following the main wheel failure, Virgin performed a fleet-wide inspection on this wheel-type. Two other wheels in service at that time had tie bolts loose enough to be moved by hand. One of these wheels was relatively new, and had not yet required a tyre change.

As another post-incident preventative safety measure, the maintenance facilities applied torque seal to all tie bolt assemblies after each tyre change. The torque seal provided a visual indication of loosening tie bolt assemblies without any additional equipment. All wheels were also inspected at regular intervals, with the longest time between inspections being 28 days in service. This interval was selected so that the majority of wheels would be inspected at least once before a tyre change (approximately 40 days in service).

When a wheel was found with broken torque seals, the wheels and tie bolt assemblies were visually inspected for damage and their breakaway torques were tested. Nuts were replaced if they failed their torque check and undamaged assemblies were put back into service. No other loose tie bolts were found following the initial fleet inspections until July 2017. From July 2017 to August 2018, the wheel inspections found:

• ten more wheels with at least one broken torque seal, including six with multiple broken torque seals
• broken torque seals on wheels that had come from each of Virgin’s maintenance facilities
• three wheels exhibiting tie bolts with broken torque seal, despite being above the minimum allowable torque
• one instance where a tie bolt with unbroken torque seal was below the minimum allowable torque.

The bolt with unbroken torque seal could indicate that not all tie bolt assemblies left the maintenance facilities with the appropriate torque. Conversely, the broken torque seals on correctly torqued nuts serve as confirmation the tie bolt assemblies loosened in service despite being appropriately tightened.

Since September 2018, there have been no more broken torque seals found during Virgin’s inspections.

Boeing advice on tie bolts loosening in service

In 2018, Boeing released a bulletin to its fleet, recognising several wheel failures involving this type of wheel and acknowledging that it was the result of tie bolt assemblies loosening in service. The publication stated:

As the nuts unscrew from the bolts, the wheel halves start to separate under the influence of tire pressure and ground loads. Eventually the wheel halves flex to a sufficient amount that the wheel ruptures…

The bulletin also stated:

The mechanism that causes the nuts to unscrew is not well understood and has been difficult to reproduce on a dynamometer during lab testing.

The bulletin described wheel-types from two different manufacturers, however both wheels had a similar single-web design, with the axle bearings contained within one wheel half. Boeing believed that the issue was unique to these two wheel-types. The bulletin stated that if the preload was ‘less-than-optimal, the nuts may start backing out, or ’unscrew’ from the bolts as the wheel rolls under load.’ Boeing identified two issues that were associated with loosening. From the digest:

Bolt preload. Strict adherence to the bolt/nut tightening procedures has been shown to minimize the likelihood of bolt loosening.

Bolt thread lubricant. Testing and in-service experience has shown that certain thread lubricants are more likely than others to result in nut loosening.

Wheel manufacturer service bulletins

The wheel manufacturer released two optional service bulletins that mitigated the likelihood or consequence of tie bolts loosening in service.

The first bulletin was released prior to the occurrence in July 2016, to ‘ease main wheel deflation in case of broken or missing tie bolts’ (SB C20626-32-014). This had not been implemented on the ruptured wheel. The service bulletin introduced a replacement for the inner half-wheel. The new part included grooves along the mating face of the half-wheel, so that any movement between the two halves would allow air to escape and result in the gradual deflation of the wheel. Based on estimations from the manufacturer, the new half-wheel currently has a 55-60 per cent adoption rate, on a global stock of approximately 12,000 in-service wheels.

The second service bulletin added lockwire on to the tie bolt assemblies to prevent any loosening while in service (SB C20626-32-016). This bulletin was released on 4 January 2018, however the wheel manufacturer advised that bulletin was not released in response to this occurrence.

Previous occurrences

The wheel manufacturer advised that globally, there have been three other cases in which this type of wheel has fractured, and in each case the manufacturer assessed that the fracture initiated from a loss of bolt tension. In two of those three cases, the manufacturer believed the loss of bolt tension was caused by interposition product (grease or anti-seize compound) between the mating faces of the wheel. VUH was the first and only occurrence in Australia (noting that Virgin and Qantas use this wheel-type on their 737 NG fleet).

While the number of wheel failures is known, the number of instances of loosening tie bolt assemblies could not be determined or accurately estimated. CMM instructions for disassembling the wheel did not include any instruction on recording bolt torque or inspecting for loose bolts. Instances of loosening tie bolts in Virgin’s fleet have been recorded since the occurrence, but in the global fleet the numbers are unknown.

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2. The manufacturer described the design as single-web, because the web region connecting the axle bearings to the rim is only contained in the outer half-wheel. Dual-web designs have a web between the bearing and rim on both wheel halves.
3. Breakaway torque measures the locking force of the nut. It is the torque required to loosen a nut that has been run onto a tie bolt.
4. Eddy current testing involves using a magnetic field to induce a current in the material being inspected. This is known as an eddy current. Defects or cracks in this material can then be identified, as they may manifest as a measurable change in the eddy current.

Safety analysis

Occurrence event

The location of fatigue crack initiation in the wheel was coincident with areas of high service stress and there was no evidence of corrosion or manufacturing defect. A majority of the tie bolt assemblies on the fractured wheel were found to be loose, and nuts were found to be backing off on appropriately-torqued tie bolt assemblies on other wheels during service. Previous occurrences of wheel fracture have also been associated with loss of bolt tension. It was therefore most likely that tie bolt assemblies loosening in service resulted in the fatigue cracking and rupture of VUH’s left main wheel.

The loose tie bolt assemblies were likely a result of interposition product (grease or anti-seize compound) on the mating faces of the wheel halves. This prevented the halves from abutting squarely against one another when tightening the tie bolt assemblies. The wheel halves were then able to move relative to one another while in service, causing the tie bolt assemblies to loosen, reducing the clamping forces and inducing cyclic stresses.

Prevention or detection of tie bolt loosening

All of the possible reasons Boeing and the wheel manufacturer provided for the wheel ruptures related to having a suboptimal clamping force between the two hub halves, resulting in the failure mechanism described above. Prior to this occurrence, the wheel manufacturer had updated the component maintenance manual to explicitly prohibit interposition product between wheel halves. That update did not prevent this occurrence and the operator had 12 further instances of tie bolts loosening in service. However, since the manufacturer visited both of the organisations maintaining Virgin’s wheels, they have had no additional occurrences.

While actively preventing interposition product between the wheel halves appears to have been effective at reducing instances of loose tie bolts, there are other mechanisms that could reduce wheel clamping force. For example, since the occurrence there has been at least one instance of a wheel entering service with a bolt that was below the minimum required torque. Clamping force will also be affected by broken or missing tie bolts, which was the hazard addressed by the manufacturer through the introduction of hub grooves for safe tyre deflation.

At the time of publication there were no mandatory inspections capable of reliably detecting loose or missing tie bolt assemblies in service, between tyre changes. Those on the outer main wheels would not be readily observable due to the presence of a hubcap. In the absence of inspections, the two optional service bulletins (grooves for safe deflation, and lockwire on the tie bolt nuts) existed to prevent wheel ruptures and subsequent adverse outcomes, however their optional nature reduced the service bulletins’ effectiveness as risk controls.

Findings

From the evidence available, the following findings are made with respect to the main landing gear wheel failure of Boeing 737-8FE, registered VH-VUH, at Brisbane Airport, Queensland on 4 January, 2017. These findings should not be read as apportioning blame or liability to any particular organisation or individual.

Contributing factors

• A majority of the wheel's tie bolt assemblies loosened while in service, likely due to grease or anti-seize compound between the mating wheel halves. This resulted in fatigue cracking and ultimately, wheel rupture.
• The tie bolt assemblies on this wheel-type (single-web) were more prone than dual-web wheels to loosening during service, however there were no inspections for detecting loose tie bolt assemblies in service and no effective mandated risk controls to prevent wheel rupture.

Sources and submissions

Sources of information

The sources of information during the investigation included:

• Virgin Australia Airlines Pty Ltd
• Ground crew at Brisbane Airport
• Airservices Australia
• Main landing gear wheel manufacturer
• Wheel maintenance providers.

Submissions

Under Part 4, Division 2 (Investigation Reports), Section 26 of the Transport Safety Investigation Act 2003 (the Act), the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. Section 26 (1) (a) of the Act 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 Virgin Australia Airlines Pty Ltd, the main landing gear wheel manufacturer, the wheel maintenance providers, Boeing, the Civil Aviation Safety Authority (CASA), the French Bureau of Enquiry and Analysis for Civil Aviation Safety (BEA), and the National Transportation Safety Board (NTSB).

Submissions were received from the main landing gear wheel manufacturer, the wheel maintenance providers, CASA, and the BEA. The submissions were reviewed and, where considered appropriate, the report was amended accordingly.

Purpose of safety investigations & publishing information

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 2020 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 Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this publication is licensed under a Creative Commons Attribution 3.0 Australia licence. Creative Commons Attribution 3.0 Australia Licence is a standard form licence agreement that allows you to copy, distribute, transmit and adapt this publication provided that you attribute the work. The ATSB’s preference is that you attribute this publication (and any material sourced from it) using the following wording: Source: 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.

On 7 April 2016, a GippsAero GA8-TC 320 Airvan (GA8) aircraft, registered P2-MFM, was being flown from Kiunga to Yehibi, Papua New Guinea. Shortly after touch-down and around 140m into the landing roll, the wheel and axle assembly from the left main landing gear leg separated from the aircraft. The pilot was able to bring the aircraft to a controlled stop.

Subsequent inspection of the right landing gear leg from P2-MFM found that it was cracked in the welded region of the axle assembly. Inspections of other aircraft in the operator's GA8 fleet, identified that numerous main landing gear legs were cracked in the same location.

The Civil Aviation Safety Authority (CASA) commenced an airworthiness investigation into the circumstances surrounding this issue. CASA requested technical assistance from the Australian Transport Safety Bureau (ATSB) to conduct a detailed metallurgical examination of the main landing gear axles from P2-MFM. To facilitate this assistance, the ATSB initiated an external investigation under the provisions of the Transport Safety Investigation Act 2003.

The ATSB has completed its examinations. Fatigue cracking and fracture was associated with marginal weld penetration between the landing gear parts and was influenced by the manufacturer's original welding processes. The manufacturer revised the assembly drawing in 2009, including enhancements to the welding procedure, such that the latest axle design is likely to have improved fatigue endurance. The ATSB was not aware of any instances of cracking of axles manufactured to the latest revision.

On 7 June 2016, the manufacturer released service bulletin SB-GA8-2016-169 (Issue 1) that recommended inspections of the axle area be conducted at service intervals not exceeding 110 hours.

A report detailing the ATSB's examination and findings was provided to CASA, the aircraft operator and the aircraft manufacturer, on 6 October 2016. Enquiries relating to the investigation should be directed to CASA on 131 757.

Figure 1: GA8 main landing gear, wheel and brake assembly showing the location of cracking and fracture on the axle assembly.

Source: GippsAero / modified by ATSB

What happened

On 8 August 2016, at about 0700 Central Standard Time (CST), a Beech 58 aircraft, registered VH-UZO (UZO), departed Gove Airport, Northern Territory, for a flight to Elcho Island Airport, Northern Territory. On board were a pilot and four passengers.

During the initial climb, the pilot selected the landing gear up and noted the landing gear motor stopped after a shorter time interval than expected. At this time, the passengers reported hearing a crunching sound. The pilot observed that the landing gear unsafe light remained illuminated after the landing gear motor stopped.

Rather than continue the flight to Elcho Island, the pilot returned the aircraft to hold overhead Gove Airport while they attempted to ascertain the reason for the landing gear malfunction. The pilot noted that the circuit breaker for the landing gear had tripped, so reset the circuit breaker and selected the landing gear down. The landing gear unsafe light remained illuminated and the circuit breaker tripped again. The pilot then contacted the aerodrome reporting officer (ARO) and requested a visual confirmation of the position of the landing gear. The ARO reported that all landing gear appeared to be fully retracted.

Given that normal landing gear extension had been unsuccessful, the pilot elected to conduct an emergency landing gear extension. The Beech 58 emergency landing gear extension requires the pilot to engage a handle into the landing gear gearbox positioned behind the front seats. The handle is then turned counterclockwise to manually lower the landing gear. Fully extending the landing gear takes about 50 turns of the handle.

The pilot held the aircraft to the north of Gove Airport and engaged the autopilot while they conducted the emergency landing gear extension procedure. The pilot reported that no resistance was felt through the extension handle when attempting the landing gear extension, the handle felt like it was not connected. The pilot then flew back overhead Gove Airport for the ARO to again report on the position of the landing gear. The ARO reported that the landing gear remained retracted. The pilot then resumed holding, and calculated that they had sufficient fuel to continue to hold for a further two hours and 15 minutes. While holding, the pilot contacted the company chief pilot and engineer to assist with further troubleshooting the malfunction. The engineer examined the aircraft wiring diagram and another Beech 58 parked at the airport. The engineer then described several methods to isolate various parts of the electrical system to identify any problem which prevented the landing gear from extending. Over the next two hours, the pilot tried these methods along with multiple attempts of the emergency landing gear extension procedure. Despite the pilot’s attempts, the landing gear remained retracted.

At about 0930, the pilot prepared for a wheels up landing. They briefed the passengers on the use of seatbelts, bracing position, emergency exit locations and actions to be taken after the landing. The ARO arranged for the emergency services to be in attendance. The pilot discussed with the chief pilot whether to land on the runway or adjacent dirt. As the runway provided a hard, smooth surface of known condition, the pilot elected to land on the runway. The chief pilot then briefed the emergency services on the intended actions of the pilot. The pilot reviewed the wheels up landing procedure in the pilot operating handbook (POH), and elected to conduct a flaps up landing to minimise damage.

At about 0945, the aircraft approached the runway. Just before the aircraft touched down, the pilot shut the engines down in accordance with the POH wheels up landing procedure. As the aircraft slid along the runway, smoke filled the cabin and the pilot selected the fuel off. Once the aircraft came to a stop (Figure 1), the occupants immediately exited the aircraft. The pilot directed the passengers to a safe location behind the aircraft.

No persons were injured, and the aircraft was substantially damaged in the accident.

Figure 1: VH-UZO after the wheels up landing

Source: Pilot

Pilot comments

The pilot of VH-UZO provided the following comments:

• To assist in troubleshooting the malfunction, multiple videos of the actions taken by the pilot and indications presented by the aircraft systems were sent to the engineer.
• The passengers were engaged to assist in the attempts to lower the landing gear. The passenger in the seat next to the pilot held the POH. Other passengers also attempted to wind the emergency landing gear handle.
• The passengers were directed to evacuate to the rear of the aircraft. The pilot has subsequently learned that the safer option is to direct passengers to the side of the aircraft and upwind, away from fuel vapours.

Engineering report

A post-accident examination of the landing gear system found that the gear box shaft bearing had fractured. This bearing secures and aligns the shaft worm drive, which attaches both the emergency handle mechanism and the electric motor to the gear box. Failure of the bearing allowed the shaft worm drive to disconnect from the gearing. The drive became jammed, causing further damage to the gear box. Damage to the gear box prevented normal operation and caused the electric motor to overload and trip the circuit breaker. The bearing failure also prevented the emergency handle from connecting to the gear box.

Safety message

Even though the operation was conducted single-pilot, this accident provides a good example of effective crew resource management techniques. The pilot quickly established that the available fuel endurance allowed ample time to carefully consider the circumstances and attempt to resolve the issue. They engaged company personnel, using multiple means, to provide as much information as possible and attempt to identify a solution to the malfunction and sought the assistance of the ARO to inspect the aircraft and to alert emergency services. Holding over an easily identifiable position, and using the passengers where appropriate to assist with management of the emergency, also reduced pilot workload. The pilot also prepared the passengers for the wheels up landing, this minimised the risk of injury and ensured the evacuation was controlled and orderly.

Aviation Short Investigations Bulletin - Issue 53

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 2016 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 Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this publication is licensed under a Creative Commons Attribution 3.0 Australia licence. Creative Commons Attribution 3.0 Australia Licence is a standard form licence agreement that allows you to copy, distribute, transmit and adapt this publication provided that you attribute the work. The ATSB’s preference is that you attribute this publication (and any material sourced from it) using the following wording: Source: 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.

The flight was scheduled as a dual instrument training flight. As the student brought the aircraft to a stop prior to the engine run-up the gear-up warning horn momentarily sounded, and the gear-up warning light flashed once or twice. The instructor took control and checked the landing gear warning systems.

As he could find nothing wrong and he could not reproduce the momentary warning signals he deduced that it was safe to proceed with the flight. He did decide to test the integrity of the gear warning system before returning to the Jandakot circuit area. The flight proceeded normally with all landing gear indications and warnings operating correctly during the departure, an instrument approach and overshoot and a practice engine failure. During a second approach the landing gear did not extend when selected down.

Following a period of trouble shooting the landing gear was extended using the manual extension system. Three green lights were obtained, and the aircraft was landed at Jandakot. As the aircraft cleared the runway, the gear unsafe horn started to sound and the gear-up light started to flash. The pilot stopped the aircraft, confirmed that the three green position lights were indicating down and locked, the landing gear lever was in the down position, the gear motor circuit breaker was out and that the gear relay circuit breaker was in.

Whilst he was visually checking the circuit breakers, the gear motor momentarily made a sound. The pilot considered shutting down the aircraft but before he could take any action the aircraft gently rocked, and the right main landing gear collapsed. The three green gear down and locked lights were still illuminated. Subsequent investigation disclosed that the landing gear motor relay contacts were fused together creating a permanent landing gear up circuit.

The pilot was conducting a test flight as part of the certification process required by the Civil Aviation Authority. Positive gear down and locked indications were obtained during the downwind leg of the circuit. As the aircraft touched down the pilot detected a settling of the right main landing gear and elected to go around. Inspection from the ground confirmed that there was a fault with the right landing gear. The pilot adjusted his approach so that the right gear touched down at a low speed. The gear leg collapsed on touch down and the aircraft slewed off the runway. It has not been determined why the landing gear collapsed.

On arrival in the circuit area at Wedderburn, the pilot was unable to extend the landing gear by the normal method. The failure of the gear to extend was accompanied by a very loud high-pitched noise in the headset and speaker systems. His attempts to lower the landing gear by the manual emergency system were also unsuccessful.

As a result, the pilot elected to return to Bankstown. However, on arrival in the circuit, he was unable to communicate by radio to the Tower or other aircraft. Air traffic control staff recognised the pilot was experiencing difficulties with the landing gear and called out the emergency services. When the services were in place the aerodrome was closed and the pilot advised by signal light that he was cleared to land.

The aircraft subsequently landed on runway 11 with the landing gear retracted. An investigation revealed a fault in the alternator field circuit which prevented the battery from being charged. The landing gear failed to extend by the normal means due to low battery voltage. The pilot was unable to lower the gear by the manual system because he could not gain access to the emergency gear extension handle due to an incorrectly fitted piece of interior trim. It was not determined how the trim came to be incorrectly fitted.

On arrival at Dubbo, the landing gear failed to extend when the normal down selection was made. The crew completed the checklist requirements and after consultation with their company, elected to return to Sydney. A Pan call was made to ATC as the aircraft approached Sydney. The landing gear was successfully extended via the alternate selection procedure and the aircraft arrived without further incident. Subsequent tests of the landing gear retract, and extension system revealed an intermittent fault in a control relay.

The pilot reported that the circuit and landing were normal. After about 200 metres of ground roll the nosewheel began to shimmy and the nose gear collapsed. The aircraft came to a stop on the runway and all occupants evacuated without injury.

The maintenance organisation which recovered the aircraft advised that they have identified and changed an intermittently faulty down lock microswitch. A detailed rigging check was carried out and all dimensions and tolerance were found to be in accordance with the manufacturer's specifications. The maintenance organisation was unable to duplicate the unlock condition during multiple retractions on the ground.

On rotation for take-off the crew heard a noise. As landing gear retraction was normal and the aircraft continued to operate normally the flight was continued. The aircraft had reached about 220km from Darwin when the crew were advised by a company ground engineer that a section of main gear tyre tread had been found on the runway. The Captain briefed the crew for a possible emergency landing and the flight continued to Brisbane where emergency services were put on standby.

A safe landing was made and the aircraft taxied to a parking bay where investigation revealed that the right hand inboard tyre tread had separated, causing damage to the landing gear door clamp. The tyre was a level 6 retread (having been re-treaded 6 times), the occurrence being similar to earlier problems with this make of tyre. The company removed all re-treaded tyres above the level 4 retread status for inspection.

The nose landing gear collapsed during the take off roll causing both propellers to strike the runway whilst operating at full power. Subsequent investigation revealed that a rigid hydraulic line had failed causing the loss of all hydraulic pressure. This lack of hydraulic pressure probably allowed vibrations to unlock the nose leg over-centre lock during the take off roll.

Examination of the failed hydraulic line revealed that there was thinning of the pipe wall at the failure where the pipe was supported by a 'P' clamp, possibly the result of corrosion. Significant Factors: Failure of a hydraulic pressure pipe, possibly due to corrosion, resulted in loss of all hydraulic pressure. The nose landing gear over-centre lock probably unlocked due to vibration.