Landing gear malfunction involving Airbus A320, VH-VFN, Sydney Airport, New South Wales, on 1 August 2019

On the morning of 1 August 2019, an Airbus A320, registered VH-VFN, was being operated as a regular public transport flight by Jetstar Airways (Jetstar) from Sydney, New South Wales to Gold Coast, Queensland. It was the first flight of the day for the flight crew and the third for the aircraft and cabin crew. The aircraft had previously been flown on a return flight between Sydney and the Gold Coast, landing in Sydney on runway 34R.^[1]

At 1020 Eastern Standard Time,^[2] the aircraft was pushed back and commenced taxiing to runway 34R (Figure 1). While the aircraft was taxied to the runway, the crew of a following aircraft reported to the Sydney East Ground Controller (Ground) that they had sighted an item on the ground at the intersection of taxiways B4 and C.^[3] Ground then sent an airport ground car to investigate the item, who reported 3 minutes later that the foreign object debris (FOD)^[4] had been retrieved. The FOD was a metallic component, later identified to be an apex pin of a main landing gear torque link (see the section titled Main landing gear strut and Figure 2).

Figure 1: Airport map – Sydney Kingsford Smith

Source: Airservices Australia, modified and annotated by the ATSB

At 1038, VH-VFN took off, and after retraction of the undercarriage, the flight crew received numerous messages on the electronic centralised aircraft monitoring (ECAM) system,^[5] two of which were ‘L/G DOORS NOT CLOSED’ and ‘L/G GEAR NOT UPLOCKED’. The captain elected to continue the climb out over the sea. The flight crew informed air traffic control and requested vectors to an area where they could troubleshoot the problem. They then cycled the undercarriage to the extended and then retracted positions however, the issue remained.

At 1042, Ground simultaneously contacted two Jetstar aircraft taxiing for take-off. They were instructed to contact their engineering department as ‘one of the safety officers has found a large piece of metal at the juncture of Bravo 4 and Charlie and it is believed to be from a Jetstar 320…’.

Figure 2: Apex pin as recovered from taxiway

Source: Jetstar Safety Department, annotated by the ATSB

At the same time, the Sydney Departures controller (Departures) informed the flight crew of VH‑VFN of a ‘large piece of metal’ found on a taxiway. The flight crew then told Departures they required a return to the airfield. In a follow up discussion a minute later, Departures informed the flight crew ‘they believe it might be a part of the landing gear’ to which the captain responded, ‘that would make sense’. Shortly after the aircraft entered a holding pattern off the coast.

At around 1048, a ‘hard stop’^[6] order was placed on departing Jetstar aircraft by the company as a result of the discovery of the apex pin on the taxiway.

At 1054, Departures advised VH-VFN to expect an approach to runway 25 when they were ready, but the first officer (FO) requested runway 34L because they were unsure about the serviceability of the brakes.^[7]  At this point, the FO confirmed that they had approximately 80 minutes of endurance.

In addition to the communication with Departures, the flight crew also contacted Jetstar’s Sydney line maintenance (Line Maintenance) via radio several times to discuss the landing gear issue. Around 1055, an engineer communicated to the flight crew that a part had been found but it had not been positively identified. They advised the crew to follow their standard operating procedures.

At 1059, believing there would be no further details from Line Maintenance, the aircraft left the holding pattern and was given vectors for an approach to land. The flight crew requested the airport’s emergency services be put on a local standby.^[8]

Around this time, Line Maintenance concluded the part was an A320 main landing gear (MLG) torque link apex pin and the Line Maintenance Supervisor (LMS) raised their concern with the Maintenance Operations Centre (MOC) ^[9] about the aircraft landing with such a fault. The MOC sought more information from the LMS. However, by the time MOC agreed with the LMS’s concerns, the aircraft had safely landed.

At 1110, the aircraft landed and stopped on taxiway B9 (Figure 1) where it was inspected by the airport’s Aircraft Rescue and Fire Fighting service fire commander. During the radio conversation between the fire commander and the FO, the FO discussed having a landing gear issue and that the aircraft was pulling to the left. The fire commander indicated that from their position in the vehicle, there were no visible issues with the aircraft.

The aircraft was then taxied on taxiway B to just short of taxiway B4 where Line Maintenance personnel inspected the MLG. The inspection found the left MLG^[10] torque link apex pin was missing along with two bolts from the associated damper unit (Figure 3). A brake hydraulic hose was also frayed to the point of allowing fluid to escape. The aircraft was then slowly towed to the gate.

Figure 3: Main landing gear torque link assembly upon landing

Source: Jetstar Safety Department, annotated by the ATSB

After passenger disembarkation, the aircraft was to be towed to a hangar for inspection and repair. To reduce the potential for further damage, and as no spare apex pin was available, the failed pin was reinstalled and temporarily secured in place to ensure the landing gear stayed correctly aligned.

The following afternoon, the missing head of the apex pin and one damper bolt (Figure 4) were found by airport staff adjacent to the T3 intersection of runway 34R. Fifteen days later the second missing damper bolt was found during a routine FOD inspection of runway 34R.

Figure 4: A damper bolt and head of apex pin retrieved from runway 34R

Source: Jetstar Safety Department, annotated by the ATSB

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1. Runway number: the number represents the approximate magnetic heading of the runway in tens of degrees. The runway identification may include L, R or C as required for left, right or centre.
2. Eastern Standard Time (EST): Coordinated Universal Time (UTC) +10 hours.
3. Taxiway intersection B4 and C is approximately 250 metres south of Gate 55.
4. Foreign object damage or foreign object debris (FOD) is any article or substance, alien to an aircraft or system, which could potentially cause damage. The term FOD is used to describe both the foreign objects themselves and any damage attributed to them.
5. Electronic Centralised Aircraft Monitoring (ECAM) is a system that monitors aircraft functions and relays their status to flight crew. It also produces messages detailing failures.
6. A hard stop call is a term for stopping an aircraft departing due to operational or engineering issues associated with the flight, e.g. incomplete maintenance or paperwork.
7. Runway 16R/34L is the longest runway available at Sydney airport. "> Runway 16R/34L is the longest runway available at Sydney airport."> Runway 16R/34L is the longest runway available at Sydney airport. a>
8. Local standby and emergency standby are the two levels of readiness by Aircraft Rescue and Fire Fighting service for an aircraft that has indicated a problem prior to landing at an airport. Local standby is the lower of the two levels and indicates the landing should be uneventful but the aircraft has some form of defect. Rescue services attend with a lower level of equipment based on the airport emergency plan.
9. The Maintenance Operations Centre (MOC) is Jetstar’s Continuing Airworthiness Management Organisation (CAMO) and covers Maintenance Watch and other engineering resources such as airport maintenance facilities.
10. All further references in this report to ‘MLG’ are associated with the left main landing gear unless otherwise noted.

Recorded data

Quick Access Recorder data

Quick Access Recorder (QAR) data (Figure 5) was retrieved from VH-VFN for the incident and prior flights. A review of the data from the previous two flights did not identify any anomalies.

Figure 5: Recorded data for the complete flight

During the initial climb on the incident flight, the aircraft did not successfully complete a full undercarriage retraction sequence, with the aircraft failing to sense the MLG up locks engaging and the gear doors closing. Approximately 90 seconds later the undercarriage extension/retraction was cycled with the landing gear being selected down successfully and then reselected up. Again, the aircraft failed to record a complete retraction of the MLG.

QAR data also showed that, on the taxi to the departure runway, left and right wheel brake applications were matched by temperature rises in the corresponding brakes. This was indicative of the brakes working correctly. On landing, despite application of both brakes, only the right wheel brake temperatures rose while the left brake temperatures continued to cool, implying the left wheel brakes were not functioning.

The aircraft manufacturer advised that an aircraft would remain directionally controllable at high speed due to the effectiveness of the fin and rudder, and at low speed with the nose wheel steering. With medium autobrake and the brakes of one wheelset failed, the calculated aircraft braking distance required increased from 1,360 m to 1,540 m. Sydney Kingsford Smith Airport runway 25 is 2,530 m and runway 34L is 3,962 m.

Electronic Centralised Aircraft Monitoring System data

The Electronic Centralised Aircraft Monitoring (ECAM) system displays messages to the crew via a dedicated interface and also sends the messages via the Aircraft Communications Addressing and Reporting System (ACARS)^[11]to the operator’s Maintenance Watch.^[12] The following ECAM messages related to the undercarriage were presented on the incident flight.

Table 1: ECAM Messages

Source: Jetstar Safety Department. Only messages relevant to the undercarriage and wheel brake systems are listed here. Other messages that are not of a consequence to this incident were removed for clarity.

‘L/G DOORS NOT CLOSED’ is a high priority message whereas ‘L/G GEAR NOT UPLOCKED’ is a low priority message. Higher priority messages are listed and actioned first. The order of messages may have reinforced to the flight crew that the landing gear door was at fault. Instead, the landing gear door fault was most likely a consequence of the MLG failing to uplock due to the wheels not being in proper alignment.

While not providing specific information about the nature of the fault to the flight crew, the ‘BRAKES ALTN BRK FAULT’ message reflected the system sensing the damage to the left wheel brakes.

Aircraft information

The Airbus A320 is a twin-engine, narrow body transport category aircraft that seats up to 186 passengers (depending on configuration). VH-VFN was manufactured in 2013 and had completed 21,256 flight hours and 11,687 flight cycles.

Main landing gear strut

The aircraft has a conventional tricycle undercarriage arrangement. Each retractable main landing gear (MLG) consists of two wheels and brake assemblies, on a common axle centreline, one each side of the landing gear strut (Figure 6). Rotation of the oleo-pneumatic strut is constrained by a torque link on the forward side between the two wheels.

The apex pin of the torque link connects the upper and lower torque link. This allows rotational torque loads to be transmitted between the landing gear strut and axle, while allowing free vertical movement of the shock absorber contained within the strut.

Figure 6: MLG assembly and exploded torque link assembly

Source: Airbus S.A.S. A320 series IPC Figure 32-11-11-52G (Sheet 1), modified by the ATSB

The apex pin also passes through a damper unit that attaches to the upper torque link. The head of the pin is protected from the environment by a rubber dust cap. The tail of the pin, nut, washer and locking pin assembly are protected by a coating of polysulfide sealant.

The upper and lower torque links also act as carriers for hydraulic brake hoses and an electrical harness connecting to equipment on the axles. Another link, known as the slave link, is fitted to the aft side of the strut and supports more hydraulic hoses and electrical wiring but is not designed to perform any anti-rotation function.

Aircraft damage

Detailed inspection of the aircraft MLG after the flight determined that, in addition to the failed apex pin, other items on the MLG were missing or damaged. These included:

• three hydraulic hoses damaged or frayed by contact with the wheel rim, with one hose leaking
• scoring damage to the apex damper unit following contact with the wheel rim and tyre, and two missing through bolts
• scoring damage to wheels, tyres and brakes from contact with the upper torque link and damper unit
• a cut wiring harness
• bending of the slave link.

The tyre treads had evidence of diagonal scoring which indicated that the MLG had not remained aligned with the aircraft during the landing.

Apex pin

Manufacturing process

The apex pin was manufactured from high strength steel in 2012. During its manufacture, the pin was initially roughly machined to oversize, heat treated to improve mechanical properties, and then machined to final size. The final machining included providing an undercut relief radius between the shank and the head, and formation of the head shape. Subsequently, parts of the pin underwent plating and corrosion protection processes. The part was subject to various manufacturing inspections, including a magnetic particle inspection (MPI),^[13]> which it passed (Manufacturer’s investigation).

Maintenance inspections

The MLG on the aircraft was last inspected in November 2017 as part of the aircraft’s maintenance program (AMP) 2C heavy maintenance inspection. During that assessment, the torque link was dissembled. The relevant inspection task required a check for excessive play in the hinge joints of the assembly, which it passed. The task did not require a visual or other non‑destructive inspection (NDI) of the apex pin. With Jetstar utilisation of VH-VFN, the 2C heavy maintenance inspections occurred approximately every 2.5 years and the pin had undergone this maintenance task twice.

The maintenance program also required the undercarriage to be overhauled every 10 years or 20,000 flight cycles (FC), whichever occurred first. The overhaul included disassembly of the undercarriage, with the apex pin inspected in detail and subject to an MPI if repaired. VH-VFN had yet to reach this overhaul requirement.

The apex pin had a life limit of 60,000 FC.

Pre-flight inspection

The first officer (FO) conducted the required pre-flight walkaround inspection for the incident flight which included the MLG wheels, tyres and strut. The FO did not report any anomalies with the inspection.

Licenced Aircraft Maintenance Engineers (LAME) were responsible for inspections of the aircraft on the first flight of every third day. The occurrence flight was the third flight of the day, so no formal inspection of the aircraft was carried out by a LAME.

Rectification action

Approximately two months prior to this occurrence, a European-operated A320 had an apex pin failure. Upon notification of both events, the landing gear manufacturer was advised, and an investigation launched. The pin was identified as being from the same manufacturing batch as the one installed on VH-VFN. This pin had accumulated 12,340 flight cycles. Due to the two pin failures, Airbus recalled the remaining 10 pins from this batch via individual contact with operators. Cracking was subsequently identified in five of those pins.

As a result, Airbus issued Alert Operators Transmission (AOT)^[14] A32N018-20 to operators worldwide on 23 January 2020, which recalled two batches either side of the initial batch (48 pins). The affected pins were requested to be removed at the earliest opportunity. As a result of this action, 15 pins were found cracked.

On 27 April 2020, AOT A32N018-20 Rev 01 was issued to add inspections for a further 1,940 pins. Apex pins serial numbers added in this revision were requested to undergo an MPI at the operator’s maintenance facilities. This action was reviewed by the European Union Aviation Safety Agency (EASA), and on 8 June 2020 EASA issued Airworthiness Directive (AD) 2020‑0130, which mandated compliance with Airbus AOT A32N018-20 Rev 01.

In August 2020, as part of inspections required by EASA AD 2020-0130, Jetstar identified a total of 12 aircraft with apex pins, and one spare part, listed in the Airworthiness Directive that required inspection. Jetstar found one additional cracked apex pin during the conduct of these inspections. That pin had acquired 13,402 flight cycles and was listed in Appendix 4 of AOT A32N018-20 Rev 01. The serial number indicated it was manufactured prior to the other identified fractured pins.

The ATSB identified three other occurrences overseas where pins in service listed in Appendix 4 of AOT A32N018-20 Rev 01 had been found cracked and reported.

Manufacturer’s investigation

The failed apex pin shank and head from VH‑VFN, which had accumulated 11,687 flight cycles, were sent to the MLG manufacturer, Safran Landing Systems (Safran) for inspection and analysis.

Safran conducted an investigation that included a detailed examination of the fractured parts. It concluded the cracks were initiated during manufacture of the parts and the parts failed due to cyclic fatigue. The investigation reviewed both the manufacturing processes and inspections.

Manufacturing process

Safran determined that the small radius under the head of the initial machining introduced a stress concentration during a subsequent heat treatment quench hardening process,^[15] that in some pins, resulted in the formation of a crack. The crack plane was found to align with the initial machined undercut (Figure 7). Despite a final machining process removing more material and providing a large relief radius under the head, some cracks were large enough so as not to be eliminated. Temper discolouration on the crack surface of some returned pins was indicative of the crack being initiated either during the quench process, or between the quench and temper steps of the heat treatment activities.^[16]

In summary, Safran concluded the cracks were initiated during the manufacturing process and were not caused by environmental effects in service.

The vast majority of apex pins in service were inspected as part of the AD and found not cracked (Rectification action). Safran determined that this disparity was likely associated with manufacturing variations including tool sharpness, surface finish and quench bath temperature, even though those variations were within allowable limits.

In 2014, some apex pins were found cracked during a manufacturing inspection. With no evidence of prior inspection failures nor cracks being found in-service, no action was taken against parts already produced. However, in response to the defective batch, the initial radius under the head of the machined pin was increased.

Additionally, in 2018 the company generated a design guidance document for radii in high strength steel parts prior to heat treatment in response to inspection failures in different parts.

Figure 7: Crack location

Source: Airbus S.A.S. modified by the ATSB

Manufacturing inspection

In accordance with Safran manufacturing procedures, apex pins underwent inspections during, and after, manufacture but before being released to service to ensure the part conformed to design specifications. Because the component was heat-treated to improve mechanical properties, the pin underwent an MPI. All pins in the occurrence batch passed the MPI inspection. That batch contained both pins that failed in service and five others subsequently found cracked.

The manufacturer’s investigation found that the inspector who conducted the MPI inspection on the batch of parts met qualifications, performance reviews, audits, eyesight requirements and had demonstrated their ability via MPI rejection of other parts. No personal circumstances were identified that may have influenced the ability of the inspector to detect cracked components and the organisation’s on time performance, capacity and production changes were also assessed with no unfavourable results. Part‑specific NDT technique and MPI process controls were also checked and found to conform to specification requirements.

Communications during the flight

Jetstar Operations Control Centre (JOCC) was responsible for the coordination and day‑to‑day running of the aircraft fleet. The JOCC used company HF frequencies and satellite communications to pass updated weather and operational changes to aircraft in flight. These means of communication were also used by flight crew to seek engineering support from the Maintenance Operations Centre (MOC) who had a Duty Technical Manager (DTM) at the JOCC.

Jetstar also utilised company VHF airband frequencies at major airports to pass operational information to crews of aircraft on the ground as well as those airborne in the local vicinity. Flight crew could also use this local VHF frequency to advise engineers at the airport of maintenance issues with their aircraft. Jetstar policy was that for any abnormal operations or delays, the JOCC would coordinate communications and be responsible for the company response. Thus, the JOCC was the primary point of contact in the event of any abnormal operations.

Flight crew could use company frequencies at certain times of the flight, however, during take-off and approach to land, sterile flight deck procedures^[17] were in place and flight crew did not monitor company radio frequencies, nor use ACARS.

Communication timeline

The following timeline of the occurrence and the communications during the incident was developed from multiple sources, mainly telephone calls and ATC VHF radio call records. As the Jetstar company VHF radio calls were not recorded, the times are approximate.

Table 2: Timeline

* estimated

Sydney Airports Corporation Limited (SACL) Car 4 retrieved the FOD and took the part to Qantas Engineering, as the majority of aircraft using taxiway C were Qantas operated. Qantas Engineering identified the part as from an A320. As Qantas did not operate that aircraft type, they redirected SACL to Jetstar Engineering. The part subsequently arrived at Engineering about 23 minutes before VH-VFN landed.

Linking the two events

The ATC centre in Sydney linked the FOD to VH-VFN as they received information directly on both events, albeit through different controllers. This meant the crew of VH-VFN was informed an unidentified aircraft part had been found on the taxiway less than 5 minutes after the aircraft took off and only 3.5 minutes after the FOD was reported to ATC.

Information related to the discovery of the apex pin on the taxiway and VH-VFN’s MLG retraction issues were received by separate parts of Jetstar. The organisation only had a limited time to link the two events given VH-VFN would turn off direct communication channels with the company during the approach which commenced 11 minutes before the landing.

Four Jetstar personnel (Jetstar Operations, Safety, Maintenance Watch and the Duty Captain) had a group call to brief and troubleshoot the reasons for VH-VFN returning to the airport. From the ECAM messages automatically relayed from the aircraft to Maintenance Watch, they correctly identified that the failure of the MLG leg to uplock was the primary issue and that the gear door not closing was a consequence.

Given it was the third flight of the day, they ruled out gear pins^[18]still being installed as a cause. They concluded, at 1100, no action was required until the aircraft landed. At that stage, this key group was unaware that the apex pin remnant had been found and been with Jetstar Line Maintenance for 13 minutes.

Shortly after the group call ended, Jetstar representatives made multiple attempts to contact VH‑VFN via company frequency which was indicative of the organisation successfully linking the two separate events.

Communication with aircraft

While airborne, the flight crew contacted Line Maintenance on the company VHF frequency at least twice. Line Maintenance informed the flight crew that a part had been found but it had not been positively identified. They further advised the flight crew to follow their procedures to manage the situation, which was in line with multiple Jetstar procedures manuals. The captain reported to the ATSB that they were frustrated that the part could not be identified.

The flight crew, having completed all checks, thinking that no further information from Line Maintenance would be forthcoming and believing it was only a gear door issue, initiated an approach for a return landing at Sydney Airport.

It was reported that contact with VH-VFN on the company VHF frequency was delayed in part due to reluctance to broadcast details over an open radio frequency. This was due the media and external persons often monitoring exchanges. It was mentioned in the company’s administration manual for personnel to be mindful of this fact.

Direct radio contact with VH-VFN was additionally complicated when the company frequency became congested due to the ‘hard stop’ call by Line Maintenance going out to taxiing aircraft. Multiple taxiing aircraft were using the frequency to determine the reason for the call back and requesting gates to return to.

By the time the JOCC, MOC and Duty Captain contacted the flight crew via Line Maintenance on the company VHF frequency regarding the criticality of the now identified part, the aircraft was on approach to land. Due to sterile flight deck procedures, the flight crew was no longer monitoring the company radio frequencies or ACARS messages. As a result, the only effective means of contacting the aircraft at that time was via ATC. ATC procedures permitted the passing of messages from the operator to the aircraft during times of an emergency however, an emergency had not been declared by the flight crew. JOCC personnel twice phoned the Sydney ATC, but no requests were made by JOCC to pass on safety critical information to the aircraft.

In response to this event, the operator advised it had clarified non-normal operational communication guidance for ground crews in the Airport Operations Manual. This included dedicated phraseology for gaining priority on airband frequencies to relay high priority messages.

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11. Aircraft Communications Addressing and Reporting System is a digital datalink system for transmission of short messages between aircraft and ground stations via airband radio or satellite.
12. Maintenance Watch – Most airlines have a group called Maintenance Watch. This is an engineering part of the organisation that actively monitors the airworthiness status of the company’s fleet, particularly aircraft flying. They monitor real-time aircraft data, including system warning messages, received by ACARS and arrange for aircraft swaps when aircraft become unserviceable. They also provide engineering expertise to the organisation and flight crew regarding the aircraft and its systems
13. Magnetic Particle Inspection: is a non-destructive inspection (NDI) process for detecting surface and shallow subsurface discontinuities in ferromagnetic materials. The process puts a magnetic field into the part which is distorted by the discontinuity. The change in magnetic field is highlighted by ferrous particles applied to the surface either dry or in a wet suspension.
14. An Alert Operators Transmission is Airbus service documentation that requires immediate, urgent or timely action to be taken by an operator to ensure the ongoing safe operation of affected aircraft. It is broadly equivalent to a Service Bulletin and is usually issued in response to a newly discovered service fault that may affect other operators.
15. Quench hardening is a mechanical process in which steel and cast-iron alloys are strengthened and hardened. The part is heat soaked at an elevated temperature (800-900ºC), then rapidly cooled in water, oil or air to set certain crystalline structures, locking in favourable mechanical properties.
16. Quenched parts are often tempered to reduce brittleness and increase low fracture toughness that may result from the quench hardening process. It involves reheating the part to a temperature lower than the quench step (200-700ºC) and slowly cooling to relieve internal stresses and permit some crystalline restructuring.
17. A sterile flight deck environment incorporates procedures throughout safety critical phases of flight, such as take-off and approaches to landing, during which non-essential activities and communications by flight crew are not permitted.
18. Gear pins – are pin inserted into key locations in the MLG retraction mechanism to ensure the undercarriage cannot be inadvertently retracted on the ground. Pins will be installed while the aircraft is on the ground overnight or in maintenance but not during a turnaround.

Introduction

On departure from Sydney the Airbus A320 landing gear failed to completely retract. Faced with limited specific information on the nature of the malfunction, the flight crew made the decision to return to Sydney, landing safely. The ATSB found that the failure of the left main landing gear (MLG) torque link apex pin, which was found on a taxiway, allowed the wheels to rotate out of proper alignment. This stopped that landing gear from retracting and caused damage to other systems on the MLG.

This analysis will examine the reason for the apex pin failure along with the associated inspections and maintenance programs for identifying defects. In addition, it reviews the actions taken by the flight crew and ground-based support infrastructure during the incident.

The ATSB found the flight crew response to the incomplete undercarriage retraction and subsequent landing was conducted in a proficient manner.

Apex pin manufacturing

The apex pins were manufactured by initially machining an oversize profile, which was then heat treated and finally machined to size including a large relief radius under the pin head. The identification of temper scale on the crack face from the heat treatment was conclusive evidence that the crack found on the occurrence pin (and several others) was induced as part of the quenching process. The initiated cracks were large enough they were not completely removed by the subsequent machining process.

While it was established that the radius of the initial machining under the head was too small to prevent heat‑related cracking, most pins did not crack. It was concluded that another factor, or factors, were required, in combination with the size of the radius, to initiate a flaw. These factors included variations in cutting tool sharpness, undercut surface roughness and/or the quenching bath temperature, even though these features may have been within prescribed tolerances.

Manufacturing inspection

The manufacturer’s investigation of the post‑manufacturing inspections focused only on the initial batch of 12 pins, two of which had failed in service and five pins were subsequently found cracked. The review of the human factors and organisational elements of the company at the time could not determine why these cracks were not found in this batch during the routine non‑destructive testing (NDT) magnetic particle inspection (MPI).

Before the manufacturer’s investigation was completed, and as a result of the airworthiness directive, 15 pins were found cracked from adjacent batches and four pins found cracked from other batches. These additional pins indicate it was unlikely cracking in pins at manufacture was a one-off event. The identification of cracks in 2014 that resulted in a manufacturing change also indicates that it was possible for cracks to be identified at manufacture.

Torque link disconnect

Following initiation of a crack during manufacture, the head of the apex pin fractured from the shank due to cyclic fatigue. The exact time of this event could not be determined. This left the apex pin shank free to migrate from the MLG upper and lower torque links thereby allowing them to disconnect just after pushback from the gate. The apex pin shank was shed onto the taxiway approximately 250 m from the pushback position.

The consequence of the torque link disconnection during the taxi and take-off was the misalignment of the MLG. During the flight the MLG was retracted twice. In both cases, either the MLG failed to lock in the retracted position or the MLG did retract successfully but the aircraft failed to sense this, i.e. sensor failure. Given the potential for misalignment without the apex pin, incomplete retraction of the MLG was considered the most likely reason. That conclusion is reinforced by the Captain’s comment regarding the unusual airflow noise. Damage to other systems including the brake lines on the MLG was further evidence the wheel set rotated out of alignment, contacting the upper torque link and adjacent equipment.

Risk analysis conducted after the incident regarding the loss of brakes on one wheel set concluded that an aircraft would remain directionally controllable at high speed due to the effectiveness of the fin and rudder, and at low speed with the nose wheel steering. The braking distances were determined to have increased by approximately 10-15 per cent.

While the disconnected torque link reduced the directional stability and braking performance due to the misaligned axle and failed left brake set, the degradation was manageable, and the aircraft landed safely. The use of runway 25 would have been acceptable, however the crew’s choice to request and use the longer runway 34L was prudent.

Flight crew decision making

The failed apex pin was collected from the taxiway approximately 40 minutes prior to VH-VFN landing. Within 15 minutes of the part being collected from the taxiway, the crew were advised by ATC that the object was likely associated with an aircraft’s landing gear. At about this time, the pin was delivered to Jetstar Line Maintenance. While Line Maintenance were identifying the part, Jetstar Operations Control Centre (JOCC) was troubleshooting the reported landing gear ‘failure to uplock‘ issue with VH-VFN. Being unaware of the failed pin and based on information available to them, the JOCC determined the aircraft could be examined after landing.

After completing troubleshooting procedures, with no additional issues identified, the flight crew elected to return to Sydney Airport. During this time, the captain requested additional information on the part found on the taxiway from ATC and Jetstar however, at the time of those requests it had yet to be positively identified. While the flight crew perceived the issue to be associated with the landing gear door, they mitigated their limited information by requesting the longer runway and for emergency services to be on standby. In this instance, these measures were not required however, they were effective in enhancing safety for a landing with an unknown problem.

Communication

For airborne Jetstar aircraft, communication with the company was primarily received and coordinated by the JOCC through HF, ACARS or Satellite communication methods. For operational reasons, the company had a local company VHF frequency at many airports that flight crew could use to communicate with local ground and engineering staff for efficient running of the business.

While this was a valid and efficient secondary line of communication, the JOCC had no visibility of these communications. For this reason, Jetstar procedures required all entities of the company to inform the JOCC of diversions, delays, or defects. Any communications with airborne aircraft beyond normal routine operational messages were required to go through the JOCC/Duty Captain to ensure consistent and appropriate messages were passed to flying aircraft.

Messages to the flight crew before the formal identification of the part followed the operator’s procedures/policy of only passing factual information to the aircraft crew. Having considered the provided information, the crew elected to return to Sydney Airport. While that decision was reasonable in the context of what the crew knew, the aircraft had 80 minutes of fuel remaining and there was no immediate urgency to return to the airport. It also meant that JOCC and Line Maintenance had a significantly reduced timeframe to identify and determine the severity of the fault. As such, an opportunity was lost for the organisation to gain a more complete picture of the situation while the aircraft was still airborne.

The company procedures prevented troubleshooting advice and non-factual information being passed to the aircraft. The policy was based on prior experiences of non-standard technical advice and workarounds being passed to aircraft, sometimes with detrimental effect.^[19] However, advice to the flight crew that people on the ground were working to identify the part and a suggestion that the aircraft continue to hold until that time, if able, would have been valuable to the crew on this occasion. The captain stated in an interview that had they known the identity of the part, they would have changed their approach to the landing preparations, including preparing the cabin for an emergency landing and enacting airport emergency procedures.

Jetstar policy and procedures required Line Maintenance to notify the JOCC when it was realised the part may be from an airborne aircraft despite Line Maintenance being in contact with the flight crew. As the aircraft was airborne, it was the JOCC’s responsibility to coordinate communication with VH-VFN. While that process was essential for ensuring accurate and coordinated information was passed to aircraft’s flight crew, on this occasion, a potential message to the aircraft was delayed while the part’s identity was clarified and confirmed by the JOCC.

Following positive identification that the detached pin created a potentially hazardous situation, ground personnel endeavoured to get a message to the flight crew. However, they were unable to contact them because the flight crew had started the approach to landing and, as per procedures, stopped monitoring the company frequency. Expeditious messaging to aircraft needs to be balanced with coordinated and accurate communication.

Failed apex pin temporarily reinstalled

Due to unavailability of spares, Jetstar temporarily reinstalled the failed apex pin shank into the torque link. This action was to enable safe towing of the aircraft to the maintenance hangar however, it had the potential to damage evidence on the fracture surface. At that time, the apex pin shank was the only piece of evidence available. Ultimately, the apex pin head was located, and analysis of both fracture surfaces determined the failure mechanism.

Notwithstanding unavoidable situations, where possible, preservation of evidence is vital in determining the circumstances of an occurrence and identifying safety issues that may present a hazard to continued operations.

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19. ATSB AO-2018-056 Depressurisation and crew incapacitation, Boeing 737-376SF, VH-XMO

From the evidence available, the following findings are made with respect to the landing gear malfunction involving Airbus A320, VH-VFN that occurred on departure from Sydney Airport on 1 August 2019.

Contributing factors

• During the manufacture of the apex pin, the initial machined profile led to unintended stress concentrations at the quench stage of the material heat treatment process that resulted in the part cracking. The crack was not removed by the final machining process. (Safety issue)
• Despite apex pins being subject to magnetic particle non-destructive inspections during manufacture, for reasons that could not be identified, this inspection did not detect the crack that was present in the occurrence pin.
• The head of the apex pin failed due to cyclic structural fatigue, which led to disconnection of the left main landing gear (MLG) torque link. This left the MLG strut free to rotate out of fore/aft alignment.

Other factors that increased risk

• The MLG brakes failed due to the strut rotating and a wheel contacting and fraying a hydraulic hose. This most likely occurred during take-off.
• Despite the failed part and aircraft being positively identified by elements within Jetstar, a message was unable to be conveyed to the flight crew who returned for landing unaware of the true nature of the undercarriage defect and the associated risks. The additional information would have improved crew decision making.

Other findings

• The undercarriage failed to fully retract by not engaging the gear uplocks and not permitting the gear doors to fully close. This was likely due to the wheels and axle being out of alignment.
• The torque link disconnect, the consequential landing gear misalignment and brake failure had the potential to reduce directional control and braking performance on touchdown. However, the degradation was manageable, and the aircraft landed safely.
• The failed apex pin was temporarily reinstalled by engineers to enable the towing of the aircraft off the taxiway and back to a gate for disembarkation. This had the potential to damage evidence on the fracture surface. At that time, the pin shank was the only piece of evidence available to understand the failure mechanism as the pin head had not yet been located.

Crack initiated during manufacture

Safety issue number: AO-2019-039-SI-01

Safety issue description: During the manufacture of the apex pin, the initial machined profile led to unintended stress concentrations at the quench stage of the material heat treatment process that resulted in the part cracking. The crack was not removed by the final machining process.

Safety action not associated with an identified safety issue

Additional safety action Jetstar Airways

In response to this event, the operator advised it clarified non-normal operational communication guidance for ground crews in the Airport Operations Manual. This included dedicated phraseology for gaining priority on airband frequencies to relay high priority messages.

Sources of information

The sources of information during the investigation included:

• the captain of the incident flight
• Jetstar Airways Pty Ltd
• Airbus
• Safran Landing Systems
• Bureau d’Enquêtes et d’Analyses (France)
• Airservices Australia
• Sydney Airport Corporation Limited
• recorded data from the Quick Access Recorder on the aircraft.

Submissions

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

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

• the flight crew
• Jetstar Airways Pty Ltd
• Airbus
• Safran Landing Systems
• Bureau d’Enquêtes et d’Analyses (France)
• Civil Aviation Safety Authority

Submissions were received from:

• a flight crew member
• Jetstar Airways Pty Ltd
• Airbus
• Safran Landing Systems
• Bureau d’Enquêtes et d’Analyses (France)

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