Qantas VH-OQL

A debris warning before departure

On 27 March 2014, Qantas Airbus A380-842 VH-OQL was operating a scheduled passenger flight from Dubai to London. Its four engines were Rolls-Royce Trent 900s. There were 484 passengers and 19 crewmembers onboard, a total of 503 people. The aircraft departed Dubai International Airport at approximately 2147 UTC. Takeoff and the first part of the climb were normal.

The No. 3 engine had generated a debris advisory on the preceding flight to Dubai. Engine health monitoring recorded that message at approximately 1859 UTC. The engine continued operating without an abnormal parameter change requiring the crew's attention. During the transit at Dubai, the defect was deferred under item 79-35-01 of the onboard Minimum Equipment List. That item referred to the ENG3 OIL CHIP DETECTED message, and the aircraft was released to service.

Shutdown and return

At approximately 2,000 ft, the crew received an ENG FAIL warning for the No. 3 engine, the right inboard engine. It had shut down without command. All engine indications had been normal before the shutdown. A second chip detector debris message was recorded at approximately 2149 UTC, followed by the shutdown and engine failure warning.

The crew declared PAN and leveled the aircraft at 6,000 ft. They tried a starter assisted relight, which was unsuccessful. According to the crew statement, the engine's high pressure stage did not rotate. The commander then initiated fuel jettison for a return to Dubai. The aircraft landed uneventfully at approximately 2249 UTC, after about an hour in flight. None of the passengers or crewmembers was injured.

A break in the drive system

At the parking stand, maintenance personnel found several metal chips on the engine master chip detector. The engine showed no external damage. Its fan could windmill freely, and inspections from the front and rear found no metal in the exhaust jet pipe. The post-flight report recorded engine failure code 7100W230, which directed troubleshooting toward an engine flameout.

Manual cranking provided evidence of a disconnected drive. The external gearbox turned, but the high pressure spool did not rotate with it. The engine's fuel and oil pumps received their drive through that gearbox. Losing that drive during flight would stop the pumps and shut the engine down. An initial inspection found internal damage in the step aside gearbox, and the aircraft could not return to service after the inspection findings.

The failed engine, serial number 91137, had accumulated 12,871 hours and 1,173 cycles since new. During a June 2013 shop visit, a damaged step aside gearbox bearing support had been replaced. The engine subsequently operated for 2,224 hours and 198 cycles, all on VH-OQL. The chip detector was sent for laboratory examination after the incident, but the debris first observed on its magnet was no longer attached when it arrived.

Movement, wear and fatigue

Examination found fractured cast webs in the step aside gearbox's driven bevel bearing support, together with damage and displacement of the driven bevel gear. The internal, external and transfer gearboxes showed no primary or secondary damage during borescope inspection. Minor displacement at two transfer gearbox bearing locations was judged non-contributory.

The angled drive shaft had met the balance acceptance limits during manufacture, although its radial run-out had been accepted slightly above the upper tolerance. Testing after the incident found its balance outside the acceptance limits. A major quality investigation confirmed the imbalance and traced it to variability in the balancing process and insufficient controls to keep that process consistent.

Rolls-Royce and Avio examined a cracked lower bearing support from another engine. The crack had started at stress concentration features created by a wear step. Examination of the fracture surface showed high cycle fatigue. Analysis of the bearing joint found that the specified bolt torque supplied insufficient clamping to prevent movement during engine operation. The investigators identified that as the primary root cause. Shaft imbalance increased the movement and rate of wear; excessive radial run-out also contributed to imbalance at high speed.

The GCAA concluded that mechanical damage in the step aside gearbox caused loss of drive to the external gearbox. The engine shut down as drive to the fuel pump, oil pumps and generators was lost. Analysis showed that greater joint clamping, combined with a correctly balanced shaft within dimensional limits, would prevent movement and leave a reserve margin in the existing design.

Changes to assembly and inspection

Avio introduced higher bolt torque, revised tightening instructions and a new balancing procedure for production and overhaul. It also introduced optical measurement of shaft run-out, applied it to new and previously operated shafts, and rejected and replaced a small number with dimensional anomalies. Cleaning, inspection and assembly instructions were revised, including measures to remove residual aluminium debris from the bearing interface.

Rolls-Royce published service bulletin RB.211-72-J023 on 9 December 2014 to increase tightening torque at 20 gearbox bolt joints. Other bulletins required hardware removal, reassessment and on-wing inspections. The report recorded completed hardware removal from 13 in-service engines and completed replacement on ten engines affected by the possibility of residual aluminium debris.

The GCAA recorded 266 engine inspections: 11 had cracks, ten had loose bolts, 17 had frettage and 218 had no findings. Work continued on washer seating and dimensional tolerances at the lower bearing joint. Shafts using a three-plane balance process became available on 28 January 2016. The completed GCAA summary report was issued on 22 March 2017. It recorded no safety recommendations.