On 9 May 2011, a Qantas Airways Limited Boeing 747-400 aircraft, registered VH-OJH, was enroute from Sydney, NSW to Singapore, when during climb from 36,000 ft to 38,000 ft, the crew noted abnormal indications from the No.4 engine. The indications included an increase in both the exhaust gas temperature and vibration levels. The flight crew reduced the engine’s thrust, however the vibration continued near maximum levels and the engine was subsequently shut down.
The aircraft continued to Singapore for a safe landing and disembarkation of the passengers and crew.
What the ATSB found
The increase in the exhaust gas temperature and vibration from the No.4 engine was a direct result of the failure and separation of a single intermediate-pressure turbine blade. The turbine blade had fractured following the initiation and growth of a fatigue crack from an origin area near the blade inner root platform. Detailed modelling and analysis was undertaken by the engine manufacturer following the occurrence, and while the root cause for the IP turbine blade failure was not fully identified at the time of this report, it was considered that the wear and loss of material from the turbine blade outer interlocking shrouds had reduced the rigidity and damping effects of the shroud and may have contributed to the high-cycle fatigue cracking and failure. The manufacturer has advised that they are continuing work to understand the underlying mechanism of the failure and will advise the ATSB if any further information is obtained.
What has been done as a result
The engine manufacturer issued non-modification service bulletin (NMSB) 72-G739 in October 2011, which instructed operators to perform a once-around-the-fleet inspection of IP turbine blades for missing shroud interlock material. Accomplishment of this task was recommended by June 2012, taking advantage of any earlier planned maintenance opportunities. The operator indicated that they had completed inspections across the fleet with no instances of excessive wear identified.
Operators and maintainers of Rolls-Royce RB211-524 engines are alerted to the potential for wear and degradation of the intermediate-pressure turbine blade interlocking shrouds, with the possibility that this mechanism, if not detected and addressed, could lead to turbine blade cracking and loss. Service experience has shown, however, that the probability of an intermediate-pressure turbine blade failure (from any mechanism) is extremely low, with only three reported occurrences across the RB211-524 engine operating history. While blade separation will likely cause malfunctions necessitating an in-flight engine shut down, the associated risks to the safety of continued flight are minor, in the context that such failures are very likely to be contained (i.e. no liberated debris) and procedures for managing engine failures in transport-category aircraft are detailed and effective.