Following take-off from Los Angeles International Airport (LAX), the crew of the B747-438 aircraft noticed a severe airframe jolt while conducting a climbing left turn. The cockpit instruments indicated that the number-1 engine exhaust gas temperature was rising through 900 degrees C. Passengers also reported flames emanating from the number-1 engine tailpipe.
The crew shut down the number-1 engine and returned the aircraft to LAX for a one- engine inoperative landing.
An initial investigation carried out by the operator's maintenance personnel revealed that there had been an apparent failure within the engine's high-pressure compressor (HPC) assembly. The engine was removed from the aircraft and transported back to the operator's engine maintenance facility in Australia, where a more detailed examination was carried out. An Australian Transport Safety Bureau (ATSB) metallurgist was present for that examination.
The engine was a Rolls Royce RB211-524G2-T-19/15 turbofan engine. The designation `T', in the engine model number, indicated that the engine had been manufactured with a core engine from the larger Rolls Royce `Trent' engine series. The inclusion of the `Trent' core had enabled the engine to be more fuel-efficient and operate at a lower exhaust gas temperature.
The `Trent' core engine was split into numbered modules. The three modules of interest to the investigation were: Module 33, the intermediate case module; Module 41, the high-pressure system module; and Module 51, the intermediate and low-pressure turbine assembly module.
The engine had a nominal overhaul life of 30,000 hours or 4,000 cycles. At the time of the failure the engine was well within its overhaul life, having been in operation for a total of 13,922 hours and 1,395 cycles. It had not undergone any major maintenance.
The ATSB Technical Analysis report on the engine failure, (see Appendix A), indicated that the engine failure had resulted from the liberation of a single blade from the first-stage HPC rotor in Module 41. The blade release had resulted in extensive damage to the engine. The friction from the liberated blade impacting the surrounding blades on the HPC rotor resulted in a titanium fire within the compressor assembly.
A close inspection of the remains of the liberated blade root stub showed evidence of fatigue cracking and loss of the forward trailing edge corner of the blade dovetail root block. None of the other blades within the first-stage HPC assembly showed any visible evidence of cracking when inspected with the unaided eye. Minor collateral damage was also evident to components in Module 51 and Module 33 resulting from the blade failure.
The manufacturer was aware of three similar failures of the HPC blades in the RB211-524G/H-T series of turbofan engines. In those failures, cracking in the blade root area was believed to have resulted from uneven friction on the blade root bedding surfaces due to a breakdown in blade root lubricant. The manufacturer further indicated that damage in the blade root area that led to local stress concentrations, such as scores and sharp edges, might also have contributed to those blade failures.
The evidence from the ATSB investigation indicated that the failure mode in this incident was the same as the three other known failures in RB211-524G/H-T turbofan engines.
The engine manufacturer issued Service Bulletin SB72-E181 on 6 August 2003 to provide a long-term solution to the uneven blade root friction problem. That SB introduced a revised dry film lubricant (DFL) to be used as an alternate blade root lubrication coating. Incorporation of the SB is indicated by a change in blade part number.
The manufacturer has also revised the manufacturing process for the HPC blades, changing the blade root machining process from broaching to milling.
A new design HPC blade drum has been developed to prevent blade loading slot cracking. Service Bulletin SB72-E106 was issued on 7 March 2003 to include the application of the improved DFL to the first stage HPC blade root dovetails of those blade drums at manufacture.
As an interim measure, the manufacturer has revised the inspection limits for blade root damage and scores, and has instigated blade root DFL removal and replacement at each module overhaul. An ultrasonic, non-destructive-inspection procedure, that can detect blade root cracking without removing the engine from the aircraft, has also been developed.
The manufacturer reported that an update to these solutions would be supplied to all operators.
Technical Analysis Report: Examination of an RB211-524G-T Turbofan Engine Compressor Failure
Boeing 747-438, VH-OJU
An Australian registered Boeing 747-438 aircraft operating a regular passenger transport flight sustained the failure of an engine shortly after takeoff from Los Angeles, USA. The engine was subsequently shut-down and the aircraft returned for an uneventful landing.
The failed engine was a Rolls Royce RB211-524G2-T model. Preliminary inspection by the operators maintenance personnel found evidence of extensive internal mechanical damage within the high-pressure compressor section of the engine and as a result, returned the engine to Australia for inspection and overhaul.
The Australian Transport Safety Bureau examined the engine following its disassembly into primary modules. The engine had failed as a result of the liberation of a single blade from the first-stage high-pressure compressor section. That failure subsequently precipitated a titanium metal fire within the compressor, extensively damaging the following stages and rendering the engine inoperative.
The engine manufacturer has attributed three previous failures of RB211 high-pressure compressors to the loss of blades from the first-stage rotor. The blade losses were all associated with fatigue cracking of the dovetail root connection. The manufacturer identified uneven centrifugal loads on the blade roots as a significant factor in the development of blade cracking; possibly exacerbated by patchy root friction and minor mechanical imperfections in the critical blade root transition region.
Evidence from the current investigation indicated the nature of the failure to be very similar to the previously reported events.
|Date:||15 December 2002||Investigation status:||Completed|
|Time:||1430 hours UTC|
|Location:||Los Angeles, Aero.|
|Release date:||16 January 2004|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||The Boeing Company|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Minor|
|Departure point||LAX airport, Los Angeles USA|
|Departure time||1420 hours UTC|
|Destination||JFK airport, New York USA|
|Role||Class of licence||Hours on type||Hours total|