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Summary

Summary

While cruising at flight level 220 between Kimmi and Ablok, approximately 220 NM north-west of Cairns, the crew of the Dash 8 heard a muffled sound that was followed by an increase of the left engine inter turbine temperature. The crew reduced the engine power and returned the aircraft to Cairns.

The engine examination revealed that the reported problem was the result of a failure of the number 5 bearing. The bearing cage contained two cracks and was open. Some roller pockets in the bearing cage were enlarged and distorted, which allowed the rollers to rotate in the plane of the cage. Some other rollers were immobilised and contained large flats. Six of the 12 rollers had a smaller diameter than the rest. Their surface was abraded and smeared, consistent with heavy contact loads.

Because of several similar failures, the engine manufacturer had issued service bulletin number 21472R2, on 25 February 1999, introducing an improved number 5 bearing. The service bulletin advised that engines of serial numbers subsequent to the failed engine were already fitted with the new bearing. It further recommended that number 5 bearings on all other Pratt & Whitney PW100 engine models should be replaced when the engines were disassembled and access was available. The improved bearing had not been installed on this engine.

 
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