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Analysis

Summary

The manufacturer reported that the reduction gear box input shaft gear tooth spalling would have been noticed during the borescope check, detailed at 72-00-00 in the maintenance manual. The shaft was not subject to ongoing boroscopic inspections for gear tooth spalling, due to the previous incorporation of the requirements of SB 20246 into the engine. The manufacturer's laboratory report indicated that the shaft had less than the required surface carburising thickness, and that it had failed from fatigue originating from the root of the gear teeth area. There are methods available for the non-destructive testing of the case hardening depth of gears and gear shafts. These techniques could possibly be used to identify below-specification gear shafts at overhaul, allowing them to be removed from service prior to failure.

The manufacturer stated that the reduction gearbox chip-detector should have warned the operator that the shaft was spalling. The operator, however, despite carrying out regular was not aware that there was a developing problem.

The manufacturer's maximum boroscopic inspection period of 300 hours for a shaft found to have `within limits spalling', indicates that the spalling was not considered to be a fast-growing problem. Had the boroscopic inspections been carried out on this shaft, the spalling of the gear teeth would probably have been identified in time to prevent this occurrence.

The operator had SOAP tested the engine for some time prior to this failure, but this had been discontinued at the suggestion of the manufacturer's representative. Had the SOAP program been continued, this may also have detected the impending failure.

The circumstances of this incident are consistent with a catastrophic failure of the right engine reduction gearbox input shaft, following spalling of the gear teeth over a prolonged period. This resulted in a turbine overspeed and loss of power from the right engine. Consequently, the reuse of a shaft of this part number and accumulated time in service is an issue that should be reviewed by the manufacturer.

The cabin attendant continued with the pre-recorded briefing after becoming aware that some passengers were experiencing difficulty with hearing parts of the presentation. It may possibly have been more effective for the attendant to have terminated the electronic presentation at that point and completed the remainder orally.

 
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