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Safety Action

Summary

At the request of and under the direction of BASI, the engine manufacturer investigated several engine failure modes during the investigation, including the misalignment of the reduction gearbox carrier housing assembly bearing bores

Following the accident the engine manufacturer has introduced an improved design N2 spur gear nut retainer and updated the overhaul assembly instructions for this component. The new nut retainer was incorporated to prevent tang fatigue failures. The improvement to the nut retainer was instituted as a result of an investigation into nut retainer failures in the U.S. Army UH-1 helicopter fleet. This investigation was initiated in April 1994.

An improved nut retainer was introduced on 13 Mar 1996 (Illustrated Parts Catalogue Temporary Revision No 6). Updated assembly instructions involving the match marking of the retainer and gearshaft, lubrication of the nut and a doubling of the torque on the nut were also introduced on 13 Mar 1996 (Overhaul Manual Temporary Revision No 16). A Service Bulletin (T5313B/T5317-0081) requiring replacement of nut retainers with improved retainers within 300 hours or two years was issued in May 1996. The Service Bulletin cites several instances of spur gear nut retainer separation in the U.S. Army T53 fleet. An Airworthiness Directive (AD 97-07-05) mandating the requirements of the Service Bulletin was issued by the U.S. Federal Aviation Administration in June 1997.

 
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