Accident Overview
While established in an out-of-ground-effect hover at 150 feet above ground level, the pilot experienced a partial loss of engine power. The helicopter subsequently executed a hard landing during the resulting autorotation.
Investigation Findings
Postcrash examination of the engine revealed that the turbine had suffered oil starvation due to a failure of the oil pump. The failure originated from the aft bearing (gear case cover) of the FCU/oil pump drive spur gear. The ribbon-type ball separator had disintegrated; it was found in two pieces and deformed in the bottom of the gearbox. The balls were ejected into the gear case, and one was ingested by the oil pump, causing internal jamming. This jamming led to the shearing of the splined adapter that drives the oil pump.
The ball separator was manufactured from austenitic stainless steel instead of the specified AISI 1010 material, and it exceeded allowable hardness specifications.
Maintenance Background
Allison CEB 1228, issued November 20, 1985, calls for bearings with the ribbon-type, pressed steel separator to be replaced with bearings incorporating a one-piece machined steel separator. Compliance was required at the next accessory gearbox repair when the gearbox was opened. The mechanic reported that he had replaced the number 2.5 bearing and the spur adapter gear shaft 265 hours and 4 months before the accident, which would have provided access to the gearbox to accomplish the bulletin.