NTSB Identification: LAX97LA039.
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Nonscheduled 14 CFR
Accident occurred Tuesday, November 12, 1996 in HANA, HI
Probable Cause Approval Date: 12/15/1997
Aircraft: McDonnell Douglas 369D, registration: N8625F
Injuries: 5 Uninjured.
NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.
During flight, the pilot heard a loud noise from the engine compartment, accompanied by the engine out light & horn. He entered an autorotation, & the helicopter rolled over during touchdown on mountainous terrain. Disassembly of the engine revealed the forward splines of the Spur Adapter Gearshaft (SAG), which connected the compressor & turbine module shafts, had fractured, & the compressor module was disconnected from the turbine section. The SAG retaining ring was found to have machined into the compressor impeller, which allowed the SAG to shift aft. Aft movement of the SAG then resulted in heavy wear on the splines of both the Compressor Adapter Coupling (CAC) & the SAG, which resulted in a fatigue failure of the SAG splines. The CAC & impeller were of an old configuration. Allison Commercial Engine Bulletin 1325 recommended replacement of the old CAG, SAG, impellers, & retaining rings with models of a newer design to 'improve spline wear characteristics.' Allison engineers opined that the typical retaining ring to impeller machining was due to the same retaining ring being returned to the engine during overhaul, though evidence of this was not apparent in the engine examination.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: subsequent failure of the spur adapter gearshaft (SAG), after 'retaining ring to compressor impeller machining' occurred and allowed the SAG to shift aft. The mechanism which induced the ring to impeller machining was not determined. Mountainous terrain in the emergency landing area was a related factor. Full narrative available
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