Incident Overview
During flight, the pilot heard a loud noise emanating from the engine compartment, accompanied by the illumination of the engine out light and the activation of the engine out horn. In response, the pilot initiated an autorotation. However, during touchdown on mountainous terrain, the helicopter rolled over.
Engine Examination
Disassembly of the engine revealed that the forward splines of the Spur Adapter Gearshaft (SAG), which connects the compressor and turbine module shafts, had fractured. Consequently, the compressor module became disconnected from the turbine section. The investigation found that the SAG retaining ring had machined into the compressor impeller, allowing the SAG to shift aft. This aft movement resulted in heavy wear on the splines of both the Compressor Adapter Coupling (CAC) and the SAG, ultimately leading to a fatigue failure of the SAG splines.
Component Configuration
The CAC and impeller were identified as being of an old configuration. Allison Commercial Engine Bulletin 1325 recommended replacing the old CAC, SAG, impellers, and 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 during the engine examination.