What happened
During a downwind swath run, the helicopter suffered an unexpected engine failure. The pilot initiated an autorotation to manage the loss of power. Despite the attempt to land safely, the aircraft touched down with significant force, resulting in a hard landing that caused substantial damage to the airframe.
The investigation
A detailed inspection of the helicopter's engine was conducted to determine the root cause of the failure. Investigators found that the nut responsible for retaining the gear to the crankshaft end had backed off. This loosening allowed the gear to separate completely from its intended position, which directly caused the engine to quit.
Findings
The investigation revealed a critical maintenance oversight. The safety mechanism designed to butt up against the face of the retaining nut was not in place. Without this safety feature, the nut was able to loosen and back off during operation, leading to the catastrophic separation of the gear and the subsequent engine failure.