What happened
A commercial pilot completed a standard westerly approach and landed the helicopter atop a mountain at 4,000 feet MSL. While the pilot was reducing the throttle to the flight idle position in preparation for engine shutdown, the aircraft experienced a severe airframe vibration.
The investigation
A post-accident inspection determined that the main rotor transmission had been torn from its lower isolation mount assembly, resulting in substantial damage to the transmission deck. Components including the main rotor drag pin assembly, the main rotor driveshaft, the forward coupling assembly, and related fasteners were analyzed at the NTSB Materials Laboratory. The laboratory findings showed no evidence of a progressive fracture mechanism; instead, all fracture faces exhibited characteristics of overstress separation.
Additionally, the NTSB retained the main rotor transmission isolation mount for evaluation and testing by the manufacturer. An FAA airframe certification engineer participated in this evaluation and reported that the isolation mount was operating slightly below acceptable stiffness limits. However, because the mount had sustained damage during the accident, an exact stiffness value could not be determined.