Incident Overview
A commercial helicopter pilot reported completing a normal westerly approach and landing on a mountain at an elevation of 4,000 feet above mean sea level. As the pilot reduced the throttle to the flight idle position in preparation for engine shutdown, a severe airframe vibration occurred.
Damage Findings
A postaccident inspection revealed that the helicopter's main rotor transmission had been torn from the lower isolation mount assembly. The transmission deck sustained substantial damage.
Component Examination
The main rotor drag pin assembly, main rotor driveshaft, forward coupling assembly, and associated fasteners were sent to the NTSB Materials Laboratory for inspection. The examination found no evidence of a progressive fracture mechanism. All fracture faces displayed features typical of overstress separation.
Isolation Mount Evaluation
The main rotor transmission isolation mount was retained by the NTSB and sent to the manufacturer for evaluation and testing. An FAA airframe certification engineer attended the evaluation process and reported that the isolation mount operated slightly below acceptable stiffness limits. However, the engineer noted that because the isolation mount sustained damage during the accident, an accurate stiffness value could not be obtained.