What happened
During the flight, the helicopter pilot observed that the main rotor revolutions per minute (RPM) began to decrease while the engine RPM simultaneously increased. In response to these abnormal indications, the pilot initiated an autorotation procedure in an attempt to maintain control of the aircraft.
Despite the pilot's efforts, the helicopter was unable to complete a safe landing and impacted a wooded area. The impact resulted in significant damage to the airframe as the helicopter came to rest among the trees.
The investigation
Following the accident, investigators examined the helicopter to determine the mechanical cause of the rotor system failure. A detailed inspection revealed that a bolt lug located on the end of the linear actuator had failed. This specific component is responsible for engaging the belt drive mechanism within the transmission system.
The examination determined that the bolt lug had failed due to fatigue, which likely led to the loss of proper drive engagement and the subsequent drop in main rotor RPM observed by the pilot during flight.