Accident Summary
A pilot with approximately 14,000 hours of flight experience was conducting aerial side-trimming of trees and bushes along a pipeline right-of-way. The helicopter was operating at an altitude of about 200 feet, using an external saw. During the trimming operation, the pilot encountered a left yaw and perceived an engine failure. In response, the pilot released the external load and attempted an autorotation from an out-of-ground effect hover at 200 feet above ground level. The subsequent ground impact resulted in crushing damage to the airframe, consistent with a high rate of descent.
Engine Examination
A detailed examination of the engine was conducted to investigate the reported loss of power. The inspection focused on the Rolls-Royce manufactured compressor adapter coupling, which had been installed approximately 414 hours prior to the accident. The examination revealed that the coupling had disconnected the compressor assembly from the turbine assembly. The disconnection was caused by fatigue cracking that originated from fretting on the pilot diameter of the coupling.
Probable Cause
The probable cause of the accident is the fatigue cracking of the Rolls-Royce manufactured compressor adapter coupling, initiated by fretting on the pilot diameter, which led to the disconnection of the compressor assembly from the turbine assembly and subsequent loss of engine power.