Accident Details
During a flight carrying an external load of logs, the helicopter was in level flight approximately 120 feet above ground level when the main rotor mast separated. Following the separation, the fuselage descended into a tree-covered slope and broke into three sections. The pilot sustained serious injuries.
Investigation Findings
Laboratory examination of the main rotor mast provided evidence that the mast failed due to fatigue. The fatigue crack originated at the lower ring groove of the damper splines, where burrs had been left during the machining of the part.
Component History
The mast had accumulated 5,644.4 flight hours at the time of the accident. The last overhaul was performed at 3,660.8 hours. The manufacturer's retirement requirements for the mast were 15,000 flight hours or 300,000 RIN (Retirement Index Number). The FAA calculated the RIN number to be 291,065.
Conclusion
The accident was caused by the fatigue failure of the main rotor mast at the lower ring groove of the damper splines, where machining burrs were present.