Primary finding
Probable cause
The pilot’s failure to maintain clearance from terrain during a toe-in landing to sloping terrain which resulted in a loss of helicopter control.
Investigator assessment
Analysis narrative
The pilot reported that, while the helicopter was in a stable hover against a mountain slope [toe-in landing], there was suddenly a “big bang”, and the helicopter began to descend and violently shuddered. He attempted to maneuver away from the slope, but the helicopter slid down the slope then came to rest on its left side. The pilot suspected a loose rock may have fell into the main rotor system or the main rotor blades contacted the slope. The helicopter sustained substantial damage to the fuselage, main rotor, and tail cone. The pilot reported that there were no preaccident mechanical malfunctions or failures with the helicopter that would have precluded normal operation.