What happened
A helicopter, originally manufactured in France for the French Army Air Force and later imported to California, was performing contract work for the Bureau of Land Management in southern Utah. The mission supported an eradication program for feral cattle. While attempting to land on a sloping rock outcrop, the pilot reported that the aircraft began to oscillate. Upon pulling up on the collective, the aircraft experienced a sudden structural failure.
The impact of the event caused the transmission and rotor system to separate from the fuselage, and the engine also separated from the airframe. One main rotor blade struck the cabin. Of the occupants on board, 5 individuals were able to exit the aircraft under their own power. However, 1 fatality occurred when one occupant was struck by a rotor blade and remained unconscious in his seat.
The investigation
The investigation revealed that the landing surface was a bowl-shaped rock formation where the right main landing wheel was positioned on a slope of approximately 5 degrees, with the nose wheel sitting 3 to 5 inches low.
A post-accident examination identified several maintenance discrepancies, including improper tire inflations, incorrect main landing gear hydraulic shock strut pressures, and differential functioning of the main rotor blade drag dampers. A manufacturer's representative noted that these components have narrow operational tolerances required to prevent ground resonance.
Findings
The pilot reported experiencing at least two incipient ground resonance events in this helicopter during the previous 18 months.