Incident Overview
A helicopter operating under contract for the Bureau of Land Management in southern Utah sustained a catastrophic failure after landing on a sloping rock outcrop. The helicopter, originally manufactured in France for the French Army Air Force, had been imported to California and issued a Certificate of Airworthiness in the normal category by the Anchorage, Alaska, FAA Flight Standards District Office. At the time of the incident, it was supporting an eradication of feral cattle program.
Sequence of Events
The pilot reported touching down on a sloping rock surface when the helicopter began to oscillate. In response, the pilot pulled up on the collective control, after which the aircraft "suddenly self-destructed." The ground resonance event resulted in the separation of the transmission/rotor system and the engine from the fuselage. One main rotor blade impacted the cabin. Of the occupants, five exited the helicopter under their own power; however, one remained buckled in the seat and was unconscious after being struck by a rotor blade.
Post-Accident Examination
A post-accident examination revealed numerous maintenance discrepancies. These included improper tire inflations, improper main landing gear hydraulic shock strut pressures, and differential functioning of the main rotor blade drag dampers. A representative of the helicopter manufacturer stated that all of these items have narrow operational tolerances if ground resonance events are to be avoided. The pilot disclosed experiencing at least two incipient ground resonance events with this helicopter during the previous 18 months.
Landing Surface Conditions
The landing surface was described as a "bowl shaped" rock formation. The right main landing wheel was positioned up-sloped approximately 5 degrees, and the nose wheel was 3 to 5 inches nose-low relative to the slope.