Incident Overview
During a planned series of marketing demonstration flights conducted for three United States Marine Corps officers, a helicopter suffered a main rotor drive system disconnect. The aircraft subsequently entered an uncontrolled descent and collided with terrain in a hilly desert environment. According to the investigation source, this was the third and final flight of the series.
Post-Crash Examination
Examination of the helicopter’s over-running clutch components after the crash revealed that the cages and sprags were bound in their normal engaged positions. Further inspection of the drag strips—of which three were present—found that two exhibited abrasion signatures across their entire width, measuring 0.07 inches. The abrasion areas generally matched the geometry of the adjacent ball bearing race.
Engineering Analysis
Review of engineering drawings for the clutch indicated that for a drag strip to contact the adjacent bearing race, the clutch assembly would need to protrude from its normal relative position by approximately 0.20 inches. Heavy oxidation on the components obscured potential relative running position markings between the inner and outer races, preventing a precise determination of their in-service alignment.
Conclusion
While the physical evidence suggests abnormal contact within the clutch assembly, the investigation source did not specify a probable cause for the main rotor drive system disconnect or the subsequent collision. No information regarding aircraft type, registration, operator, occupant counts, injuries, or environmental conditions was provided in the source.