What happened
A ferry flight was underway when the pilot, cruising at 1,200 feet MSL for approximately one hour, experienced a sudden rightward yaw of the nose. The pilot was unable to control this yaw and subsequently initiated an autorotation at approximately 70 mph to reach a nearby field. During the forced landing, the helicopter struck a fence.
There were no injuries reported in the accident.
The investigation
Mechanical examination of the aircraft revealed that the aft pinion nut of the main transmission had disengaged. This failure allowed the tail rotor splined drive sleeve to separate from both the pinion and the tail rotor drive shaft. Investigators were unable to determine the specific reason why the aft pinion nut backed off. Records indicated that the most recent annual inspection had been completed on June 7, 1996, and the aircraft had flown 66 hours since that inspection.