Overview
A ferry flight of a helicopter ended with a forced landing after the pilot lost yaw control. The helicopter had been cruising at 1,200 feet MSL for over an hour when the nose began yawing to the right. Unable to counteract the yaw, the pilot initiated an autorotation at approximately 70 mph toward a field. During the subsequent landing, the helicopter struck a fence.
Examination Findings
Post-accident examination revealed that the aft pinion nut on the main transmission had disengaged. This allowed the tail rotor splined drive sleeve to separate from the pinion and the tail rotor drive shaft, resulting in a loss of tail rotor drive. The investigation did not determine the reason for the pinion nut backing off.
Maintenance History
The last annual inspection was completed on June 7, 1996. Since that inspection, the helicopter had accumulated 66 hours of flight time.