On December 4, 1996, at 1635 eastern standard time, a Hughes 269C, N1105P, was substantially damaged when it collided with a fence during a forced landing near Mayfield, Kentucky. The certificated commercial pilot and passenger were not injured. Visual meteorological conditions prevailed and a flight plan was not filed. The ferry flight was conducted under 14 CFR Part 91, and originated in Henning, Tennessee, at 1530. The intended destination was Indianapolis, Indiana. The pilot reported that he was ferrying the helicopter to the new owner. He said that the had been cruising at 1,200 feet MSL for about an hour, when the nose of the helicopter began to yaw to the right, followed by a change in aircraft noise. He attempted to correct the yaw by applying full left pedal, but was unsuccessful. He entered autorotation at about 70 mph, and attempted a forced landing to the middle of a field. During touchdown, the helicopter collided with a fence. The helicopter was examined after the accident by an FAA Inspector. The examination revealed that the aft pinion nut of the main transmission disengaged during flight, which disengaged the tail rotor splined drive sleeve from the pinion, and the tail rotor drive shaft. It was not determined why the aft pinion nut backed off causing the drive spline to disengage. The last annual inspection was completed on June 7, 1996, and the helicopter had accumulated 66 hours since the annual inspection.