On October 31, 1994, at 1630 hours Pacific standard time, a Bell UH-1E helicopter, N5088J, sustained a hard landing about 12 miles southwest of Willits, California, while executing an autorotation. The pilot was conducting a local visual flight rules external logging operation under Title 14 CFR 133. The helicopter, operated by the pilot, doing business as D&R Helicopters, Marysville, California, sustained substantial damage. The certificated commercial pilot, the sole occupant, sustained minor injuries. Visual meteorological conditions prevailed. The flight originated from a privately owned helipad near Willits, California, at an undetermined time. The pilot submitted the required Pilot/Operator Aircraft Accident Report, National Transportation Safety Board Form 6120.1/2. He said he had been conducting logging operations for 6.2 hours before the accident. The accident flight was the last flight of the day and he had just ". . . sit a log down in the log landing area . . ." and the helicopter was hovering about 200 feet above the ground. The helicopter was ". . . clear of all trees by at least 250 feet in all directions . . . ." He then heard a loud noise emanating from the rear of the helicopter and then the helicopter pitched forward and began spinning to the right. The pilot said he tried to roll the throttle off, but was unsuccessful. He reduced the rate of the right spin by lowering the collective. The helicopter's main rotor blades struck a tree when it was between 15 and 20 feet above the ground. A Federal Aviation Administration (FAA) airworthiness inspector from Oakland [California] Flight Standards District Office conducted the on-scene investigation. The inspector reported that the pilot dropped a log at the accident area and began a climb. During the climb, the helicopter's 90-degree tail rotor gearbox struck a tree. The pilot immediately executed an autorotation, but sustained a hard landing. At the insurer's request, Scanning Electron Analysis Laboratories, El Segundo, California, conducted a metallurgical examination on the tail rotor's 90-degree gearbox attach bracket. The metallurgist reported in a telephone interview conducted on March 23, 1994, that the examination revealed no evidence of any fatigue on the bracket's fractured surfaces. The fractured surfaces displayed overload characteristics.