On October 24, 1998, at 1130 central daylight time, a Hiller UH-12C, N9739C, owned by a corporation and operated by a commercial pilot, sustained substantial damage during a forced landing four miles east of the Clinton Municipal Airport, near Clinton, Iowa. The pilot said that he lost tail rotor control and initiated an autorotational landing into a field. He and the passenger reported no injuries. The 14 CFR Part 91 sightseeing flight was operating in visual meteorological conditions. No flight plan was on file. The local flight departed Clinton about 1120. An examination of the helicopter after the accident revealed that the "90 degree" gearbox had suffered a failure of the internal gears. When disassembled it was determined that the drive gear and the pinion gear in the transmission were both missing teeth. A check of the maintenance log books for the helicopter indicated that the last overhaul and maintenance on the "90 degree" gearbox occurred 195 hours prior to the failure. The helicopter had received an inspection the month prior to the accident with no note of maintenance on the gearbox or gears. The helicopter had accumulated about 33 hours since the most recent inspection.