On June 2, 1998, at 1815 central daylight time, a Hiller UH-12E helicopter, was substantially damaged during a forced landing following a main transmission failure while maneuvering near Delaplaine, Arkansas. The commercial pilot, sole occupant of the helicopter, was not injured. The helicopter was owned and operated by Wilson Flying Service, Inc., of Walnut Ridge, Arkansas. Visual meteorological conditions prevailed for the local Title 14 CFR Part 137 aerial application flight for which a flight plan was not filed. The flight originated from a nearby farm approximately 5 minutes prior to the accident. The pilot reported that he was in the process of completing his sixth aerial application pass while dispensing herbicides on rice fields using east to west spray swaths. The helicopter was traveling in an easterly direction at an airspeed of 60 knots, while maintaining a constant 5 foot clearance over the rice field when the failure occurred. The pilot further stated that the helicopter was operating normally until he heard a loud grinding and growling noises coming from the area of the main transmission. The grinding noise, which was accompanied by a severe vibration, was proceeded by "the needles splitting and a marked increased in engine RPM." Suspecting an imminent transmission failure, the pilot turned the helicopter to the right to avoid a 50 foot tall cottonwood tree, as he simultaneously initiated a forced landing to an area short of a ditch and a tree line. During the touchdown in the soft ground, the left landing gear skid broke, the main rotor blades impacted the ground and the tailboom was severed by the rotating rotor blades. The helicopter came to rest in the upright position on a heading of 240 degrees, with both skids and crosstubes collapsed. Maintenance records that were provided by the operator, were reviewed by a FAA inspector. Entries in the records indicate that the main transmission (part number 23700-9, serial number 617), which has a recommended TBO of 1,200 hours, had accumulated 1,048 hours since its last overhaul in 1993. According to the manufacturer, the 23700-9 transmissions are subject to a recurring inspection of the oil filter every 50 flight hours in accordance with Airworthiness Directive (AD) 81-17-03. According to the maintenance logbooks, the last inspection in accordance with AD 81-17-03 was completed 340 hours prior to the accident, on March 26, 1996, at 708 Hobbs hours (1,908 hours total airframe hours). Examination of the transmission by a manufacturer's representative revealed "a massive failure of the first stage planetary gear system due to lack of lubrication." See enclosed manufacturer's report dated June 17, 1998, for details of the examination.