On July 4, 2003, approximately 0850 Pacific daylight time, a Sikorsky UH-19D, N37799, registered to and operated by Americopter Aviation Services as a 14 CFR Part 137 aerial application flight, experienced a loss of engine power followed by a collision with terrain near La Grande, Oregon. Visual meteorological conditions prevailed at the time and no flight plan was filed. The helicopter was destroyed by impact damage and a post-crash fire. The commercial pilot, the sole occupant, was not injured. The flight originated from an off airport location about five minutes prior to the accident. In a written statement, the pilot reported that during a turn while spraying a fungicide on a potato field, he heard and felt three or four "clunking sounds in the airframe." The pilot continued the pedal turn to the left and then straightened out to approach the field. Rotor rpm was deteriorating as he flew over a paved highway. The pilot stated that he had no auto-rotating capability and landed hard in the field. The helicopter bounced once then touched down again coming to rest on its left side. The pilot exited the helicopter, that was already on fire, which had begun on the first impact with the ground. The fire eventually consumed the wreckage. Two Federal Aviation Administration Inspectors from the Boise, Idaho, Flight Standards District Office responded to the accident site and examined the wreckage. The inspectors reported that continuity was established to both tail rotor gear boxes. The main transmission was destroyed by the fire as well as a majority of the engine. The free wheeling side of the clutch assembly and the main drive shaft assembly nut were found disconnected. Maintenance records indicated that the overhauled clutch and fan assembly was installed on April 24, 2001 at a hobbs time of 1215.3 hours and total airframe time of 3943.0 hours. The last annual inspection was accomplished on September 4, 2002, at a hobbs time of 1380.0 hours and airframe total time of 4107.7 hours. At the time of the accident, the hobbs time was 1,494 hours. Approximately 278.7 hours had been accumulated since overhaul of the clutch and fan assembly. Overhaul is accomplished approximately every 600 hours. The clutch assembly, coupling, bearing support and drive plate were sent to the National Transportation Safety Board Materials Laboratory for further inspection. The specialist reported that examination of the clutch revealed that a portion of the retainer was missing, revealing the shield underneath, and resolidified material had come to rest on the bearing housing. The examination also revealed damage to the thread and the splines, on the portion of the drive housing protruding through the bearing housing. Damage was limited to the rearmost two threads and consisted of bands of material deformed rearwards. The bands were aligned with the groove between the splines where the mating splines of the coupling would be engaged. The specialist reported that closer examination of the threads revealed evidence of rubbing or smearing on their flanks, normally indicative of engagement with a mating thread, was not found. The only surface features were circumferential lines, consistent with the use of a thread cutting tool, and small isolated patches of corrosion. The coupling and bearing support were still wire-locked together. The threads on the forward side of the bearing support are normally assembled to the threads of the clutch assembly's drive housing, and splines on the coupling engage with the drive housing splines. Examination of the forward faces of the coupling and bearing support, revealed that the crown of the first full form thread on the bearing support had been sheared away from the thread in a forward direction. The sliver produced was approximately 2 inches long. The specialist reported that a closer view revealed that the sheared portion of the first full form thread was tapered along its length and consisted only of the crown portion of the thread. The flanks of the three threads aft of the sheared thread displayed circumferential lines, consistent with the use of a thread cutting tool, and small isolated patches of corrosion. There were no indications of rubbing or smearing on the flanks that would normally indicate engagement with a mating thread. Examination of the one fractured tang on the drive plate revealed a rough grainy surface, consistent with an overload event.