ERA11LA216

The aerial application flight had departed and was enroute to apply agricultural product to a nearby field; the engine rpm increased uncommanded and the pilot performed a forced landing in an open field. The pilot reported that after the helicopter came to rest, he exited and put his hand on the transmission, which was hot. He then felt the transmission cooler, which he stated was similar in temperature to the surrounding air temperature. Seven days before the accident, a continued airworthiness inspection was performed on the helicopter; the main transmission gearbox oil was drained, and the main rotor transmission was removed to facilitate engine removal. Particulate matter may have been inadvertently left in or around the main rotor transmission during its removal. During the flight the particulate matter may have become lodged in one of the journals inside the transmission and caused its failure, which then allowed the engine to overspeed and the helicopter to lose lift. However, the investigation was not able to conclusively determine the cause of the failure because the transmission could not be secured for a teardown.

On March 25, 2011, at 1238 eastern daylight time, a Hiller UH-12E helicopter, N4033P, operated by Kritter Cropdusting, Inc., was substantially damaged during a forced landing following an uncommanded high engine rpm and a loss of lift, while surveying an area for spraying near Culpeper, Virginia. The certificated commercial pilot was not injured. The aerial application flight was conducted under the provision of 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed and no flight plan was filed for the local flight. The flight originated from a nearby open area on the pilot's property about 1215. The pilot reported that he was flying approximately 150 to 200 feet above ground level when he noted an uncommanded high engine rpm; the helicopter turned approximately 60 degrees in heading, and experienced a loss of lift. He reduced the collective and initiated an auto rotation. After the helicopter came to rest, the pilot exited the helicopter, and reported that the transmission was "hot" and the transmission cooler was "the same temperature as the outside air temperature." According to photographs provided by a Federal Aviation Administration (FAA) inspector that responded to the accident location, the helicopter was resting on the hopper that was utilized to carry the applicant and it was leaning about 45 degrees to the left. The tailboom was severed and the main rotor blades were damaged. The pilot, age 55, held a commercial pilot certificate, with ratings for airplane single-engine land, rotorcraft-helicopter, and instrument airplane. The pilot reported to the FAA inspector that he accumulated 12,382 total hours of flight experience and 1,500 hours in the accident helicopter make and model. His most recent FAA second-class medical certificate was issued on October 12, 2010. The helicopter was manufactured in 1982 and was equipped with a Lycoming VO-540-C2A, 315- horsepower engine. At the time of the accident, it was registered with the FAA in the standard-restricted category and used for aerial application. The helicopter's most recent continued airworthiness inspection and engine overhaul was completed on March 18, 2011. At that time, the helicopter had accrued about 3,340.4 total hours of operation and had a Hobbs meter time of 250.6 hours. During the inspection the main rotor transmission was removed to facilitate engine removal and overhaul. The transmission oil was drained, the oil filter was removed and inspected, with "no metal detected." The transmission gear box was reinstalled and serviced with oil as required in the Hiller Maintenance Manual. An engine run and flight test were completed and "all test results [were] within parameters" and "all systems were operating normally." The helicopter had flown 1.8 hours since the last inspection and engine overhaul.