Hiller UH-12E N4033P accident near Culpeper, Virginia

Probable cause

Mechanical failure of the transmission for reasons that could not be determined because the transmission could not be torn down.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for ERA11LA216 for the full record.