On July 11, 1996, a Hiller UH-12E (registration N102HT) operated by James E Willoughby was involved in an aviation accident near Cottondale, FL. Investigators recorded the probable cause as: failure of the main rotor drive torsional coupling and inner gimbal ring, which resulted in jamming of the flight controls during an attempted autorotation. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter experienced vibration and a loud bang two minutes into climbout. Pilots initiated autorotation but lost flight control response, impacting inverted. Post-crash examination revealed failure of the main drive clutch and engine gimble ring.
Incident Overview
Approximately two minutes after beginning a climbout, the pilots of a helicopter detected a vibration in the airframe followed by a loud bang. In response, they entered an autorotation and selected a field for a forced landing. As the pilots attempted to control the helicopter, the flight controls did not respond to inputs. Control was subsequently lost, and the helicopter impacted the ground in an inverted attitude.
Wreckage Examination
Examination of the helicopter revealed that the main drive clutch and engine gimble ring with the torsional coupling had failed. Debris from the failed components was not recovered for examination. A review of maintenance logs showed that the drive coupling assembly had accumulated 511 hours of total service since new. Additionally, wreckage examination disclosed that the entire transmission assembly had rotated from its normally installed position, and the flight control rods were displaced from their normal positions.