On November 29, 1989, a Hiller UH-12E (registration N9762C) operated by Glacier Helicopters was involved in an aviation accident near Raymond, WA. Investigators recorded the probable cause as: The failure of an impeller section due to fatigue, which resulted in additional engine damage and loss of power. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter lost power due to an impeller fatigue failure while hauling an external load uphill, forcing the pilot to autorotate onto inclined terrain where the aircraft rolled over.
What happened
The accident occurred during an external load operation involving a helicopter. While the aircraft was hauling a load uphill toward the designated landing zone, the pilot reported hearing a loud noise followed by a significant loss of engine power. In response to the mechanical failure, the pilot immediately released the external load and initiated an autorotation. The helicopter landed on inclined terrain, where it subsequently rolled over upon impact.
The investigation
A thorough teardown of the engine was conducted to determine the cause of the power loss. Examination revealed that a section of the impeller had failed due to fatigue. This structural failure resulted in additional damage to the engine components and directly caused the loss of power experienced by the pilot during flight.
Probable cause
The failure of an impeller section due to fatigue, which resulted in additional engine damage and loss of power.