Hughes OH 6 N37929 accident near Bullfrog, Utah

Probable cause

A loss of engine power while the helicopter was hovering for reasons that could not be determined because postaccident examination of the engine and rotor drive system revealed no evidence of a mechanical malfunction or failure that would have precluded normal operation.

NTSB analysis

During a hover about 50 ft above ground level over steeply sloped terrain, the helicopter suddenly made an uncommanded left yaw, and the rotor rpm started to drop. The pilot lowered the collective and performed an autorotation to a rock ledge. After touching down on the ledge, the helicopter tipped over and tumbled downslope about 60 ft, and the tailboom was severed from the helicopter.

The rotor blade damage observed during the postaccident wreckage examination was consistent with low-rotor energy at the time of ground impact. Postaccident examination of the engine, rotor drive system, and fuel control unit revealed no anomalies that would have precluded normal operation. The engine's damaged external components were replaced, and a test run was conducted. The engine started and produced full power. The helicopter's operating manual stated that an uncommanded left yaw and a drop in rotor rpm could be indications of a loss of engine power. Given the uncommanded left yaw and drop in rotor rpm during the flight, it is likely that the engine lost power; however, the reason for the loss of engine power could not be determined.

Text from the NTSB. Open the NTSB docket for WPR14GA176 for the full record.