Bell 47G 2 N975BH accident near Oshkosh, Wisconsin

Probable cause

The loosening and separation of the throttle linkage nut, which resulted in a loss of throttle control and subsequent hard landing after autorotation.

NTSB analysis

The airline transport pilot maneuvered the helicopter on the downwind leg of the airport traffic pattern and attempted to reduce power for landing; however, the engine power remained excessively high. The pilot attempted to further reduce the engine power by turning off one magneto and applying carburetor heat, but the power remained too high to safely descend. The pilot then entered a sideslip, which allowed the helicopter to descend to about 100 ft above the runway. He turned the engine power off and autorotated to the ground. The helicopter landed hard in a tail-low attitude. The main rotor struck the tail and the tail rotor assembly separated from the helicopter.

A postaccident examination found that the throttle linkage nut had loosened and separated from its fitting, causing the throttle to remain open. The helicopter's maintenance manual did not specify a torque value for the throttle linkage nut, and there were no maintenance procedures requiring the nut to be loosened or removed during overhaul. The pilot indicated that a complete overhaul of the helicopter was completed about 37 hours before the accident flight.

Text from the NTSB. Open the NTSB docket for CEN16LA294 for the full record.