NTSB Factual Report

On July 30, 2016, about 1500 central daylight time (CDT), a Bell 47 G2, N975BH, experienced a loss of engine power control during descent into Whittman Regional Airport (KOSH), Oshkosh, Wisconsin. The helicopter sustained substantial damage to the main rotor blades and tail. The airline transport-rated pilot and two passengers on-board were not injured. The helicopter was privately registered and operated by the Experimental Aircraft Association (EAA) under the provisions of 14 Code of Federal Regulations Part 136 as a commercial air tour flight. Visual meteorological conditions prevailed for the flight, and no flight plan was filed. The local flight originated at Pioneer Airport (WS17), Oshkosh, Wisconsin.

The pilot explained that tour flights departed from Pioneer Airport (WS17), part of the Oshkosh Airport Complex, and flew over the EAA AirVenture Fly-In grounds then returned to the airport. Upon return to the airport, the pilot entered the downwind leg of the pattern and attempted to reduce power for landing. The power would only decrease to 3100rpm. The throttle was moved 'full travel' with no power change. The pilot explained that he wanted to maintain the collective to avoid over speeding the engine, but was unable to slow or descend. He switched to the right magneto and applied full carburetor heat to reduce the engine power; however, the power did not reduce enough to descend. After flying the traffic pattern "several times" attempting to descend, the pilot described putting the helicopter into a slip with the right pedal. He stated it was the only way to descend because when the pedals were in a neutral position the helicopter would climb with the engine still producing full power. He circled at the end of runway 9 to descend in altitude, getting to approximately 100 feet agl before shutting off the ignition and conducting a low altitude auto rotation. The helicopter impacted the ground "hard" on the back side of the skids, the main rotor blade struck the tail and the tail rotor assembly separated from the tail boom. The helicopter skidded to the left side of the runway and came to rest in the grass.

The pilot stated there were no indications of a flight control problem in prior flights the day of the accident, or during the accident flight until returning to the airport at the completion of the tour.

The pilot indicated a complete overhaul of the helicopter was completed 37.6 hours before the accident flight. An on-scene inspection of the helicopter by the FAA and NTSB personnel revealed the throttle linkage nut had loosened and separated from its fitting, this separation would result in complete loss of throttle control. The maintenance manual for the Bell 47 G-2 did not specify a torque value for this nut given there were no maintenance procedures requiring the nut to be loosened or removed.

NTSB 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.