Loss of Throttle Control and Hard Landing at Oshkosh

On July 30, 2016, a Bell 47G-2 helicopter, N975BH, was operating at Oshkosh, Wisconsin, as part of a commercial air-tour operation over the EAA AirVenture grounds. The airline transport-rated pilot was flying with two passengers when a throttle-control problem developed during the approach to Wittman Regional Airport.

The pilot was on the downwind leg and attempted to reduce engine power for landing. The engine remained at excessively high power. He tried additional measures, including turning off one magneto and applying carburetor heat, but the power did not reduce enough to allow a normal descent.

The pilot then entered a sideslip and brought the helicopter down to about 100 feet above the runway. He turned the engine power off and entered autorotation. The helicopter reached the runway in a tail-low attitude and landed hard. The main rotor struck the tail, and the tail-rotor assembly separated. The pilot and both passengers were not injured, although the helicopter sustained substantial damage.

Investigators found that the throttle linkage nut had loosened and separated from its fitting. That left the throttle in the open position and prevented normal power control. The helicopter's maintenance manual did not specify a torque value for the throttle-linkage nut, and the overhaul procedures did not require that nut to be loosened or removed.

The pilot reported that the helicopter had undergone a complete overhaul about 37 flight hours before the accident. The NTSB determined that the loosening and separation of the throttle-linkage nut caused the loss of throttle control and the subsequent hard landing after autorotation.

This accident is a useful example of how a control-system problem can produce an unusual emergency. The helicopter had plenty of engine power, but the pilot could not command it normally. The recovery depended on reducing energy, descending to a manageable height and transitioning to autorotation before landing.

Accident Facts

Recorded line: “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.”

Sources

NTSB Aviation Investigation Final Report, CEN16LA294.

NTSB accident record: CEN16LA294, N975BH, Bell 47G-2.

https://www.youtube.com/watch?v=jpdAEA2C1Bw