FTW02LA247 · Eurocopter AS350B3

The commercial pilot was in cruise flight with the auto-pilot engaged when the red "GOV" light illuminated and the audio sounded. He disengaged the autopilot and resumed manual control of engine RPM. As the pilot was turning toward his home base, the pilot re-engaged the autopilot, and the engine RPM began to increase and decrease. The pilot was unable to control the RPM and initiated an autorotation to a field. The helicopter landed hard separating the tail boom and fracturing two arms of the main rotor starflex. The helicopter was equipped with a Turbomeca Ariel 2B series engine, which incorporated an electronic engine control unit (EECU) that electronically controlled engine fuel flow via a Hydromechanical Metering Unit (HMU). Data retrieved from the helicopter's flight data computer revealed that the fuel-metering unit valve had malfunctioned. Further investigation revealed that there had been 10 previous incidents worldwide where there was an internal failure in the EECU, which resulted in a red "GOV" warning light illuminating during flight. When this light illuminated, it caused the fuel metering valve to freeze in the position it had at the time of the detection, and therefore engine power could no longer be automatically controlled. The HMU consisted of a resolver and a stepper motor. The resolver measured/sensed the position of the fuel metering valve. The stepper motor took commands from the EECU and physically moved the metering valve. In automatic mode, the HMU compared the resolver information and stepper motor information, as to the position of the fuel -metering valve. When a cumulative mismatch of greater than 7 degrees occurred, the red "GOV" light illuminated and the pilot had to revert to manual mode for throttle operation. Further examination of the Hydromechanical Metering Unit (HMU) revealed that the stepper motor rack was too short and needed to be lengthened.

Verbatim NTSB analysis and probable cause from the NTSB dataset