Incident Summary
According to the pilots, the helicopter was flying at an altitude between 400 and 500 feet above ground level and at an airspeed between 75 and 80 knots when the engine lost power. The instructor-pilot took control of the helicopter, jettisoned the external load, and performed an autorotation to a nearby corn field. He reported that the engine "didn't sound like it was running and the engine out was activated."
Landing and Rollover
The helicopter touched down and slid 6 to 8 feet when one of the skids dug into the earth. The helicopter then turned to the left about 45 degrees and rolled over onto its right side.
Post-Accident Examination
Examination after the incident revealed that the power turbine governor (PTG) spring had fractured in fatigue. The fracture originated from a drawing mark and was the result of deficiencies in the manufacturing process.
PTG Spring Function
The PTG spring "provides force opposing those exerted by rotating flyweights on the position of the governor lever. Failure of the spring would result in a false pneumatic signal to the fuel control and a corresponding reduction in fuel flow. The effect of the false pneumatic signal would be a pronounced reduction in fuel flow. At the reduced fuel flow, the engine could not sustain flight, and the symptoms of the reduced fuel flow might easily be interpreted by the pilot as an engine failure."