Incident Overview
A flight instructor and a student pilot were conducting autorotation training in a helicopter. The instructor indicated that they were practicing autorotations from about 1,500 to 500 feet above ground level. After the practice, the instructor decided to return to the departure airport. En route, the instructor idled the engine to simulate an engine failure for the student.
Loss of Engine Response
When the student pilot attempted to recover from the simulated engine-out condition, the helicopter engine did not respond to any power inputs. The instructor heard a low rotor RPM warning horn and immediately took over the flight controls. The instructor then initiated an autorotation, flared the helicopter, leveled it, and touched down in an uneven field. The helicopter came to rest on its side.
Post-Accident Engine Examination
The engine was removed from the airframe and transported to a certified engine overhaul facility, with oversight from the National Transportation Safety Board (NTSB) and the Federal Aviation Administration (FAA). At the facility, the engine was operated through several parameters, including engine start, magneto check, and a series of rapid throttle advancements from idle to full power. No abnormalities were noted during these tests.
A differential compression check was performed, yielding the following readings: cylinder one: 15/80 PSI; cylinder two: 15/80 PSI; cylinder three: 20/80 PSI; cylinder four: 15/80 PSI. The cylinders were subsequently removed and sent to ECI (Engine Components Inc.) for further examination of the pistons and cylinders, with FAA oversight. The pistons, piston pins, and combustion chambers of all four cylinders displayed discoloration consistent with operation at high engine temperatures.
Findings
The investigation documented the sequence of events and the post-accident engine examination results. The engine test runs did not reveal any functional anomalies. The compression readings were low, and the cylinder components showed evidence of high-temperature exposure. No probable cause was stated in the source material.