What happened
During an instructional flight intended to demonstrate a practice autorotation, the instructor pilot directed the student to reduce the throttle to idle to simulate an engine failure at altitude. During this maneuver, after the throttle had been reduced, the instructor realized that the engine had actually failed.
The crew completed an autorotation into a grass field with a slight upward slope. Upon landing into the wind, the helicopter slid backward approximately 15 feet. This movement caused the tail boom to flex upward and be struck by the main rotor. The helicopter sustained damage to the tail boom, all three main rotor blades, and the tail rotor drive shaft. There were no injuries reported.
The investigation
A subsequent engine run was conducted under the supervision of two FAA inspectors. This test revealed no mechanical anomalies that would have prevented normal operations. While the inspectors noted that the idle RPM and idle mixture had been poorly adjusted for the prevailing altitude and temperature, they determined these settings did not cause the loss of engine power.
Findings
The investigation concluded that the loss of power was caused by the instructor pilot's failure to follow cautions specified in the helicopter's flight manual.