Scenario
The flight instructor and a private pilot receiving instruction were practicing landings. During the downwind leg, the instructor asked the student what his reaction would be if the engine quit. They then entered an autorotation from 5,200 feet mean sea level (600 feet above ground level) at an airspeed of 75 knots.
Sequence of Events
The student turned the helicopter into the wind, which caused a loss of 15 knots and a drop in rotor rpm to 96 percent. Noticing the rpm drop, the instructor added power, but there was no increase in rpm. The altitude had dropped to 4,900 feet msl (300 feet agl), and the rate of descent was increasing. The instructor kept the collective control down and pulled back on the cyclic control to transfer speed into rotor rpm and altitude. The engine and rotor rpm needles "were married at 93% and 95%." He increased collective to reduce the rate of descent but the throttle was fully open. The low rotor rpm horn never shut off, and there was no audible increase in rpm.
Final Descent and Impact
The instructor stated that as they passed 50 feet agl, the rpm was so low that "the helicopter was close to stall." He leveled off, but the helicopter struck the ground and bounced several times, collapsing the skids. It rolled over on its left side, shearing off the main rotor and tail rotor blades. The fuselage skin was also wrinkled.
Investigation Findings
Nothing was found that would have precluded the development of engine power. Based on an instructor interview, the FAA concluded that the flight instructor should have had the student make a straight ahead autorotation instead of a turning autorotation. The student allowed rpm to drop so that it would have been difficult, if not impossible, to recover.