Primary finding
Probable cause
The student pilot’s failure to maintain rotor rpm and airspeed and the flight instructor's inadequate remedial action to regain helicopter control during the straight-in autorotation which resulted in a hard landing.
Investigator assessment
Analysis narrative
The helicopter flight instructor was performing straight-in autorotation with power recovery maneuvers with a student pilot. On the third autorotation, the flight instructor stated that the student started the maneuver at 700 ft above ground level (agl) with the main rotor rpm at 100%. During the glide, when the RPM decayed to 90% and the airspeed decreased to 60 knots, the instructor intervened, and he attempted to recover the lost RPM. About 100 ft agl, the instructor prepared for a roll-on landing. The helicopter subsequently landed hard, rolled over on it’s left side and sustained substantial damage when the main rotor blades severed the tailboom. The instructor reported that there were no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.