Primary finding
Probable cause
The flight instructor's failure to maintain helicopter control which resulted in a hard landing while preforming a practice autorotation.
Investigator assessment
Analysis narrative
The instructor reported that during the instructional flight, he was manipulating the controls and accepted the option to land on the taxiway; he decreased the throttle for a full touchdown auto rotation. About 50 ft above the ground (agl), he flared the helicopter and leveled off about 5 to 10 ft agl. The instructor reported that everything seemed "normal" at this point. He then raised the collective, but the descent was not arrested, so he raised the collective full up. The helicopter landed hard and shook violently. The instructor closed the throttle to which stopped the fuel supply. The tailboom was substantially damaged. The flight instructor reported that there were no mechanical anomalies with the helicopter that would have precluded normal operation. The instructor added that he felt a slight tailwind after exiting the helicopter and that the tail wind caused a "vortex ring state" which stalled the main rotor blades.