Incident Overview
A flight instructor reported that during an autorotation training exercise, a student initiated the maneuver and completed a 180-degree turn at approximately 250 feet above ground level (agl). The instructor observed a noticeable lateral vibration of the airframe and a rapid increase in the descent rate.
Instructor Response
Taking control of the helicopter, the instructor advanced the throttle and noted that the rotor and engine rpm needles were "joined" just below 100 percent rpm. He expressed confusion over why the helicopter exhibited a descent rate greater than 1,500 feet per minute despite the engine at full power and rotor rpm at 100 percent. Neither the instructor nor the student heard the LOW ROTOR RPM warning.
Attempted Recovery and Impact
The instructor attempted to flare the helicopter, but reported insufficient time to arrest the descent. The helicopter subsequently impacted terrain and rolled over.
Post-Accident Findings
A post-accident examination revealed no mechanical anomalies. Dust devils were observed in the area after the accident occurred.