Accident Overview
A helicopter experienced a rollover after conducting an autorotative landing on a sloped mountainside. According to statements from the pilot, the helicopter entered a descent when the student pilot failed to maintain adequate main rotor revolutions per minute (rpm) while orbiting a mountain peak. The student relinquished control to the certified flight instructor (CFI), who attempted but was unsuccessful in restoring the requisite rpm. The CFI then performed an autorotative landing on downsloping mountainous terrain. Subsequently, the helicopter rolled over.
Pilot Observations
Neither pilot reported hearing the low rotor rpm warning horn, observing illumination of any annunciator light, or feeling any unusual vibration during the event.
Wreckage Examination
During the subsequent wreckage examination, the low rotor rpm warning systems were found to be functional. No evidence of any preimpact mechanical malfunction was discovered with the airframe or the drive system. The engine was run up to full power, and no malfunctions were noted.
Operational Conditions
At the time of the accident flight, the helicopter was operating near its maximum certificated gross weight and under conditions of high density altitude, specifically over 4,300 feet.