What happened
The private pilot had initially rented the helicopter for a local flight. During the pre-flight phase, the pilot practiced several maneuvers on the airport grounds. After completing these exercises, the commercial pilot and passenger assumed control of the aircraft. The commercial pilot then executed a 180-degree practice autorotation directed toward a taxiway.
During the descent, the main rotor (M/R) revolutions per minute (RPM) decreased to 380 RPM. Approximately 30 feet above ground level (AGL), an attempt was made to recover power. However, the sink rate remained too high for a successful recovery. Consequently, the tail boom sustained damage from a strike by a main rotor blade.
The investigation
The investigation focused on the mechanical condition of the helicopter following the incident. Examination revealed that the tail boom had been struck by a main rotor blade during the failed power recovery attempt. The specific mechanical failure mode was identified as the result of the low rotor RPM and high sink rate during the critical phase of the autorotation.
Findings
The primary factors contributing to the accident included the high sink rate during the final phase of the practice maneuver and the subsequent main rotor blade strike. The decrease in M/R RPM to 380 RPM prior to the power recovery attempt indicated insufficient energy retention for a safe transition from autorotation to powered flight at that altitude.
Safety message
Pilots practicing autorotations must ensure sufficient altitude is maintained to allow for adequate rotor RPM buildup and sink rate reduction before attempting power recovery. Low-altitude practice maneuvers carry increased risk of mechanical damage if the recovery parameters are not met.