Guimbal CABRI N401SH accident near Santa Ana, California

Probable cause

The flight instructor's delayed application of power during a power recovery following an autorotation, which resulted in an unrecoverable low rotor rpm during the landing flare and subsequent loss of control.

NTSB analysis

The flight instructor and private pilot receiving instruction (pilot) initiated a practice autorotation from about 700 ft in the airport traffic pattern. As the helicopter reached about 150 ft, the instructor slightly opened the twist grip throttle to initiate a power recovery but realized shortly thereafter that the applied power was insufficient to arrest the helicopter’s autorotative descent. The instructor applied additional power, and as the helicopter reached 40 ft, the pilot receiving instruction raised the collective, but the helicopter continued to descend in a flat attitude. The instructor then advanced the twist grip to full power but reported that the engine power did not increase as expected. The low rotor rpm horn activated, the nose yawed left, the helicopter rolled to the left, and the main rotor blades impacted the ground, resulting in substantial damage.

A detailed examination of the collective control, twist-grip throttle, and governor control system revealed no pre-impact mechanical anomalies. It is likely that the flight instructor could not apply sufficient power to abort the practice autorotation following the pilot’s collective input, which resulted in an unrecoverable low rotor rpm condition and subsequent loss of control.

The helicopter’s flight manual stated that the lowest recommended height for aborting an autorotation was 300 ft; however, the instructor initiated the recovery at 150 ft, which resulted in reduced time and altitude available to recognize that the engine was not responding as expected and continue with a power-off, touchdown autorotation. In a service letter published after the accident, the manufacturer advised against initiating a power recovery from an autorotation in the flare, as it could result in a loss of yaw control upon application of maximum engine torque.

Text from the NTSB. Open the NTSB docket for WPR18LA252 for the full record.