Robinson Helicopter R22 BETA N771MM accident near Lake Worth, Florida

Probable cause

A main rotor stall due to low rotor rpm, which resulted in an uncontrolled descent into terrain. Contributing to the accident was the flight instructor’s delayed remedial action.

NTSB analysis

The purpose of the instructional helicopter flight was to review advanced maneuvers in preparation for the student pilot's upcoming stage check. The student stated that he and the flight instructor had conducted 3 approaches and landings before initiating a practice autorotation. He stated that as the helicopter descended through 100 ft in the autorotation, the instructor applied throttle in an attempt to recover, but the engine did not respond. A witness stated that, about halfway through the helicopter's autorotative descent, it appeared to momentarily level off before abruptly entering a nose-down attitude and descending to ground contact. A surveillance video showed the helicopter descending rapidly at a steep angle in the last 2 seconds before impact. The helicopter impacted terrain about 700 ft north of the runway threshold, fatally injuring the instructor and seriously injuring the student. Postaccident examination of the helicopter and a test run of the engine revealed no mechanical anomalies that would have precluded normal operation.

The flight school's published procedure for practice autorotations instructed the pilot to initiate the maneuver first by lowering the collective, then reducing the throttle to idle. The practice of reducing the throttle to idle was contrary to manufacturer guidance for this maneuver, which stated that the throttle should be adjusted only enough to allow for a small tachometer needle separation in order to reduce the chance of inadvertent engine stoppage during the maneuver. The practice of reducing throttle to idle introduced greater susceptibility to a loss of engine power, though it could not be determined whether a loss of power occurred before the accident.

Based on conflicting statements from the student, it could not be determined who was controlling the helicopter during the entry into and throughout the autorotation before about 100 ft. The helicopter's trajectory described by a witness and as captured on surveillance video suggested a rapid, uncontrolled descent during the final portion of the autorotation, consistent with a main rotor stall; likely as a result of a premature application of collective pitch. This allowed the rotor rpm to decay below the normal operating range at an altitude that was insufficient for power recovery. In the event that the helicopter did experience a loss of power during the maneuver, the helicopter should have been able to attain a safe landing following a steady-state autorotation. The helicopter manufacturer published notices to pilots warning that main rotor stall due to low rotor rpm could occur rapidly, at any airspeed, and that if allowed to develop, recovery could become "virtually impossible."

Text from the NTSB. Open the NTSB docket for ERA15FA085 for the full record.