Accident Sequence
While performing aerial photography operations at an altitude of approximately 80 feet above ground level and an estimated groundspeed of 20 knots, the helicopter's engine suffered a partial loss of power. The pilot immediately executed the emergency checklist and configured the aircraft for an autorotation. During the descent, the pilot reported that the rotor and engine would not disengage, and rotor rpm continued to decrease. The helicopter subsequently struck the ground with considerable force, resulting in damage to one main rotor blade and the tail boom.
Post-Accident Examination
Following the accident, the engine was ground-tested at idle power and operated normally. A teardown and inspection of both the engine and clutch assembly revealed no mechanical anomalies or pre-existing faults.
Operational Considerations
A review of the helicopter's height-velocity diagram indicated that, under the conditions present at the time of the incident, the aircraft was operating outside the recommended safe operating envelope.