Accident Details
A helicopter conducting aerial photography of a large airport expansion project experienced a loss of control while maneuvering in a hover. The helicopter approached the construction site in an out-of-ground-effect hover taxi with a quartering left tailwind. It then turned right and slowed to a stationary hover about 250 feet above ground level, now in a direct tailwind condition.
Once stabilized in the hover, the pilot initiated a rapid 180-degree pedal turn to the right around the mast. After a brief pause, a second rapid right pedal turn was begun. The second turn was performed at a faster rate than the first, and the helicopter continued into a spinning, vertical descent until it struck the ground.
Post-Accident Examination
Post-accident examination of the helicopter revealed no mechanical anomalies. The collective control was found in the full up position, and the main rotor blades exhibited signatures consistent with low rotor speed (rpm) at ground contact.
Pilot Experience
Company flight records indicated the pilot had accumulated 531 hours of total flight experience, including 60 hours in airplanes. The pilot's experience in the accident helicopter make and model was 87 hours.
Relative Wind Considerations
A relative wind chart published by the helicopter manufacturer depicts a compass rose with a shaded area indicating winds from between 120 degrees and 240 degrees, at speeds above 5 knots and up to 17 knots. A note on the chart warns that an unanticipated right yaw may occur when operating in those shaded areas.
According to FAA Advisory Circular (AC) 90-95, any maneuver requiring operation in a high-power, low-airspeed environment with a left crosswind or tailwind creates an environment where unanticipated right yaw may occur. The AC further advises that there is greater susceptibility for loss of tailrotor effectiveness in right turns, and the phenomenon may occur in varying degrees in all single main rotor helicopters at airspeeds less than 30 knots.