N6057

Following an uneventful preflight inspection of the helicopter, the flight instructor and the student pilot boarded the helicopter for a local instructional flight. About 10 seconds after the student pilot lifted the helicopter into a stable hover with a left crosswind, the helicopter began a slow yaw to the right that the student was unable to arrest through the application of the left anti-torque pedal. As the rate of the yaw increased, the flight instructor took control of the helicopter and attempted to stop the yaw with the application of additional left anti-torque pedal, to no avail. The flight instructor then reduced engine power, arresting the yaw, and performed an autorotation to the ground after an estimated 1 3/4 total revolutions. As the helicopter touched down, it rocked backwards, and the tail rotor struck the helicopter's ground handling dolly, resulting in substantial damage to the tail rotor. The flight instructor did not report any pre-impact mechanical malfunctions or failures of the helicopter. Archived weather data showed that wind velocity about the time of the accident was 15 knots, with gusts to 24 knots. According to Federal Aviation Administration Advisory Circular 90-95, pilots are advised to be aware that, "…if a considerable amount of left pedal is being maintained, a sufficient amount of left pedal may not be available to counteract an unanticipated right yaw." Additionally, pilots should, "Stay vigilant to power and wind conditions."