Accident Narrative
The helicopter was substantially damaged shortly after landing when an isolated wind gust, funneled by a ravine, caused the aircraft to become light on its skids. The subsequent weather vaning tendency, due to the tailwind orientation of the gust, resulted in a loss of control. The pilot reported that, shortly after touchdown, the helicopter began to shake, then jump and rotate on its skids in a clockwise direction for about 30 degrees. Once the helicopter rotated 30 degrees to the right, it immediately experienced violent movements counterclockwise and proceeded to thrash around in all directions. The pilot noted a sensation of the tail going up with a rolling motion to the left side. The helicopter eventually came to rest back on its skids.
Witness Account
A witness to the accident reported that the helicopter had landed and was on the ground for 15 to 20 seconds when the wind came up from the bottom of the hill, causing the trees to blow. The witness observed the helicopter tilt to the right, and the main rotor struck the pavement. The helicopter then rotated to the left before coming to rest. The witness noted that the winds appeared calm both before and after the accident.
Post-Accident Examination
A post-accident examination of the helicopter revealed no anomalies consistent with a pre-accident failure.
FAA Handbook Reference
The Federal Aviation Administration (FAA) Rotorcraft Flying Handbook states that as an engine turns the main rotor system counterclockwise, the helicopter fuselage turns clockwise. A tail rotor is often incorporated to counteract this torque-induced turning tendency. The handbook notes that when a helicopter becomes light on its skids, torque tends to cause the nose to swing or yaw to the right. With a tailwind, the helicopter is affected by weathercock stability, causing it to attempt to weathervane into the wind. In the absence of an opposing pedal input, the helicopter may start an uncommanded turn either to the right or left depending on wind direction.