Aircraft Accident/Incident Report

Lake Ozark, Missouri 65049
Friday, May 16, 2003 19:30 CDT

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 started to shake, then jump around and rotate (still on the skids) in a clockwise direction for about 30 degrees." He noted that once the helicopter rotated 30 degrees to the right, "it immediately started violent movements counterclockwise and proceeded to thrash around in all directions." He continued: "It felt as if the tail went up … with a rolling to the left side with a great deal of thrashing around." He noted that the helicopter came to rest back on its skids. The private heliport was adjacent to a wooded ravine. A witness to the accident reported that the helicopter had landed and was on the ground for 15 - 20 seconds when the wind came up from the bottom of the hill and the trees started blowing. He reported the helicopter tilted to the right and the main rotor struck the pavement. The helicopter then rotated to the left prior to coming to rest. The witness noted that the winds appeared to be calm both prior to, and after, the accident. A post-accident examination of the helicopter revealed no anomalies consistent with a pre-accident failure. The Federal Aviation Administration (FAA) Rotorcraft Flying Handbook states: "As an engine turns the main rotor system in a counterclockwise direction, the helicopter fuselage turns clockwise." A tail rotor is often incorporated into helicopter designs in order to counteract this torque-induced turning tendency. The handbook notes that, as 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 in which the helicopter attempts 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 upon the wind direction, according to the handbook.

An inability to maintain control of the helicopter during the wind gust and brief tail wind. The ravine, wind gust and tailwind were contributing factors to the accident.