CHI03LA128

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.

On May 16, 2003, at 1930 central daylight time, a Robinson R44 helicopter, N227G, piloted by a commercial pilot, was substantially damaged shortly after landing at a private heliport in Lake Ozark, Missouri. The helicopter was standing with its engine running at the time of the accident. The flight was being conducted under 14 CFR Part 91 and was not on a flight plan. The pilot and passenger reported minor injuries. The flight departed the Cape Girardeau Regional Airport (CGI), Cape Girardeau, Missouri, approximately 1730.

The pilot's written statement reported that he had completed a landing to a private heliport under a light wind. He stated that the collective was fully down. Shortly after touchdown, the pilot reportedly heard a grinding noise coming from the rear of the helicopter. He noted that as the noise increased in intensity, "the helicopter started to shake, then jump around and rotate (still on the skids) in a clockwise direction for about 30 degrees." He reported the movement of the helicopter began about 4 seconds after full down collective was reached.

The pilot reported that once the helicopter had rotated 30 degrees to the right, "it immediately started violent movements counterclockwise and proceeded to thrash around in all directions." He continued: "The next few seconds are a bit of a blur but 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 landing area was a portion of a local motel parking lot, which had been set aside for use as a private heliport. Approach to the area was made over a wooded ravine adjacent to the lot. The motel and a store were located on each side of the parking lot.

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 was conducted. The helicopter exhibited scrape marks and bucking of the skin and sub-structure on all sides of the fuselage. The engine firewall was deformed and buckled at the aft end. The skids were deformed, segments were broken off, and the attachment fittings were bent or broken. Both windshields and the left front door window were broken out.

The tail boom was separated from the fuselage. The lower vertical stabilizer was bent and crushed upward. The tail rotor drive shaft was fractured. Tail rotor transmission continuity was verified.

The main rotor blades were broken off completely within two feet of the hub. The blades exhibited bending and portions of the skins were delaminated. Main rotor gearbox continuity was confirmed by rotating the forward portion of the tail rotor drive shaft. Corresponding rotation of the main rotor shaft was observed.

Cyclic and collective linkages were intact and produced the correct responses. Throttle control continuity was verified. The anti-torque pedals were intact and free to move. Movement in the pedals produced a corresponding motion in the attached linkage.

The hydraulic actuators and hydraulic pump were tested. Although they had sustained damage, they were found to be functional. There was no evidence associated with a pre-accident failure or malfunction.

Engine continuity was verified and compression obtained on all cylinders. The spark plugs electrodes were light gray in color, consistent with normal wear.

Weather conditions recorded at the Lee C. Fine Airport (AIZ), located 4 miles southeast of the accident site, at 1935, were: overcast clouds at 800 feet above ground level, 7 miles visibility and winds from 060 degrees at 12 knots, gusting to 19 knots.

The Federal Aviation Administration (FAA) Rotorcraft Flying Handbook describes helicopter flight characteristics and operating techniques. The 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.