On April 13, 2005, approximately 1130 central daylight time, a Robinson R44 II single-engine helicopter, N7535S, was substantially damaged during engine shut-down at the West Woodward Airport (WWR), near Woodward, Oklahoma. The private pilot and his passenger were not injured. Visual meteorological conditions prevailed, and a flight plan was not filed for the personal flight, which was being conducted under the provisions of 14 Code of Federal Regulations Part 91. The flight departed from the Purcell Municipal Airport (3O3), near Purcell, Oklahoma, approximately 1100, with WWR as its intended destination. In a written statement, the pilot reported that upon arrival to WWR, he elected to land the helicopter into the prevailing "strong northerly wind". Immediately after touching down on the portable helipad, the pilot initiated the prescribed "cool down procedures." After "sufficient time", the pilot disengaged the clutch, waited 30-45 seconds, and then shut-down the engine. With the cyclic locked in the center position, the clutch light went out, as the rotors were slowly winding down. As the pilot reached back to apply the rotor brake, "a strong wind gust caused the rotor to lift and strike the tail boom." The pilot further reported that at the time of rotor blade impact, "the engine was turned-off and the clutch was totally disengaged." A review of photographs that were provided to an NTSB investigator revealed structural damage to the tail boom, as well as the main rotor blade. At 1130, the automated weather observing system at WWR reported wind from 350 degrees at 11 knots, gusting to 15 knots, 10 statute miles visibility, a clear sky, temperature 15 degrees Fahrenheit, dew point 6 degrees Fahrenheit, and a barometric pressure setting of 30.13 inches of Mercury.