On September 24, 2000, about 1730 Eastern Daylight Time, a homebuilt Rans-12, N91337, was substantially damaged while landing at the Warrenton Airpark, Warrenton, Virginia. The certificated private pilot was seriously injured. Visual meteorological conditions prevailed and no flight plan was filed for the personal flight conducted under 14 CFR Part 91. According to the pilot, as he arrived in the airport area, he observed a thunderstorm "moving in" from the west/northwest, at a distance of about 10 miles. He entered the traffic pattern for the north runway and did not experience any noticeable turbulence. As he was turning left base for final approach, about 150-200 feet above the ground, a gust of wind came over the trees and "flipped the airplane over." The airplane descended, impacted trees, and crashed onto the runway. The pilot additionally stated that he was at an indicated airspeed of 50-55 mph when he encountered the wind gust and that the stalling speed of the airplane was 37 mph. Federal Aviation Administration (FAA) Airplane Flying Handbook, FAA-H-8083-3 stated, "After turning onto the base leg, the pilot should start the decent with reduced power and an airspeed of approximately 1.4 Vso." FAA Advisory Circular AC 61-67B stated, Stall and Spin Awareness Training, "Turbulence can cause an aircraft to stall at a significantly higher airspeed than in stable conditions. A vertical gust or windshear can cause a sudden change in the relative wind, and result in an abrupt increase in angle of attack. Although a gust may not be maintained long enough for a stall to develop, the aircraft may stall while the pilot is attempting to control the flightpath, particularly during an approach in gusty conditions. When flying in moderate to severe turbulence or strong crosswinds, a higher than normal approach speed should be maintained. In cruise flight in moderate or severe turbulence, an airspeed well above the indicated stall speed and below maneuvering speed should be used." The winds recorded at an airport located 5 miles southwest of the accident site, at 1703, were from 350 degrees at 5 knots. At 1723, the winds were from 350 degrees at 6 knots, and at 1744, the winds were from 010 at 8 knots.