On July 16, 1999, at 0955 mountain daylight time, a Navion B, N5304K, registered to and operated by the pilot as a 14 CFR Part 91 personal flight, landed hard at the Sulphur Creek Ranch airstrip, located about 30 miles northwest of Stanley, Idaho. Visual meteorological conditions prevailed at the time and no flight plan was filed. The airplane was substantially damaged and the private pilot and flight instructor were not injured. The flight departed from McCall, Idaho, about 30 minutes prior to the accident. The pilot reported that he was participating in a mountain flying seminar that consisted of practicing takeoffs and landings at high density altitude to short dirt/grass airstrips in canyons and valleys. The pilot stated that he circled the airstrip twice to assess the landing field. The pilot reported that he entered the pattern at about 900 feet above ground level for landing on runway 26 with a tailwind of about ten knots. The pilot stated that just prior to flaring, the airplane was about 20 to 30 feet above ground level, when it suddenly went straight down. The pilot applied full power, however, the airplane continued to descend and landed hard on all three landing gear. The landing roll was uneventful and the pilot taxied the airplane to parking to check for damage. The pilot reported that he felt that a sudden down draft or high tailwind gust caused the airplane to drop like it did. The flight instructor reported that the pilot did not have time to react to the tailwind gust which resulted in the aircraft's rapid descent and subsequent hard landing. The Idaho Airport Facilities Directory recommends "land runway 26 (upstream), takeoff runway 8 (downstream); one way strip."