On February 5, 1999, at 1343 hours mountain standard time, a homebuilt Rotorway Exec 162, N88XS, owned and operated by the pilot, encountered a wind gust at the Ryan Field, Tucson. The student pilot reported that he had been hovering in the area and was not having any difficulty handling the controls when suddenly he lost control, the right skid impacted the ground, and the experimental helicopter rolled over. The helicopter was substantially damaged during the flight, which was performed under 14 CFR Part 91, and the student pilot received a minor injury. Visual meteorological conditions prevailed at the time of the solo instructional flight that originated from the airport about 1230. In the pilot's report, he stated that during the hour preceding the accident the local wind speed was between 0 and 3 miles per hour. However, suddenly "a severe gust of wind (estimated at 25 - 30 mph) came from 90 degrees . . . and hit me on my left side." A witness, who was standing on the ramp about 50 yards from the accident site, reported observing the helicopter hovering 2 feet above the ground when an estimated 20 to 25 knot wind gust pushed the helicopter sideways. Thereafter, its right skid touched the ground and it rolled over.