On August 25, 1994, at 1915 hours central daylight time, a Tierra-2, 2 seat registered ultralight aircraft, N6161Q, operated by Robert Idler of West Des Moines, Iowa, impacted wooded, hilly terrain 8 miles southwest of Des Moines, Iowa, and was destroyed. The Airline Transport rated pilot and passenger received minor injuries. No fire occurred. Visual meteorological conditions existed at the time of the accident and no flight plan was filed. The flight operated under 14 CFR Part 91, originated from Chandler Field on the Gene Wilson Farm in rural Prole, Iowa, at 1900 central daylight time. The pilot reported that he took off in clear weather and was overrun by the gust front of an approaching thunderstorm which was located between 7 and 15 miles away. The aircraft entered an uncommanded right descending turn, which the pilot reported he was unable to counter with full left aileron input. He also reported that the engine operated normally until the aircraft impacted trees.