The student pilot/owner reported that he had about 329 total hours of flight experience, including 250 hours in single-engine airplanes, 58 hours in ultralight aircraft, and 21 hours in gliders. He stated that the accident aircraft was a modified version of the Kolb Firefly ultralight. The single-seat aircraft was registered with the Federal Aviation Administration (FAA) as an experimental amateur-built "Buzzard 1 Motorglider." A week before the accident, the aircraft was assembled by a certificated aircraft mechanic. The pilot reported that he had previously flown the aircraft one time, for about 30 minutes. During the accident flight takeoff roll, the pilot noticed that the aircraft did not "feel right," but he opted to continue. About 250 feet above the ground, the outboard right wing "started to flutter in a violent way," and in response, the pilot reduced engine power and aircraft speed. He then "lost all control." The aircraft impacted the ground in a field, and seriously injured the pilot. About 1 hour after the accident, skydivers noticed the wreckage, and summoned emergency services to rescue the pilot. Examination of the wreckage by an FAA inspector revealed that the bolt that was supposed to attach an aileron pushrod to the outboard aileron bellcrank was lying loose in the bottom of the wing bay; neither the nut nor the cotter pin was recovered. Examination of the opposite aileron bellcrank revealed that the corresponding bolt and nut were not secured by a cotter pin, despite provisions for one. The mechanic who assembled the aircraft stated that he did not install cotter pins in either of the two noted bolts. The pilot stated that he did not inspect the aircraft after assembly, or before the flight.