After being assembled, the ultralight vehicle was taxi tested, and on 9/19/83, it was flown on its 1st flight. The next morning it was taxi tested again. At approximately 0855 PDT, the pilot took off on a local flight. At approximately 0945, after making a touch-and-go landing, the pilot was turning final for another landing when the ultralight began an uncommanded, smooth, left roll at about 200 to 300 ft AGL. Corrective aileron was applied, but the ultralight continued to roll left and subsequently crashed in a steep nose down attitude. An exam revealed there was continuity to all flight controls. Both wings and the fuselage tube were damaged during the accident. The left wing trailing edge tube had failed at a bolt hole where the left rear strut brackets were installed. Additional washers had been installed on the bolt and the self-locking nut had been torqued until the threads on the nut were beyond the threaded portion of the bolt. There was evidence that the rear spar had separated and the bolt had rotated approximately 20 degrees prior to impact. No airspeed indicator installed.