SEA83FU009

After being assembled, the ultralight vehicle was taxi tested, and on 9/19/83, it was flown on its 1st flt. The next morning it was taxi tested again. At aprx 0855 PDT, the plt took off on a local flt. At aprx 0945, after making a touch-and-go landing, the plt 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 & subsequently crashed in a steep nose down attitude. An exam revealed there was continuity to all flt controls. Both wings & 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 & 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 & the bolt had rotated aprx 20 deg prior to impact. No airspeed ind installed.