The pilot had flown the ultralight vehicle to his employer's house and landed in the back yard. During the landing, the vehicle sustained damage which included a bent main landing gear axle, a sheared kingpin bolt, and a cracked tail boom to tailmount sleeve. A hardened bolt was purchased at an automotive parts store and partial repairs were made; however, the damage to the tail boom was not detected. The ultralight was preflighted and the pilot took off. After circling once and waving, he started on the return flight. Shortly after that, witnesses heard the engine slow or stop as the ultralight pitched nose down into a steep dive and crashed. An exam of the wreckage revealed that the right tail boom tube had separated in flight. During impact, it had penetrated the horizontal stabilizer sail without bending. The left tail boom was bent during impact. A picture of the ultralight (taken before takeoff) revealed that the horizontal stabilizer was slightly angled from the horizontal axis of the ultralight.