On December 15, 1995, at 0423 eastern standard time (est), a Swearingen SA-226AT, N31AT, operated by Air Transportation Inc. of El Paso, Texas, was substantially damaged when it landed gear up at Detroit Metropolitan Wayne County Airport, Detroit, Michigan. The pilot reported no injuries. Instrument meteorological conditions prevailed for the flight, which operated on an IFR flight plan. The 14 CFR Part 91 repositioning flight had departed Flint, Michigan, at 0401 est, with an intended destination of Louisville, Kentucky. The pilot reported that shortly after departing Flint, the airplane's left engine started to surge. The airplane also began experiencing intermittent electrical surges causing the instrument panel lights, cabin lights, and radios to go OFF and ON. The pilot stated that he decided to divert to Detroit and land because the airport had emergency equipment available. The pilot stated that he did not advise ATC of any problem while diverting because he believed that he could land without incident. The pilot did not attempt to secure the left engine before landing because it was still developing some usable power. The pilot stated he placed the gear select handle in the DOWN position and he observed three green gear down and locked lights. He stated that prior to touchdown, he positioned both power levers to flight idle and no gear warning horn sounded. The airplane landed on the runway gear up. Postaccident examination and testing of the landing gear and electrical power revealed no abnormalities. The landing gear emergency extension functioned properly. The landing gear indicating system showed a safe gear indication when the gear was extended during examination. Substantial damage to the gear doors was observed, but no damage to the landing gear was observed.