On December 21, 2005, about 1638 central standard time, an experimental amateur-built Elliott Quad City Ultralight Challenger II, N115BT, operated by an unlicensed pilot, sustained substantial damage on impact with terrain following a flat spin encountered on base turn to Phillips Field Airport (MO23), near California, Missouri. The personal flight was operating under 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed at the time of the accident. No flight plan was on file. The pilot sustained minor injuries. The local flight originated from MO23 about 1500. The pilot, in part, stated: On Dec. 21, 2005, I ... lost control, went into a stall-spin, and landed nearly flat in the ditch just south of Little Moniteau Road. ... The accident was caused by pilot error. I forgot that the owner's manual said that "Solo flight is restricted to the front seat." ... I had started practicing landings on Dec. 21, 2005, at 3:00 PM. ... After two test flights I decided to see if I could taxi the plane from the rear seat. I did that and I noticed that I could see the entire instrument panel from the rear seat. I intended to taxi some more, but then impulsively I decided to take off. I forgot that one either has to solo from the front seat or add weight to the front seat to solo from the rear seat. ... As I turned base to the west I was still trying to trim the plane and lost attention to airspeed. It must have dropped below 40 mph. This plane stalls at about 32 mph. I was trying to turn left final but the plane swung right, then entered a stall-spin from about 400 ft. AGL. I tried two different maneuvers to get out of the spin, but they did not work. I put the nose down to regain airspeed, then I released the controls, to no avail. Then I kept some backpressure to keep the plane spinning flat, and it spiraled down and hit the ground about 4:38 PM. ... The pilot's safety recommendation was, "More training in type would have been useful. A placard could be required to "Solo from front seat only"."