On July 25, 1993, at 1510 eastern daylight time, an unregistered Maxair two-seat ultralight aircraft, owned and operated by Rufus E. Hinton, crashed while making a forced landing, following loss of engine power near Wimauma, Florida, while on a 14 CFR Part 91, instructional flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft received substantial damage and the nonrated pilot/instructor and nonrated dual student received serious injuries. The flight originated at Wimauma, Florida, on July 25, 1993, at 1440. The nonrated flight instructor stated he was giving an orientation flight to the student. After about 30 minutes of flying they preceded back to the airport. They descended from 600 feet to 200 feet and leveled off. Suddenly, the engine seized. He initiated a forced landing to a field. At an altitude of 20 feet above the field the airspeed decreased rapidly and the aircraft stalled. The aircraft then descended uncontrolled and crashed. Postcrash examination of the engine revealed the no. 1 cylinder spark plug to be fouled by unknown matter. The no. 1 piston had score marks on each corner. According to the mechanic who inspected the engine, the score marks are typical of a seizure of the piston in the cylinder caused by rapid cooling of the engine due to a rapid descent at idle power. (See attached FAA inspector statement and mechanic statement.) The aircraft was not registered with the FAA at the time of the accident. The aircraft was qualified to receive a United States Ultralight Association exemption to FAA registry requirements. The pilot had applied for the exemption from USUA; however, at the time of the accident the exemption had not been issued.