The aircraft spun into the ground from about 500 ft AGL. The vehicle had been observed by witnesses to be in slow flight making turns to the right. After 180 degrees of turn, the nose pitched down to about 60 degrees and the left wing went down to about 75 degrees of bank angle. After 4 and 1/2 turns, the ultralight impacted the ground in an open field just east of the airport. The pilot was seriously injured and does not remember the accident, according to a witness who talked to him at the scene. The pilot stated that he was demonstrating spins to prospective buyers and a piece of the propeller came off and hit him in the head, knocking off his helmet and rendering him unconscious. This happened at 1200 ft, he said in a telephone interview. The pilot's helmet was found 8 ft behind the pilot at the scene. One blade of the propeller was broken off at the root of the blade and was found alongside the propeller in the wreckage. One of the first persons on the scene said that the pilot had his helmet on at impact.