According to tower controller statements, the airplane completed the right downwind leg of the traffic pattern, then flew a "short" base leg. As it began turning on to the final approach, about 100 feet above the ground, the airplane yawed, rolled right, and nosed down into the ground. Postflight examination of the airplane revealed substantial damage to the fuselage and wings. The pilot was seriously injured and could not recall the events that transpired during the accident sequence. A subsequent examination of the wreckage performed by a Federal Aviation Administration inspector found no preexisting mechanical anomalies with the airplane.