The pilot reported that he was flying straight and level at 200 feet above ground level(AGL) and about 55 knots when the aircraft sideslipped to the left and nosed over. The nose continued to drop to the almost vertical position and the speed built up to 60 knots. At about 100 feet AGL, the pilot attempted to pull out of the dive but the aircraft did not respond and struck the ground in the almost vertical nose down attitude. Eyewitnesses reported that the aircraft was in the circuit area when they saw a wing drop and the nose of the aircraft fall sharply. The AUF Safety Officer reported that the flying surfaces of the aircraft were undamaged and he suspected that a stall/spin DEPARTURE was experienced by the pilot. In addition to cuts and bruises, the pilot suffered compound fractures to both legs and his right arm. This accident was not the subject of a formal on scene investigation.