The pilot flew alongside a vehicle travelling along a gravel road. The two occupants of the vehicle knew the pilot well and they were not surprised to see the aircraft. The aircraft was then flown across the road in front of the vehicle at an approximate height of 250 feet. Whilst continuing their progress along the road, the occupants observed the aircraft perform a "wing over" manoeuvre. The aircraft failed to recover from the resultant steep dive, striking the ground at approximately 65 degrees nose down. The two occupants of the vehicle extracted the pilot from the crushed cabin. Detailed examination of the wreckage failed to reveal any abnormalities.