WHILE HE WAS TAKING A D-10 DOZER OVER A 4 FOOT DROP, THE DOZER BLADE CAUGHT THE GROUND CAUSING THE DOZER TO SUDDENTLY STOP IN AN STEEP POSITION. THE SUDDEN STOP AND THE STEEP INCLINE, CAUSED T HE OPERATOR TO BE THROWN FORWARD STRIKING HIS HEAD ON A METAL BEAM IN THE TOP OF THE CAB. THE OPERATOR WAS IN AN AIR RIDE SEAT AND WEARING A SEAT BELT.