CREW WAS HAULING 20""X10' VENT PIPE INTO MINE USING TIMBER TRUCKS AND ELECTRIC LOCI. MECHANIC WAS RIDING ON TIMBERTRUCK BEHIND TIMBERTRUCK WITH VENT PIPE ON IT. WHILE TRAVELLING INTO MINE A VENT PIPE HOOKED A TIMBER PUSHING THE VENT PIPE BACK HITTING THE MECHANIC ON HIS LOWER LEFT LEG. ACCIDENT IN MAIN HAULAGE DRIFT