ATTEMPTING TO REMOVE 3"" PIPE COUPLING FROM FRONT OF WATER TANK MOUNTED ON PETERBILT TRI-AXLE WATER DISTRIBUTION TRUCK. STANDING BETWEEN TRUCK CAB & MOUNTED TANK WITH LEFT FOOT ON TRUCK FRAME & RIGHT FOOT PRESSING ON THE HANDLE OF A 36"" PIPE WRENCH, JAWS SLIPPED CAUSING LOSS OF BALANCE & FALLING, BUTTOCKS/ PELVIS HITTING TRUCK FRAME FIRST THEN LANDING IN SITTING POSITION ON GROUND.