A B737-300 INCURRED A MAJOR FUEL SPILL DURING A WING AFT BOOST PUMP REPLACEMENT. BOOST PUMP REMOVAL MANUAL SHUTOFF VALVE FAILED OPEN.

Narrative

JUN/MON/03; AN ACFT ARRIVED WITH A 1 DAY OLD MEL ON ITS L AFT FUEL BOOST PUMP. ALTHOUGH IT WASN'T REQUIRED TO BE FIXED; MAINT CTL TOLD US TO CHANGE THE BOOST PUMP TO CLR THE MEL. A FELLOW MECH AND I ARRIVED AT THE ACFT AND PROCEEDED TO REMOVE AND REPLACE THE PUMP. AFTER GAINING ACCESS TO THE PUMP; I CLOSED THE SHUTOFF VALVE. I REMOVED THE ELECTRICAL CONNECTOR AND LOWER MOUNTING HARDWARE; WITH THE LOCKING ARMS STILL IN PLACE. I BEGAN TO DRAIN FUEL FROM THE PUMP CASE INTO A BUCKET WHILE OTHER MECH WAS PREPARING THE NEW PUMP FOR INSTALLATION. WE BOTH FELT THE CASE WAS EMPTY OF FUEL. THEN; WITH OTHER MECH HOLDING THE BUCKET FOR ANY EXCESS FUEL; I POSITIONED THE LOCKING ARMS AWAY FROM THE PUMP ASSEMBLY AND REMOVED THE PUMP FROM THE WING. THIS IS WHEN WE KNEW THE SHUTOFF VALVE HAD FAILED IN THE OPEN POS. FUEL WAS POURING OUT OF THE CAVITY AT SUCH A RATE WE HAD TO CLR THE WING AREA AND START EMER PROCS. PWR WAS SHUT OFF AND THE ACFT WAS EVACED. WHEN THE FUEL LEVEL DROPPED TO WHERE IT WAS NOT LEAKING FROM THE ACFT; THE FIRE DEPT ADVISED US THAT IT WAS CLR FOR US TO FINISH OUR WORK TO CLOSE THE OPENING. WE THEN REPLACED THE FUEL PUMP WITHOUT INCIDENT AND CLRED THE MEL; ADDING A MAINT NOTE FOR THE DEFECTIVE FUEL SHUTOFF VALVE. THERE WERE NO INJURIES AND AN ESTIMATED 900 GALS OF FUEL SPILLED ONTO THE RAMP AREA. THE MAJORITY OF FUEL WAS CONTAINED BEFORE REACHING THE STORM DRAINS; AND 100% OF THE FUEL WAS RECOVERED ON ARPT PROPERTY. I KNOW THE CAUSE OF THIS INCIDENT WAS THE FAILURE OF THE FUEL SHUTOFF VALVE. THE CONTRIBUTING FACTOR WAS THAT WE DID NOT KNOW THE PUMP CASE STILL HAD FUEL IN IT; AFTER IT APPEARED TO BE DRAINED. TO CORRECT THIS PROB IN THE FUTURE; I SUGGEST REMOVING THE CASE DRAIN/RELIEF VALVE FROM THE PUMP ASSEMBLY TO CONFIRM THERE IS NO REMAINING FUEL; AND VERIFYING THE CASE DRAIN/RELIEF VALVE HAS NOT FAILED OR BECOME BLOCKED. IF THE FUEL SHUTOFF VALVE DOES FAIL; FUEL WOULD TRICKLE OUT A 1/4 INCH DRAIN HOLE VERSUS THE 3 INCH FUEL SUPPLY LINE.