A B747 FLC FLIES THEIR ACFT WITH AN INOP FUEL DUMP SYS OVER THE ACFT GROSS WT ALLOWED AS PER THE MEL ON 2 SEPARATE LEGS.

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SUBJECT: FAILURE TO FOLLOW MEL PROCS. CREW RPTED FOR DUTY AT MIAMI OPS. WHILE THE FO AND I WERE GOING OVER THE PAPERWORK IN OPS; THE FE WAS CHKING ON THE ACFT STATUS WITH MIAMI MAINT. WHEN THE FE WAS FINISHED IN MAINT HE CAME UP TO OPS WHERE THE FO AND I WERE GOING OVER THE PAPERWORK. HE RPTED TO ME THAT THERE WERE 3 DMI'S ON THE ACFT. 2 OF THE DMI'S WERE INCONSEQUENTIAL; ONE WAS A #4 MAIN TANK REFUELING VALVE. I ASKED THE FE IF THERE WERE ANY SPECIAL PROCS AND HE SAID NO. THIS TOOK PLACE IN THE MIAMI OPS OFFICE. IN THE COCKPIT I ASKED AGAIN IF THERE WERE ANY SPECIAL PROCS -- ONCE AGAIN I WAS TOLD NO. I TOOK HIS WORD WITHOUT CHKING MYSELF. A ROUND TRIP FLT WAS CONDUCTED WITHOUT INCIDENT. THE FLT WAS FROM MIAMI TO BOGOTA BACK TO MIAMI. THE MEL ITEM THAT REQUIRED A SPECIAL PROC WAS A #4 MAIN TANK REFUELING VALVE. I BELIEVE THAT MIAMI MAINT MISLED THE FE WHO IN TURN TOLD ME WHAT MAINT HAD SAID WITHOUT REALLY CHKING. HE NEVER INDICATED TO ME THAT HE WASN'T SURE OR HAD NOT LOOKED IT UP. HAD HE; WE ALL COULD HAVE GONE OVER IT TOGETHER. MIAMI MAINT SAID THAT ONE JETTISON PUMP WAS TO BE CONSIDERED INOP (WHICH IS NOT THE CASE). THE MEL PROC SHOULD HAVE LIMITED OUR TKOF WT TO MAX ZERO FUEL WT OF 590000 LBS. BECAUSE WITH THE #4 MAIN TANK REFUELING VALVE DMI'ED OPENED; THE FUEL JETTISON SYS MUST BE CONSIDERED INOP; NOT JUST ONE JETTISON PUMP INOP; BUT THE WHOLE SYS! THEREFORE; BOTH TKOFS -- ONE AT 714000 LBS AND THE OTHER AT 639000 LBS -- WERE NOT LEGAL. AS ALWAYS; THE CAPT IS RESPONSIBLE. I SHOULD HAVE CHKED THE MEL MANUAL MYSELF.