B727 CARGO HAD THE #3 FUEL GAUGE DEFERRED. IN REFUELING NECESSARY TO XFER FUEL FROM #3 TANK TO #2 AND AFTER FUELING XFER A KNOWN AMOUNT BACK INTO #3. SO NOTED ON WALKAROUND DEFUEL VALVE OPEN. FUELERS ADVISED EVERYTHING BACK TO NORMAL AND THE FLT DEPARTED. ARR BERGAMO; ITALY; DEFUEL VALVE OPEN.

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I AM THE FE ON A B727 CARGO ACFT. WE WERE OPERATING WITH THE #3 FUEL GAUGE PLACARDED INOP. THEREFORE; UPON ARR TO BRUSSELS; WE HAVE TO PUMP ALL THE FUEL INTO THE #2 FUEL TANK TO OBTAIN A KNOWN QUANTITY; THEN PUMP A KNOWN QUANTITY INTO TANK #3. 3 OF OUR OWN MECHS WERE AT THE ACFT AND WERE IN THE PROCESS OF DOING THIS WHILE I WAS PERFORMING MY PREFLT DUTIES. WHEN I DID MY WALKAROUND; THE MECHS HAD THE MANUAL DEFUELING VALVE OPEN AND THE FUELING PANEL OPENED BECAUSE THEY WERE STILL XFERRING FUEL. THE RAMPERS THEN FINISHED LOADING US AND I CLOSED THE CARGO DOOR AND PUT UP THE TAILSTAND; AND WENT BACK INTO THE COCKPIT AND GAVE THE RAMPERS OUR PAPERWORK. THEN THE MECH CAME UP INTO THE COCKPIT AND SAID HE HAD XFERRED A KNOWN QUANTITY OF FUEL INTO TANK #3; THAT TANK #3 WAS OPERATING NORMALLY; THAT HE HAD CLRED THE WRITE-UP AND THAT THE ACFT WAS ALL CLOSED UP AND READY TO GO. HE THEN LEFT THE COCKPIT; CLOSED THE FORWARD ENTRY DOOR AND PULLED AWAY THE STAIRS. I DID NOT GO BACK OUTSIDE AND VISUALLY CHK THAT THE 3 MECHS HAD CLOSED THE MANUAL DEFUELING VALVE. UPON ARR AT BERGAMO; ON MY POSTFLT; I SAW THE MANUAL DEFUELING VALVE WAS OPENED AND REALIZED IT HAD BEEN OPEN THE ENTIRE FLT. I TOLD THE CAPT WHO WROTE IT UP AND ONE OF OUR MECHS INSPECTED IT AND CLRED IT. THIS COULD HAVE BEEN AVOIDED IF I HAD FORCED THE MECHS TO BRING BACK THE STAIRS; AND VISUALLY INSPECTED WHAT THEY HAD BEEN WORKING ON MYSELF; EVEN WHEN IT IS OUR OWN MECHS. THIS IS WHAT I DID THE REST OF THE WK AFTER THIS INCIDENT AND WILL CONTINUE TO DO FROM NOW ON.