AN MD80 ACR FLC HAS FUEL XFER FROM THE R TANK TO THE ALREADY FULL L TANK AND THEN OUT OF THE OVERFLOW VENT UNDER THE R WINGTIP. THE FLC DECLARED AN EMER AND RETURNED TO LGA WHERE THE FUEL XFER VALVE WAS DISCOVERED TO BE STILL OPEN.

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WHILE ACCOMPLISHING THE AFTER TKOF CHKLIST I NOTED THAT THE R FUEL GAUGE WAS DECREASING RAPIDLY. AT SAME TIME A PAX RPTED TO FLT ATTENDANT SEEING FUEL COMING OFF R WING. WE DECLARED EMER; RETURNED TO EWR; UNEVENTFUL LNDG. I RECALLED THAT JUST PRIOR TO DOING WALKAROUND EXTERIOR INSPECTION; A MECH TOLD ME HE WAS ABOUT TO XFER FUEL. DURING WALKAROUND; I NOTICED FUEL PANEL WAS OPEN. LATER; THE MECH MET THE CAPT AND TOLD HIM THE XFER WAS COMPLETE. APPARENTLY THE MECH FAILED TO CLOSE THE DEFUELING VALVE AND THIS CAUSED FUEL TO BE XFERRED FROM R TANK TO L TANK; WHICH WAS FULL; AND THEN VENTED OVERBOARD. COULD BE PREVENTED (HOPEFULLY) BY REQUIRING LOGBOOK ENTRY FOR FUEL XFERS; NOTING THE RESETTING OF SWITCHES AND VALVES. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS MD80 FO SAID THAT HE DID NOT WATCH THE MECH CLOSE THE FUELING PANEL AFTER HE HAD COMPLETED THE FUEL XFER. HE DID; HOWEVER; WATCH THE REOPENING OF THE PANEL TO THE XFER VALVE. THE RPTR FELT THAT THE FLC AND ATC HANDLED THE PROB FAIRLY WELL. HE SAID ONE PECULIARITY WAS WITH THE LAST DEP CTLR WHO; AFTER THEY HAD DECLARED AN EMER AND REQUESTED TO RETURN TO LGA; CONTINUED TO GIVE THEM THEIR NORMAL DOWN RTE CLRNC. THE CTLR DID THIS 3 TIMES; ACCORDING TO THE RPTR; BEFORE HE REACTED PROPERLY. THE RPTR SAID THAT; NOW THAT THE EVENT IS OVER; HE FINDS THIS AN AMUSING HUMAN FACTORS STUDY.