A B727-200 FE FUELED THE ACFT INCORRECTLY AND THE ACFT TOOK OFF WITH INSUFFICIENT FUEL. THEY RETURNED TO POINT OF DEP.

Narrative

AS THE FINAL LOADING OF THE ACFT WAS BEING ACCOMPLISHED; I ASKED THE CAPT FOR THE FUEL LOAD TO PUT ON THE AIRPLANE. HE TOLD ME 28000 LBS AND I THEN WENT DOWNSTAIRS TO THE FUELING STATION TO BEGIN THE FUELING PROCESS. AT THIS POINT; I MISTAKENLY CALCULATED THE DISTRIBUTION TO BE 7000 LBS IN EACH TANK (#1; #2 AND TANK #3). UNFORTUNATELY; THIS WAS CAUSED BY MY EXPERIENCE IN FUELING AIRPLANES WITH 4 WING TANKS. I THEN PROCEEDED TO FUEL THE AIRPLANE TO 21000 LBS OF FUEL. SINCE THE #2 TANK QUANTITY GAUGE IN THE COCKPIT WAS ON DMI BECAUSE IT READ ZERO; AND THE GAUGE IN THE REFUELING PANEL WAS WORKING OK; THE ERROR IN THE FUEL ON BOARD COULD NOT BE NOTICED BY GLANCING AT THE FE FUEL PANEL. FURTHERMORE; SINCE THE #2 FUEL QUANTITY SYS WAS ON DMI; I DRIP STICKED THE TANK TO BE SURE THERE WAS 7000 LBS IN #2 TANK. AT THE PRE-START CHKLIST; THE CAPT AND COPLT EACH REPEATED THE REQUIRED FUEL AND '28000 LBS ON BOARD' WHICH I ALSO REPEATED. AT THAT MOMENT NONE OF US QUESTIONED HOW MUCH FUEL WAS IN #2 TANK BUT THE CAPT DID ASK IF I HAD PUT A KNOWN QUANTITY OF FUEL IN THE #2 TANK; WHICH I CONFIRMED WITH 'YES;' SINCE AT THIS POINT I STILL HAD '21000 LBS' IN MY HEAD. UPON REACHING TOP OF CLB; WE MADE OUR ROUTINE FUEL CHK. SINCE ALL OF THE TANKS WERE EVEN AT 7000 LBS IN MERIDA AND THE FUEL FLOWS HAD BEEN KEPT CONSTANT THROUGHOUT THE CLB; TANKS #1 AND #3 EACH READ ABOUT THE SAME AMOUNT AND I USED AN AVERAGE TO CALCULATE THE FUEL IN #2 TANK. THIS TOTAL CAME OUT TO ABOUT 12600 LBS WHICH WHEN CHKED AGAINST THE FLT PLAN SHOWED US TO BE MORE THAN 7000 LBS BEHIND. AT THIS POINT WE REALIZED THERE WAS SOMETHING WRONG AND THE CAPT ASKED ME HOW MUCH FUEL I PUT ON BOARD. I REPLIED 21000 LBS AND TA THAT POINT WE DECIDED TO RETURN TO MERIDA. FACTORS AFFECTING THIS OCCURRENCE WERE THE FACT THAT ON THE PREVIOUS DAY WE HAD TO RETURN TO BLOCKS DUE TO THE WX RADAR NOT OPERATING. AFTER A FEW HRS DELAY WE DEPARTED TO BELIZE AS SCHEDULED. MERIDA WAS SUPPOSED TO BE NEXT AND THEN RETURN TO MIAMI. ON THE WAY TO BELIZE; OUR RADAR FAILED AGAIN AND THEN THERE WAS A PROB WITH THE FUEL SYS. THE #2 TANK WAS ALREADY ON DMI AND I WAS FINDING IT TO BE IMPOSSIBLE TO KEEP TANKS #1 AND #3 BALANCED. THE MORE I TRIED; THE WORSE IT WOULD GET. WE WERE DEEPLY CONCERNED BECAUSE WE DIDN'T KNOW IF WE WERE LOSING FUEL OUT OF #1 OR IF IT WAS MIGRATING INTO #2 AND WE DIDN'T KNOW SINCE #2 WAS ON DMI. WHEN WE LANDED IN BELIZE. WE HAD TO SPEND THE NIGHT TO WAIT FOR MAINT TO COME IN THE MORNING. THE NEXT DAY; THE AIRPLANE WAS FIXED; EXCEPT THE #1 XFEED VALVE WAS FOUND TO BE FAULTY AND ALSO PLACED ON DMI AND WE PROCEEDED TO MERIDA. ONCE IN MERIDA WE HAD TO WAIT 4 HRS FOR FREIGHT TO ARRIVE SO AS SOON AS IT DID AND THE FUEL TRUCK ARRIVED TOO; WE COULDN'T WAIT TO GET ON OUR WAY. SO WHEN THE PROPER FUEL LOAD HAD TO BE VERIFIED ON THE PRE-START CHKLIST; I JUST REPEATED WHAT I HEARD BECAUSE I WAS CATCHING UP ON PAPERWORK.