FLC OF A LEARJET; LR35; TOOK OFF WITH THE L FUEL TANK LEFT OPEN AFTER REFUELING. TWR CTLR ALERTED CREW TO FUEL BEING VENTED OVERBOARD AND A RETURN TO LAND WAS MADE.

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WE GOT CALLED FOR AN ON DEMAND CHARTER TO LEAVE HPN TO GO TO SAT. WE WERE BASED IN BUF AND BY THE TIME WE GOT THINGS TOGETHER WE ARRIVED IN HPN AND OUR PAX WERE ALREADY THERE. I ORDERED OUR FUEL LOAD; PICKED UP THE CATERING; BRIEFED OUR PAX; LOADED THE LUGGAGE; PAID THE BILL; MADE THE NECESSARY PHONE CALLS; ETC. ON MY WAY IN TO GET SOME OF THE LUGGAGE; I PERSONALLY CHKED THE R FUEL CAP AND THOUGHT TO CHK THE L CAP BUT IN ORDER TO SAVE TIME I DECIDED TO CHK THE L CAP AFTER LOADING THE LUGGAGE. DURING ALL OF THIS THE COPLT WAS GETTING THE CLRNC AND CHKING THE RTE. AFTER LOADING THE LUGGAGE I PROCEEDED TO BOARD THE PAX AND THEN SHUT THE DOORS; MISSING THE FACT THAT I DID NOT CHK THE FUEL CAP. DURING THE TKOF CLB THE TWR INFORMED US OF FUEL COMING FROM THE L WING. I LOOKED OUT TO SEE THE FUEL CAP WAS DRAGGING BEHIND THE FUEL PORT AND FUEL ESCAPING FROM THE PORT. WE INFORMED THE TWR WE WERE RETURNING AND MADE A L TFC PATTERN. DURING THE FINAL APCH I THOUGHT ABOUT BEING OVER GROSS WT BUT ELECTED TO MAKE A LNDG FOR VARIOUS REASONS. THE LNDG WAS WITHOUT INCIDENT AND I PUT THE FUEL CAP ON AND WE TAXIED BACK FOR DEP. MY FIRST SLIP UP WAS I SHOULD HAVE CHKED BOTH CAPS AT THE SAME TIME AND THAT THE FUELERS ARE HUMAN ALSO. I NEED TO SLOW DOWN AND TAKE A COUPLE OF MINS TO MAKE SURE EVERYTHING IS TAKEN CARE OF AND IF POSSIBLE DESIGNATE THE OTHER PLT TO SOME OTHER DUTIES FIRST.