CL600 FLT CREW DISCOVERS SHORTLY AFTER TKOF THAT THE RIGHT WING IS LOSING FUEL DUE TO A LOOSE FUEL CAP. FLT CREW RETURNS FOR AN UNEVENTFUL LANDING.

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WE ARRIVED AT ZZZ TO PICK UP PAX AND REFUEL FOR A TRIP OUT E. SEVERAL OF THE PAX HAD ARRIVED; THE PF WAS FLT PLANNING AND THE PNF WAS HELPING THE PAX WITH THEIR LUGGAGE. THE LINE SVC HAD BEEN ADVISED OF THE FUEL LOAD; OF 14800 LBS (DON'T REMEMBER WHAT THE GALS WERE). WHILE LOADING THE ACFT THE LINE SVC PERSON ASKED ME FOR 'A KEY FOR THE GAS CAPS.' I TOLD HIM HE DIDN'T NEED ONE AS WE WERE A SINGLE POINT REFUEL SYS. I ALSO TOLD HIM THAT I WOULD OPEN THE FUEL VALVES FROM THE PANEL ON THE INSIDE OF THE ACFT; WHICH I DID. I ALSO SPOKE WITH ANOTHER FUELER WHO HAD TAKEN CHARGE OF THE FUELING AND SAID TO HIM 'I DIDN'T KNOW WHAT THE OTHER GUY WAS THINKING BECAUSE HE ASKED FOR A KEY FOR THE FUEL CAPS.' I WAS ADVISED THAT HE WAS NEW. WE CONTINUED TO LOAD THE ACFT WITH PAX; LUGGAGE AND FUEL. WHEN THE LOADING WAS COMPLETE I CLOSED THE FUEL VALVES FROM THE INSIDE FUELING PANEL. BEFORE I CLOSED THE CABIN DOOR I LOOKED FOR CHOCKS ON THE WHEELS; AND CHKED THE PRESSURE REFUEL DOOR CLOSED ALONG WITH THE OUTSIDE FUEL ACCESS PANEL. AT THAT TIME I DID NOT NOTICE ANYTHING UNUSUAL ABOUT THE FUEL CAP ON THE R WING. WE TAXIED OUT FOR DEP. JUST AFTER I HAD SELECTED THE GEAR UP THE TWR CALLED AND INDICATED THAT THE R ENG APPEARED TO BE ON FIRE; BECAUSE THERE WAS SMOKE. I LOOKED AT THE R WING AND I COULD SEE THAT THE FUEL CAP WAS HANGING ON BY THE LANYARD. I THEN ADVISED THE OTHER PLT AND THE TWR. WE MADE THE DECISION TO RETURN AND ASKED FOR THE OPPOSITE RWY TO THAT WHICH WE TOOK OFF. EVEN THOUGH FUEL WAS EVAPORATING FROM THE WING IT WAS NOT YET AN EMER SITUATION AS WE HAD A LOT OF FUEL ON BOARD; HOWEVER WE ASKED FOR PRIORITY HANDLING. THE TWR CLRED US TO LAND. WE DID A TEARDROP BACK TO THE ARPT AND LANDED WITHOUT INCIDENT. THE ENTIRE FLT WAS 4 MINS LONG; AND WE LOST ABOUT 1000 LBS OF FUEL ACCORDING TO OUR GAUGES. IN REVIEWING THE CIRCUMSTANCES WITH THE FBO; I WAS TOLD THAT THE FUELER WHO ASKED FOR THE KEYS TO THE GAS CAPS WAS NEW. IT WAS OBVIOUS TO ME AT THAT POINT THE INDIVIDUAL HAD NOT RECEIVED PROPER TRAINING FROM THE FBO AND PROBABLY SHOULD HAVE BEEN SHADOWED BY THE OTHER FUEL PERSON. WHAT IS CERTAINLY WORTH NOTING HERE IS THE AMOUNT OF FUEL THAT WE HAD ON BOARD THE ACFT. WITH 14800 LBS OF FUEL ON THE ACFT THE FUEL CAP WAS PROVIDING SOME SORT OF SEAL; SINCE IF IT WAS LOOSE WE WOULD HAVE HAD FUEL LEAKING ALL OVER THE RAMP AT THE TIME OF FILLING. SOON AS WE BROKE GND THE CAP MUST HAVE POPPED FROM THE WING BENDING. WHAT IS DISTRESSING IS THE FACT THE INDIVIDUAL DID NOT SEEK SOMEONE OUT TO HELP REPLACE THE FUEL CAP AFTER HE HAD OPENED IT. NEITHER PLT NOR THE SECOND REFUELER NOTICED THAT THE CAP HAD BEEN OPENED. THIS IS A LEASED ACFT; AND DID NOT COME WITH THE KEYS FOR THE FUEL CAPS. I HAD ASKED FOR KEYS HOWEVER OWNERSHIP DECLINED TO PROVIDE THEM. IN OUR PREVIOUSLY OWNED ACFT WE HAD LOCKED THE FUEL CAPS FOR SECURITY PURPOSES. NO ONE SAW THE FUELER OPEN THE FUEL CAP ON THE R WING; PROBABLY BECAUSE WE WERE LOADING THE ACFT FROM THE OTHER SIDE. THE FBO IS NOW ACUTELY AWARE OF THE LACK OF TRAINING OF THEIR EMPLOYEE AND AS FOR ME AND MY OTHER PLT WE TOO ARE ACUTELY AWARE OF WHAT HAPPENED AND HAVE INCORPORATED EXTRA DILIGENCE WITH REGARD TO FUELING OPS WHILE USING SINGLE POINT.