ACFT FUEL EQUIP PROB MANDATES A RETURN AND LAND TO REPLACE THE FUEL CAP THAT WAS NOT SECURED.

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WHILE CLBING OUT OF PHILADELPHIA; I HAPPENED TO LOOK OVER AT THE R WING AND NOTICE THAT THE FUEL CAP WAS OFF AND TRAILING BY THE SAFETY CHAIN. I IMMEDIATELY INFORMED THE CAPT AND HE REQUESTED TO RETURN TO PHILADELPHIA WITH ATC PRIORITY HANDLING. IT DID NOT APPEAR AS THOUGH WE WERE LOOSING FUEL. WE WERE VECTORED FOR THE APCH AND LANDED RWY 9L WITHOUT INCIDENT. WHEN WE ARRIVED AT THE ACFT; IT WAS LATE INBOUND. I CONDUCTED THE PREFLT AND DID NOT NOTE ANY PROBS. DURING MY PREFLT THE FUELER WAS SIMULTANEOUSLY FUELING THE ACFT WITH HIS TRUCK PARKED OFF OF THE R WING. I SPECIFICALLY RECALL THAT HE PULLED THE HOSE UNDER THE ACFT TO FUEL THE L SIDE AND THAT HIS LADDER WAS STILL AT THE R WING. NEXT I BOARDED THE PAX; CLOSED THE DOOR AND CHKED THE CABIN AS I MOVED FORWARD TO THE COCKPIT. THEN I BEGAN WORKING ON THE WT AND BAL AS THE CAPT RAN THE BEFORE START CHKLIST AND STARTED THE R ENG. AFTER I COMPLETED THE WT AND BAL; HE STARTED THE L ENG; WE TAXIED AND DEPARTED. UPON RETURNING; SOMEONE APCHED THE AIRPLANE WING AT THE RAMP BEFORE ANY CREW MEMBER OR COMPANY EMPLOYEE HAD AN OPPORTUNITY TO INSPECT THE CAP. COMPANY DISPATCHED A MECH WHO INSPECTED THE WING; INSPECTED AND CHANGED THE CAP; CHKED FOR WATER IN THE FUEL AND MADE THE APPROPRIATE LOGBOOK ENTRIES. SUPPLEMENTAL INFO FROM ACN 237708: THE FUEL CAP IS NOT INCLUDED IN ANY CHKLIST AND WAS NOT OBSERVED WHILE RESPONDING TO ANY ITEMS THE CHKLIST DID INCLUDE. THE R FUEL CAP IS NOT VISIBLE FROM THE CAPT'S SEAT AND THE FUEL CAP CAN BE HIDDEN FROM THE FO'S SIGHT BY THE PROP. I FEEL THAT A SIMPLE CHANGE TO THE BEFORE START CHKLIST TO INCLUDE A VISUAL CHK OF THE FUEL CAPS COULD HAVE AVOIDED THIS MISHAP.