ACR LTT ACFT WAS FORCED TO RETURN FOR LNDG AFTER FUEL TANK CAPS WERE OBSERVED COMING LOOSE.

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FUELER RPTED THAT SEVERAL FUEL CAPS HAVE A TENDENCY TO 'POP BACK UP' AFTER FUELING. I WOULD LIKE TO SEE A PROGRAM WHERE THESE GND SVC PEOPLE INFORM THE CAPT SO THAT WORN OR FAULTY EQUIP CAN BE REPLACED/REPAIRED. AFTER SWAPPING ACFT AT LAST MIN; AT THE CONCOURSE; WE DEPARTED MEMPHIS 36L. AT ROTATION PAX IN SEAT NUMBER AND CAPT SEAT NUMBER WITNESSED L FUEL CAP LIFT OUT OF POS ON THE L WING. THE CAPT IN CABIN INFORMED FLT ATTENDANT OF THE LOOSE FUEL CAP WHO IN TURN NOTIFIED THE COCKPIT. RETURN TO MEMPHIS LNDG 36L WITHOUT INCIDENT. TAXIED TO GATE WHERE ACFT WAS MET BY MAINT AND FUELER. L ENG WAS SHUT DOWN; THE DISCREPANCY WAS ENTERED INTO MAINT LOG BE CAPT. FUELER RESECURED CAP AND MAINT INSPECTED FOR DAMAGE; FOUND NONE; AND CLRED THE OPEN WRITE-UP IN MAINT LOG. WE DEPARTED MEMPHIS ONCE AGAIN AND COMPLETED THE FLT WITHOUT INCIDENT. THE FUELER RPTED THAT THE FUEL CAP LOCKING LEVEL WAS IN THE SECURE/FLUSH POS. DURING PREFLT BOTH FUEL CAPS WERE INSPECTED BY THE CREW FROM THE COCKPIT AND WERE OBSERVED TO BE IN THE SECURED POS.