Narrative:

At gate, the fueler started to hook up for fueling. After consulting the captain, we agreed to skip fueling, and I went and told fuelers to stop. At the time, the right fuel cap was off, but he hadn't started pumping. He replaced the cap and drove off, and we boarded passenger. The captain did a walkaround inspection, and we closed up and started engines. As part of our before-taxi checklist, we visually checked that the fuel caps were in place, and they were. I could not see if the lock tab was latched, but I assumed it had been, since the cap was in place. Upon arrival in ZZZ, the fueler noted that the right cap was missing. It must have come loose sometime during flight.

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Original NASA ASRS Text

Title: A BEECH 1900D HAD THE FUEL TANK REFUELING CAP DEPART THE ACFT INFLT CAUSED BY FAILURE TO LOCK CAP.

Narrative: AT GATE, THE FUELER STARTED TO HOOK UP FOR FUELING. AFTER CONSULTING THE CAPT, WE AGREED TO SKIP FUELING, AND I WENT AND TOLD FUELERS TO STOP. AT THE TIME, THE R FUEL CAP WAS OFF, BUT HE HADN'T STARTED PUMPING. HE REPLACED THE CAP AND DROVE OFF, AND WE BOARDED PAX. THE CAPT DID A WALKAROUND INSPECTION, AND WE CLOSED UP AND STARTED ENGS. AS PART OF OUR BEFORE-TAXI CHKLIST, WE VISUALLY CHKED THAT THE FUEL CAPS WERE IN PLACE, AND THEY WERE. I COULD NOT SEE IF THE LOCK TAB WAS LATCHED, BUT I ASSUMED IT HAD BEEN, SINCE THE CAP WAS IN PLACE. UPON ARR IN ZZZ, THE FUELER NOTED THAT THE R CAP WAS MISSING. IT MUST HAVE COME LOOSE SOMETIME DURING FLT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.