FLC OF A DHC8 TAXIED OUT FOR TKOF WITH THE FUEL DOOR HANGING DOWN IN THE OPEN POS. ANOTHER ACFT; TAXIING IN TRAIL; FLC NOTIFIED RPTR THAT THE DOOR SEEMED TO BE OPEN. THE CREW RECEIVED THE TWR'S PERMISSION TO GET OUT OF LINE TO CLOSE THE DOOR BEFORE TKOF.

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I PROCEEDED TO CONDUCT MY OUTSIDE (EXTERIOR) PREFLT INSPECTION OF THE ACFT. THE FUELER HAD NOT YET ARRIVED DURING MY WALKAROUND INSPECTION. OUR FLT OUTBOUND FROM PHL WAS DUE OUT AT XA58. AT APPROX XX30 HRS I RETURNED TO THE COCKPIT AND COULD SEE THAT A FUEL TRUCK ARRIVED SOME TIME LATER (AROUND XY40 HRS). AT APPROX XY48 THE PAX ARRIVED. THE CAPT AND MYSELF COMPLETED THE BEFORE START CHKLIST AND WHEN CLRED BY THE RAMP PERSONNEL; STARTED THE ENGS. WE TAXIED TO RWY 17 IN PREPARATION FOR DEP AND WHILE WE WERE #3 FOR TKOF; THE GND CTLR TOLD US THAT; 'ACFT BEHIND YOU SAYS THAT YOUR FUEL DOOR APPEARS TO BE OPEN AND THE FUEL CAP IS OFF.' THE CAPT OBTAINED PERMISSION TO SHUT DOWN ON THE TXWY APPROX 100 FT FROM THE HOLD SHORT LINE FOR RWY 17. I WENT OUT AND SECURED THE FUEL DOOR AND CAP AND RETURNED TO THE FLT DECK. (WE SHUT DOWN THE #1 ENG PRIOR TO OPENING THE DOOR BUT LEFT #2 RUNNING IN START-FEATHER. WE DEPARTED PHL UNEVENTFULLY. THE DASH 8 DOES NOT HAVE A FUEL DOOR OPEN CAUTION LIGHT WHICH I BELIEVE WOULD HAVE PREVENTED THIS FROM HAPPENING; EVEN THOUGH THE FUELER WAS IN ERROR; AND THE RAMP PERSONNEL WAVED US OFF THE GATE WITHOUT NOTICING THE DOOR OPEN AND CAP HANGING DOWN. WE WERE VERY FORTUNATE THAT WE WERE IN THE LINE-UP WITH SOMEONE BEHIND US THAT DAY.