DC9 CAUSES ANOTHER ACR GAR BECAUSE THEY TAXI ONTO RWY AT DHN ATTEMPTING TO OBTAIN IFR DEP CLRNC WITH ZJX THROUGH ANB FSS.

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WE WERE TAXIING OUT OF DHN ARPT. I CALLED ADDISON RADIO FOR A CLRNC. NOTE: DHN TWR CLOSES AT XA00. I WAS INFORMED THAT I COULD NOT GET A CLRNC DUE TO A MISSING ACFT IN THE DHN AREA. I CALLED ON 118.4; WHICH IS DHN TWR AND ADVISORY FREQ IF THE TWR IS CLOSED. THE ACFT FLYING AROUND WAS AN AIRPLANE IN TRAINING. HE WAS NOT ON A CLRNC; AND HE WAS WORKING WITH ZJX DOING FLT TRAINING. WE THEN TAXIED TO THE TERMINAL AND FOUND ANOTHER ACFT AT THE GATE; GAVE RADIO HIS N-NUMBERS AND THAT WAS THE ACFT THAT WAS MISSING AND DID NOT CLOSE HIS FLT PLAN. WE THEN TAXIED OUT FOR CLRNC. I RECEIVED A CLRNC; WE LOOKED AROUND FOR TFC; DID NOT SEE ANYONE; AND PULLED ONTO THE RWY. AFTER LINING UP ON THE RWY; I SAW AN AIRPLANE ABOUT 3/4 DOWN THE RWY; WHICH WAS ABOUT 500 FT. EVIDENTLY; WE CAUSED HIS GAR. I SPOKE TO HIM ON 118.4; APOLOGIZED; AND MENTIONED THAT WE NEVER SAW HIM. HE REMARKED HE TRIED TO CALL US ON 118.4 AND I TOLD HIM WE WERE ON 122.5; DEALING WITH ADDISON RADIO BECAUSE THEY HAD GIVEN US A LIMITED DEP TIME. THERE WAS NO CONFLICT; WE NEVER SAW HIM GO AROUND EXCEPT WHEN HE SAW HIM IN THE DISTANCE. THE POINT BEING HERE IS CAIRNS (OZR) DEP AND APCH ON FREQ 125.4 CLOSES AT APPROX XE00. WHEN THEY ARE ACTIVE; WE HAVE TFC POS ALL THE TIME THROUGH RADIO; VFR/IFR. DHN CAN GET BUSY AT NIGHT. THEY HAVE MIL HELI TRAINING; MIL FIXED WING TRAINING; PLUS PEOPLE LIKE MYSELF DOING ACFT AND TESTING OUT OF IT. IN ADDITION; JAX CANNOT GET RADAR COVERAGE BELOW 3000 FT. THEREFORE; I GUESS THE POINT IS; IT WOULD SURE BE NICE IF APCH CTL COULD BE OPEN 24 HRS A DAY. NOTE: WE WERE OPERATING UNDER FAR PART 91 DOING TEST AND FERRY FLT.