A C182 PLT ENTERED DCA'S CLASS B AIRSPACE.

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UPON LEAVING OUR MISSION BASE AT HEF TO RETURN TO OUR HOME BASE GAI; WE TRIED TO CONTACT DULLES APCH FOR CLRNC THROUGH THE CLASS B AIRSPACE. HE WAS EXTREMELY BUSY AND SAID TO STAY CLR OF THE CLASS B AIRSPACE AND REMAIN VFR. WE HAD BEEN CLBING AT THAT POINT; EXPECTING CLRNC. WE IMMEDIATELY DSNDED TO 1200 FT TO STAY CLR (BELOW) THE CLASS B AIRSPACE; AND USED THE DME FROM ARMEL TO STAY APPROX 8 MI AWAY. THE CTLR CALLED BACK A FEW MINS LATER FOR OUR REQUEST AND; AT THAT TIME; HE SAID WE WERE IN CLASS B AIRSPACE UNDER THE APCH PATH TO RWY 19R AND TOLD US TO MAKE AN IMMEDIATE R TURN TO 090 DEGS. THE DME AT THAT TIME SHOWED 7.3 MI; SO I DON'T BELIEVE WE WERE IN CLASS B AIRSPACE; BUT WE WOULD HAVE BEEN VERY SOON IF WE DIDN'T TURN. NEXT TIME WE WILL REVIEW THE TERMINAL AREA CHART MORE BEFORE TKOF. IT WOULD HELP IF THE TWR AT HEF COULD COORDINATE WITH DULLES APCH AHEAD OF TIME SO THAT THERE IS NO DELAY IN FINDING OUT IF ENTRANCE INTO THE CLASS B AIRSPACE IS GOING TO BE ALLOWED. IF NOT; WE COULD HAVE MADE ALTERNATE PLANS BEFORE TKOF.