ATX PLT HAS DIFFICULTY OBTAINING CLRNC INTO THE DCA TCA.

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I WAS PIC OF AN AIR TAXI FLT FROM ABE TO DCA VFR AT 4500' MSL AND CONTACTED BWI APCH (133.0) OVER THE SUSQUEHANNA RIVER ABOUT 40 MI NE OF BWI AND REQUESTED RADAR ADVISORIES AND A HDOF TO DCA APCH FOR LNDG AT DCA. AS WE WERE HANDED OFF TO THE S CTLR (119.7) WE REPEATED THE REQUEST FOR HDOF THE WASHINGTON APCH. AT ABOUT 18 DME FROM DCA WE WERE TOLD; 'UNABLE TO EFFECT A HDOF TO WASHINGTON; DSND BELOW 2500' TO REMAIN BELOW THE WASHINGTON TCA; CONTACT THEM 124.2; SQUAWK 1200; GOOD DAY;' OR WORDS TO THAT EFFECT. AT THIS POINT WE WERE 1 MIN FROM PENETRATING THE TCA. WE TRIED 124.2 WITH NO RESPONSE; THEN 126.55; WHO RESPONDED AND ADVISED ANOTHER FREQ--I BELIEVE 119.8 OR SOME SUCH. BY THIS TIME WE WERE ABOUT 1 MI FROM THE TCA AND TURNED FROM A SOUTHWESTERLY HDG TO A SOUTHEASTERLY HDG TO REMAIN CLR. WE FINALLY RAISED WASHINGTON ON 124.2; WHO QUICKLY CLRED US INTO THE TCA. THE PROB AS I SEE IT IS CONTIGUOUS POSITIVE CTLED AIRSPACE BTWN BWI AND DCA W/O A CERTAIN HDOF PROC FOR VFR TFC. THE ARSA AND TCA ARE CONTIGUOUS SO THERE IS NO UNCTLED BUFFER TO LEAVE ONE APCH FAC AND CONTACT ANOTHER UNLESS BELOW 2500'. THERE SHOULD BE PROVISIONS FOR A HDOF OR CLRNC TO REMAIN IN ONE FAC'S CTL UNTIL ACCEPTED BY ANOTHER; WHICH IS WHAT WASHINGTON DID ON THE OUTBND LEG. IT WOULD BE VERY DIFFICULT FOR AN ACFT NOT EQUIPPED WITH DME; WHICH WE HAD; TO AVOID AN AIRSPACE VIOLATION.