PCT TRACON ISSUES A NEW XPONDER CODE TO A PLT WITH A DC ADIZ FLT PLAN THAT CAUSES QUESTIONS TO BE RAISED AND AN AFTER LNDG INVESTIGATION OF A POSSIBLE DC ADIZ VIOLATION.

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I DEPARTED MRB ON A VFR FLT TO FME AT XA00 EDT ON OCT/MON/03. I HAD EARLIER FILED AN ADIZ FLT PLAN WITH ALTOONA FSS FOR AN ENTRY INTO THE ADIZ AT XE00Z. MY XPONDER WAS SET TO 1200 ON TKOF. AT APPROX 5 MI E OF THE MARTINSBURG VOR; ESTABLISHED COMS WITH POTOMAC APCH ON 126.1; AFTER SEVERAL ATTEMPTS WHILE PROCEEDING ON COURSE; TO OPEN MY ADIZ FLT PLAN. CTLR TOLD ME THAT IT WOULD BE APPROX 5 MINS BEFORE HE COULD GET TO ME. AFTER SOME TIME; THE CTLR GOT BACK TO ME WITH 'MY TAIL NUMBER; SQUAWK XXXX; REMAIN CLR OF B AIRSPACE.' AT THIS TIME I SWITCHED MY XPONDER TO CODE XXXX AND 'IDENTED.' I THEN PROCEEDED ON COURSE; AND DID NOT HEAR ANY CALLS FROM THE CTLR. I WANTED TO BE SURE THAT THE CTLR HAD ME UNDER POSITIVE CTL; SO CALLED BACK AND RPTED POS N OF GAI. CTLR ASKED FOR POS RELATIVE TO A NAVAID AND I SUPPLIED POS RELATIVE TO MARTINSBURG AND FREDERICK. HE FINALLY TOLD ME TO SWITCH TO FREQ 126.7; WHICH I ACKNOWLEDGED AND EXECUTED. WHEN THERE WAS NO TFC ON THAT FREQ; I WENT BACK TO 126.1 TO ENSURE THAT I HAD HEARD THE FREQ CORRECTLY. THE CTLR THEN TOLD ME THAT I WAS ON THE EDGE OF HIS COVERAGE AND TO SWITCH TO 128.7 TO 'SORT OUT MY XPONDER PROBS.' ON CONTACTING 128.7; CTLR HAD ME IDENTED IMMEDIATELY; THEN CAME BACK WITH A PHONE NUMBER TO CALL WHEN I GOT ON THE GND. AT THAT POINT; FME WAS IN SIGHT AND THE CTLR TOLD ME MY POS RELATIVE TO THE ARPT AND 'FREQ CHANGE APPROVED; KEEP YOUR CODE UNTIL ON THE GND.' AFTER LNDG; I WAS INTERCEPTED BY BLACKHAWK HELI AND WAS INTERVIEWED BY THE CUSTOMS CREW. CUSTOMS CREW TOLD ME THAT IT WAS RPTED THAT I HAD VIOLATED THE ADIZ AND THAT THEY WERE CHKING IT OUT. I WENT OVER MY FLT PLAN AND RELATED THE ABOVE SEQUENCE OF EVENTS; PROVIDING REQUESTED DOCUMENTS TO THE CREW. THE CREW SEEMED SATISFIED THAT I HAD FOLLOWED PROPER PROCS FOR ESTABLISHING 2-WAY COMS WITH POTOMAC AND HAVING A CTLR SUPPLIED XPONDER CODE BEFORE ENTERING THE ADIZ. ON CONTACTING POTOMAC; I PROVIDED THE REQUESTED INFO; THEN REQUESTED A DESCRIPTION OF THE ALLEGED VIOLATION. I WAS TOLD THAT THE AIRPLANE WAS ON A DOCUMENTED 'FLT PLAN FROM MRB TO FME AND AT XE38Z HAD CHANGED CODE TO XXX; THEN VIOLATED THE ADIZ. BLACKHAWK FOLLOWING.' I DO NOT THINK THAT I VIOLATED THE ADIZ. I WAS GIVEN THE XPONDER CODE 'XXXX' BY POTOMAC ON 126.1 AND COMPLIED WITH ALL PROCS WHILE ENRTE FROM MARTINSBURG TO FME.