A BE36 PLT VIOLATED THE ZDC ADIZ AND WAS ESCORTED BY A CUSTOMS SVC ACFT TO A LNDG AT MMU.

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I DEPARTED SOMERSET ARPT (KSMQ) ON AN IFR FLT PLAN TO GAITHERSBURG; MD (KGAI). THE ACFT WAS A BEECHCRAFT BONANZA BE36. I FILED A STANDARD IFR FLT PLAN; VIA DUAT; AND WAS CLRED TO KGAI. I WAS NOT INFORMED THAT THIS INVOLVED AN ADIZ PENETRATION CLRNC. I LANDED AT KGAI AT ABOUT XA15. I CONCLUDED MY BUSINESS IN ROCKVILLE; MD; AND RETURNED TO KGAI FOR THE RETURN FLT. I ELECTED TO DEPART VFR; AS IT WAS GOOD VMC. I DID NOT FILE A FLT PLAN. MY MISUNDERSTANDING OF THE TEXTUAL TFR FOR THE ZDC ADIZ LED ME TO BELIEVE THAT A FLT PLAN AND BEACON CODE WAS ONLY REQUIRED FOR DEPS FROM POTOMAC ARPT OR HYDE FIELD. I DEPARTED KGAI AT ABOUT XF30 LOCAL TIME AND WAS SQUAWKING 1200. I WAS FLYING GPS; DIRECT TO KSMQ; AWAY FROM THE ADIZ ZONE. WE CLRED THE ADIZ ZONE IN ABOUT 10 MINS. I WAS JOINED BY A CUSTOMS SVC ACFT ABOUT 40 MINS INTO THE FLT AND ASKED TO COMMUNICATE ON 122.75. THEY ASKED ME TO LAND AT KSMQ WHICH WAS ABOUT 20 MI DISTANT. SINCE THE CUSTOMS SVC ACFT WAS UNABLE TO LAND AT KSMQ; WE AGREED TO LAND AT MORRISTOWN (MMU) WHICH WAS ABOUT 10 MI FURTHER. ON LNDG AT KMMU; I WAS INFORMED OF AN ADIZ VIOLATION BY DEPARTING KGAI WITHOUT A FLT PLAN AND BEACON CODE. I WAS ASKED TO PRESENT IDENTIFICATION; ACFT REGISTRATION; AND MY PLT'S LICENSE; ALL OF WHICH I DID. THIS I DID IMMEDIATELY; AND HE BASICALLY ASKED ME THE SAME INFO. AFTER THIS I WAS TOLD I WAS FREE TO PROCEED TO KSMQ.