ENRTE THROUGH THE DC ADIZ; C182 PLT REALIZES THAT HE HAS FAILED TO TURN HIS XPONDER FROM STANDBY TO ALT.

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AT APPROX XA00 AM; I FILED A VFR ADIZ FLT PLAN WITH LESSBURG FLT SVC FROM HEF TO CSN VORTAC; DEPARTING HEF AT XA30 AM AND RETURNING TO HEF 1 HR LATER. I CONTACTED HEF TWR AND RECEIVED A XPONDER CODE AND A POTOMAC APCH FREQ. I SET THE FREQ IN MY RADIO; SET THE XPONDER CODE AND TURNED THE XPONDER SWITCH TO STANDBY. THE WX WAS VFR AND THERE WAS A LOT OF INCOMING/OUTGOING TFC ON RWY 16L. AFTER WAITING FOR SOMETIME; I WAS CLRED TO THE SW WITH AN ALT OF 2500 FT. IN DUE COURSE; I WAS CLRED BY THE MANASSAS TWR; TOLD TO CONTACT POTOMAC APCH. I WAITED EXPECTING A CALL TO CLR ME FROM THE ADIZ. AFTER A REASONABLE TIME; WHEN I DIDN'T HEAR FROM POTOMAC APCH; I CONDUCTED AN INST AND RADIO TROUBLESHOOT EXERCISE. TO MY SURPRISE; I HAD NOT TURNED MY XPONDER FROM STANDBY TO ALT; BUT DID SO IMMEDIATELY. I FLEW TO CSN USING VISUAL REFS AND A MOVING MAP DISPLAY AND CALLED POTOMAC APCH. THE FREQ WAS VERY BUSY DUE TO TFC AND SO I FLEW AT 2500 FT TO W66; AND AGAIN CALLED POTOMAC APCH. THEY RESPONDED AND ASKED IF I HAD FILED DEP AND RETURN FLT PLANS. I RESPONDED AFFIRMATIVELY. I THEN REQUESTED CLRNC TO RETURN TO HEF AND RECEIVED A NEW XPONDER CODE WHICH I ENTERED AND WAS REQUESTED TO RPT 'ARPT IN SIGHT.' THEN POTOMAC APCH REQUESTED I CALL UPON LNDG. I RPTED 'ARPT IN SIGHT;' AND CALLED MANASSAS TWR AND LANDED ON RWY 16L. UPON LNDG I CONTACTED THE REQUESTED NUMBER AND WAS TOLD I MIGHT HAVE VIOLATED THE ADIZ AND COULD RECEIVE A CALL FROM THE FAA. TO DATE THERE HAS BEEN NO CONTACT.