A PA-28-161 PLT; IN THE PATTERN AT HEF; INEXPLICABLY CHANGED HIS XPONDER FROM THE ASSIGNED SQUAWK TO 1200; AFTER THE TWR CLOSED; THEREBY COMMITTING A VIOLATION OF THE DC ADIZ.

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I AM A NEW PVT PLT WORKING ON MY CURRENCY FOR NIGHT FLT WITH PAX. CLOSED TFC; REMAINED WITHIN TFC PATTERN FOR DURATION OF FLT. PERFORMING TKOFS AND LNDGS TO A FULL STOP AND TAXI BACK. RIGHT BEFORE MY SECOND TO LAST TKOF; CLASS D AIRSPACE TRANSITIONED TO CLASS E; UPON THE CLOSING OF THE CTL TWR. PROC WITHIN THE CLASS D; THAT LIES WITHIN THE DC ADIZ; FOR CLOSED TFC IS TO CONTACT GND BEFORE TAXI AND RECEIVE A DISCRETE XPONDER CODE. IT IS NOT NECESSARY TO FILE A FLT PLAN. DURING CLBOUT; DUE TO INEXPERIENCE; I BEGAN TO QUESTION WHAT I WAS TO DO SINCE I WAS NOW AIRBORNE IN CLASS E AIRSPACE WITHIN THE ADIZ. I DID NOT KNOW IF THE SQUAWK CODE ASSIGNED BY MANASSAS TWR WAS STILL VALID; SO I SQUAWKED 1200. I MADE 1 ADDITIONAL TKOF AND LNDG THEN TIED DOWN THE ACFT; DUE TO BECOMING MORE UNSURE IF I WAS FOLLOWING PROPER PROC. I AM ALSO EMPLOYED AT THE FBO THAT I FLY OUT OF; AND AGENTS WERE THERE ASKING QUESTIONS THE NEXT MORNING. I HAVE NOT BEEN PERSONALLY CONTACTED YET. I HAVE LEARNED TO NEVER SQUAWK 1200 WITHIN THE BOUNDARIES OF AN ADIZ. REGARDLESS OF THE MASS CONFUSION AMONG THE PLTS AND INSTRUCTORS OF THE GA COMMUNITY CONCERNING PROPER PROCS IN AND AROUND THE ADIZ; FILE A FLT PLAN. I WILL FOLLOW UP BY REVIEWING THE NOTAMS AND BY READING THE QUESTION AND ANSWER SECTION IN THE NOTAM SECTION OF THE AOPA WEBSITE. A VALUABLE LESSON LEARNED.