A PA284 PLT INADVERTENTLY FAILED TO ENTER THE CORRECT XPONDER CODE BEFORE ENTERING THE WASHINGTON; DC; ADIZ.

Narrative

I DEPARTED GAI; AN ARPT WITHIN THE DC ADIZ; WITH THE XPONDER ON BUT HAVING FAILED TO ENTER MY ASSIGNED BEACON CODE. THIS IS A VIOLATION OF THE PROCS ESTABLISHED FOR AVIATION OPS WITHIN THE ADIZ. I COMPOUNDED THE PROB BY -- AFTER BEING INFORMED BY ATC THAT THERE WAS A PROB WITH MY BEACON CODE -- READING FROM MY KNEE BOARD AND ENTERING THE CODE I HAD BEEN ASSIGNED THE PREVIOUS WKEND. I HAD SCRIBED MY CURRENT CLRNC ON THE SAME PIECE OF PAPER AS THE OLD ONE; SEPARATED BY A LINE. UNFORTUNATELY; BY THE TIME I WAS NOTIFIED THAT THERE WAS A PROB; THERE WAS ENOUGH CONFUSION IN THE CONVERSATION BTWN ATC AND ME THAT I REACTED TOO QUICKLY IN READING THE BEACON CODE INFO FROM THE PIECE OF PAPER AND ENTERING IT INTO THE XPONDER. THIS ERROR BECAME OBVIOUS WHILE I WAS ON THE PHONE WITH ATC AT MY DEST. WE WENT OVER THE EVENTS; AND I NOTICED THAT THE INFO I HAD READ WAS NOT CORRECT. TO MY RECOLLECTION; I HAVE NEVER BEFORE FAILED TO SET MY XPONDER TO SQUAWK THE ASSIGNED BEACON CODE PRIOR TO FLT; WHETHER ON AN ADIZ CLRNC OR AN INST FLT PLAN. I UNDERSTAND THE SIGNIFICANCE TO NATIONAL SECURITY OF AN ACFT ALOFT WITHIN DC'S AIRSPACE WITHOUT PROPER IDENT. THE OBVIOUS WAY TO AVOID REPEATING THIS PROB IS TO: 1) DOUBLECHK THAT NOT ONLY IS THE XPONDER ON; BUT THAT IT IS SET TO THE ASSIGNED CODE; AND 2) USE A NEW SHEET OF PAPER FOR EACH FLT.