PLT OF M20 SUFFERS DC ADIZ INCURSION WHEN INEXPERIENCED PLT PAX IN R SEAT SWITCHES XPONDER TO 1200 WHILE STILL IN THE ADIZ.

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I WAS RETURNING TO FREEWAY ARPT IN MITCHELLVILLE; MD. AFTER RPTING THE ARPT IN SIGHT; I WAS INSTRUCTED BY POTOMAC APCH TO MAINTAIN MY SQUAWK CODE UNTIL ON THE GND; FREQ CHANGE TO ADVISORY FREQ WAS APPROVED; AND I WAS ADVISED THAT TFC WAS IN THE PATTERN. I REPEATED THE INSTRUCTION TO MAINTAIN MY CURRENT CODE; ADVISED THAT I WAS SWITCHING TO ADVISORY FREQ AND WOULD LOOK FOR THE TFC. AS I WAS SETTING UP FOR LNDG; LOOKING FOR TFC; CHANGING TO THE ARPT ADVISORY FREQ AND ANNOUNCING MY POS ENTERING THE PATTERN; MY PAX (ALSO A PLT) RESET THE XPONDER TO SQUAWK 1200. I WAS NOT AWARE THAT HE HAD CHANGED THE CODE. APPARENTLY; HE MISTAKENLY THOUGHT CTLR'S AUTH TO 'CHANGE TO ADVISORY FREQ' MEANT CHANGING THE XPONDER TO 1200; THE COMMON VFR CODE. ALL OF THIS OCCURRED IN THE LAST 5 MINS OF THE FLT. WE WERE LESS THAN 1 MI FROM THE ARPT; ENTERING THE TFC PATTERN AT 1000 FT. I WAS COMPLETELY FOCUSED ON THE TASK AT HAND: SLOWING THE PLANE; SETTING UP FOR LNDG; LOOKING FOR/ADVISING OTHER ACFT IN THE PATTERN AND; MOST IMPORTANTLY; FLYING THE ACFT TO A SAFE LNDG. CHKING AND RECHKING THE XPONDER IS NOT ON THE LNDG CHKLIST. IT'S NOT GOING TO DETERMINE THE SAFETY OF THE AIRPLANE AND PAX; PARTICULARLY AT THIS MOST CRITICAL POINT IN A FLT. I WAS UNAWARE THAT THE XPONDER HAD BEEN CHANGED UNTIL I WAS CONTACTED BY PHONE BY POTOMAC APCH AFTER LNDG. THE OTHER PLT THEN REVEALED WHAT HAD HAPPENED. HE WANTS TO ASSUME FULL RESPONSIBILITY AND ACCEPT ANY PENALTY OR VIOLATION; BUT BECAUSE I WAS PIC; IT'S COMING TO ME. THE PENALTY IS A BIGGER DEAL FOR ME; AS HE RARELY FLIES. I'M THE ONE WHO FLIES ALL THE TIME IN THE ADIZ; AND CAN LEAST AFFORD SOMETHING LIKE THIS ON MY RECORD. I HAVE NO OTHER VIOLATIONS.