Narrative:

I was returning to freeway airport in mitchellville, md. After reporting the airport in sight, I was instructed by potomac approach to maintain my squawk code until on the ground, frequency change to advisory frequency was approved, and I was advised that traffic was in the pattern. I repeated the instruction to maintain my current code, advised that I was switching to advisory frequency and would look for the traffic. As I was setting up for landing, looking for traffic, changing to the airport advisory frequency and announcing my position entering the pattern, my passenger (also a pilot) reset the transponder to squawk 1200. I was not aware that he had changed the code. Apparently, he mistakenly thought controller's authority/authorized to 'change to advisory frequency' meant changing the transponder to 1200, the common VFR code. All of this occurred in the last 5 mins of the flight. We were less than 1 mi from the airport, entering the traffic pattern at 1000 ft. I was completely focused on the task at hand: slowing the plane, setting up for landing, looking for/advising other aircraft in the pattern and, most importantly, flying the aircraft to a safe landing. Checking and rechking the transponder is not on the landing checklist. It's not going to determine the safety of the airplane and passenger, particularly at this most critical point in a flight. I was unaware that the transponder had been changed until I was contacted by phone by potomac approach after landing. The other pilot 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.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.