WHILE APCHING W29; FLT CREW OF PA46 SQUAWK 1200 WITHIN THE DC ADIZ AFTER CANCELING IFR CLRNC.

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I FLEW AN IFR FLT PLAN FROM W29. I WAS PIC AND PAX; AN INST-RATED PVT PLT WAS COMMUNICATING WITH ATC AND TUNING THE RADIOS. WITH W29 IN SIGHT AND BELOW THE WASHINGTON/BALTIMORE CLASS B AIRSPACE; I TOLD MY PAX TO CANCEL IFR. HE CANCELLED THE IFR FLT PLAN WITH POTOMAC APCH APPROX 7 NM FROM BAY BRIDGE. POTOMAC APCH INSTRUCTED US TO KEEP OUR CURRENT BEACON CODE FOR THE APCH AND LNDG INTO BAY BRIDGE. I CONDUCTED A NORMAL VISUAL APCH TO RWY 11 AT BAY BRIDGE ARPT. DURING THE APCH; WHILE I WAS SCANNING THE AREA FOR TFC AND CONFIGURING THE AIRPLANE FOR LNDG; IT IS POSSIBLE THAT WE INADVERTENTLY BUMPED THE 'VFR' BUTTON ON THE #1 XPONDER; WHICH MAY HAVE SWITCHED THE IFR BEACON CODE TO THE 1200 CODE. IT IS ALSO POSSIBLE THAT WE MAY HAVE INADVERTENTLY SWITCHED FROM THE #1 XPONDER; WHICH CONTAINED THE IFR BEACON CODE; TO THE #2 XPONDER; WHICH WAS CONFIGURED WITH A 1200 CODE. BOTH MY PAX AND I HAVE FLOWN INTO THE WASHINGTON ADIZ ON NUMEROUS OCCASIONS AND ARE FAMILIAR WITH THE PROCS THAT HAVE BEEN IN PLACE FOR THE LAST 2 YRS. IN THE FUTURE; I WILL CERTAINLY BE MORE VIGILANT TO ASSURE THAT MY PAX DOES NOT INADVERTENTLY CHANGE THE BEACON CODE; ESPECIALLY WHEN I AM OPERATING IN THE WASHINGTON ADIZ.