A C177RG PVT PLT IS CITED FOR CHANGING HIS XPONDER CODE TO 1200; VFR OPS; AFTER CANCELING HIS FLT PLAN WITH PCT CTLR WHEN IN TFC PATTERN TO LAND AT FME.

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I AM A PVT PLT ON AN INSTRUMENT FLT FROM AGC TO FME. FRONT SEAT PAX IS A PRIVATE PILOT WITH INSTRUMENT RATING AND IS CURRENT. I RECEIVED A BRIEF FROM FSS AND FILED AN IFR FLT PLAN; INTENDING TO OPERATE ON IFR FLT PLAN DUE IN PART TO TFR AND WASHINGTON AIRSPACE ISSUES. DURING THE COURSE OF THE FLT NON-FLYING PLT ASSISTED WITH RADIO TUNING. UPON VISUAL APCH TO FME CTLR ASKED ONCE FOR ME TO CANCEL IFR ON THIS FREQ OR ON THE GND BY PHONE. CTLR CLRED ME FOR THE VISUAL APCH TO FME AND AGAIN INSTRUCTED ME TO CANCEL IFR ON THIS FREQ OR AS SOON AS POSSIBLE WHEN ON THE GND. I CANCELLED IN THE AIR AND THE CTLR STATED THAT CANCELLATION WAS RECEIVED AND THAT I COULD SWITCH TO ADVISORIES. I CHANGED TO THE FME UNICOM; AND NO LONGER SPOKE OR HEARD APCH. UPON LNDG ARPT STAFF ASKED ME TO CALL APCH. THE APCH SUPVR ASKED ME IF I CHANGED XPONDER CODES; I STATED NO. HOWEVER; THE NON-FLYING PLT STATED THAT HE RESET THE XPONDER TO 1200 WHEN I CANCELLED IFR. I WAS FOCUSED ON FLYING THE ACFT; SETTING UP FOR LNDG AND ASSURING THAT ALL PRELNDG CHKS WERE MADE. I FAILED TO NOTICE THE NON-FLYING PLT RESETTING THE XPONDER AND FAILED TO RECHK THE XPONDER NUMBERS DURING THE FINAL. I HAVE ASKED THE NON-FLYING PLT TO CONFIRM ALL RADIO/XPONDER CHANGES WITH ME PRIOR TO MAKING THE CHANGES.