A TB20 PLT CHANGES XPONDER CODE PREMATURELY WHEN GIVEN APPROVAL FOR FREQ CHANGE WHEN ENTERING PATTERN FOR LNDG AT FME; MD.

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MY ACFT; A TB20; WAS AT FME ON THE EDGE OF THE FLOOR TO CEILING AIRSPACE OF BALTIMORE; SO ONLY 1 SIDE OF THE RWY CAN BE USED FOR LNDG AND TKOF; PLUS THE CEILING IS ONLY 1500 FT. FOR THE YRS I HAVE BEEN USING THIS ARPT; WHEN CLRED THROUGH BRAVO AIRSPACE FROM THE N; YOU ARE NOT SURE WHAT DIRECTION OR HOW MANY PLANES ARE IN THE PATTERN WHEN YOU SAY YOU HAVE ARPT IN SIGHT. BEFORE BECOMING AN ADIZ; YOU WOULD TRY TO GET ON CTAF FREQ AND SQUAWK 1200 AS FAST AS YOU COULD; SO YOU COULD GET YOUR EYES OUT OF THE PLANE AS TO ENTER THE PATTERN ALMOST IMMEDIATELY. SINCE BECOMING AN ADIZ; AND THE CUSTOMS FOR 3 OTHER ARPTS IN THE ADIZ PLUS HAVING TO TALK TO APCH; THE PATTERN HAS BEEN VERY INTERESTING TO SAY THE LEAST. ON THIS OCCASION; WHEN RPTING ARPT IN SIGHT AND WAS TOLD FREQ CHANGE APPROVED; I; BY REFLEX; STARTED TO CHANGE XPONDER TO 1200; ONLY CHANGING 2 NUMBERS WHEN HEARING THE CTLR SAY; 'STAY ON CODE UNTIL ON GND;' WHICH I KNOW TO DO. I TOLD THE CTLR I CHANGED 2 NUMBERS; AND HE TOLD ME TO CHANGE BACK TO THE ORIGINAL NUMBERS; WHICH I DID NOT REMEMBER. I WAS APPROX 1 MI FROM FIELD BY THIS TIME; AND HE GAVE ME 2 OTHER NUMBERS TO PUT IN. WHEN I LANDED (PATTERN WAS EMPTY) AND UNICOM WAS CLOSED FOR ADVISORIES; I CALLED POTOMAC TO CANCEL ON THE GND. THE CTLR; WHO ANSWERED THE PHONE IMMEDIATELY; STARTED TO REPRIMAND ME LIKE A CHILD; AND SAID I COULD GET 90 DAYS SUSPENSION. I WAS TRYING TO GET AHEAD OF THE AIRPLANE ANTICIPATING A BUSY UNCTLED PATTERN. THE LAST NICE DAY I FLEW OUT OF TIPTON; THE CTLR COULD NOT KEEP UP WITH THE TFC; AND TOLD PEOPLE TO CIRCLE THE ARPT; THE PATTERN IS 1000 FT AND THE CEILING IS 1500 FT; VERY SCARY. ON THE DAY OF THE INCIDENT; ALL THE CTLRS WERE GREAT; AND I WAS GIVEN 3 XPONDER CODES FOR THE 1 1/2 HR TRIP FROM CONNECTICUT; WHICH BEING A HOLIDAY; HELPED ACCOUNT FOR THE LOW VOLUME.