DA20 INSTRUCTOR PLT AND STUDENT PLT DEP FME; MD; AND ENTER THE DC ADIZ WITH A VFR CODE IN THEIR XPONDER VERSUS THE ASSIGNED CODE WHEN CALLING THE LCL CTLR AT BWI.

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A FLT INSTRUCTOR AND STUDENT PLT WERE THE OCCUPANTS OF DIAMOND DA-20-C1 WHEN THE STUDENT PLT INITIATED THE TKOF CHKLIST AT TIPTON ARPT (FME); LOCATED IN THE WASHINGTON DC AIR DEFENSE IDENT ZONE (ADIZ). WHEN THE STUDENT READ THROUGH THE XPONDER PORTION OF THE CHKLIST; HE TURNED THE MODE SELECTOR SWITCH OF THE XPONDER TO THE ALT (MODE C) FUNCTION; BUT IN DOING SO; ALSO INADVERTENTLY STUCK THE 'VFR' BUTTON WITH THE KNUCKLE OF HIS R INDEX FINGER; WHICH PUT A CODE OF 1200 INTO THE XPONDER; ERASING THE CORRECT XPONDER CODE. THE VFR BUTTON IS LOCATED MILLIMETERS TO THE R AND BELOW THE XPONDER'S MODE SELECTOR SWITCH; MAKING IT EASY TO UNINTENTIONALLY STRIKE WHILE TURNING THE MODE SELECTOR. AS WE DEPARTED ON RWY 28 AND CLBED THROUGH 600 FT MSL (ABOUT 500 FT AGL); THE CFI NOTICED 1200 IN THE XPONDER INSTEAD OF ASSIGNED CODE. THE CFI INSTANTLY PUT THE CODE BACK INTO THE XPONDER AND THEN MADE IMMEDIATE CONTACT WITH BWI TWR ON 119.40. THE CFI HONESTLY NOTED THE XPONDER CODE AND THE INTENTIONS TO LAND AT BWI. THE TWR CTLR ASKED US TO 'IDENT' AND SOON THEREAFTER CLRED US INTO THE CLASS B AIRSPACE AND PLAN LNDG ON RWY 33R. FLT WAS CLRED TO LAND ON RWY 33R AND THEN WAS TOLD TO CONTACT GND CTL ON 121.90 AFTER CLRING THE RWY TO COPY A PHONE NUMBER FOR POTOMAC APCH. THE CFI WROTE THE NUMBER AND CALLED IMMEDIATELY AFTER SHUTDOWN. THE CFI EXPLAINED THE CLOSE PROX OF THE VFR BUTTON ON THE XPONDER TO THE FAA OFFICIAL AND THAT HE HAD PROMPTLY CONTACTED THE CTLR. THE FAA OFFICIAL INFORMED THE CFI THAT HE COULD EXPECT A PHONE CALL FROM THE FAA IN ABOUT 1 MONTH FOR FURTHER QUESTIONING. THIS PHONE CALL CAME 3 DAYS LATER.