AN AUTOMATIC 'VFR' FUNCTION MAY HAVE CAUSED THE CAPT OF A BE-1900 TO BE ACCUSED BY PCT OF ENTERING THE DC ADIZ WITHOUT A CODE.

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I WAS THE CAPT ON A BEECH 1900 PERFORMING A REPOSITIONING FLT FROM WASHINGTON DULLES ARPT TO MANASSAS REGIONAL ARPT. AFTER LNDG IN MANASSAS; I WAS INFORMED THAT I NEEDED TO CALL APCH CTL. WHEN I CALLED THEM; THEY TOLD ME THAT I HAD BEEN ISSUED A SQUAWK CODE BUT WE HAD SQUAWKED 1200 FOR THE ENTIRE FLT. THE MAJOR PROB WAS THAT IT OCCURRED IN THE WASHINGTON ADIZ. I WAS SURPRISED AND TOLD THEM THAT I DIDN'T KNOW HOW THAT HAPPENED; THAT MAYBE IT WAS AN OVERSIGHT. UPON REVIEWING THE SIT WITH THE FO; WE CONFIRMED WITH EACH OTHER THAT WE HAD PERFORMED THE TAXI CHKLIST WHICH INCLUDES A CHK OF THE NAV; COM; AND XPONDER SETTINGS. WE DETERMINED THAT THE 1200 SQUAWK CODE MUST HAVE COME FROM THE XPONDER'S VFR MODE. ON THE TCASII/XPONDER INSTALLED IN THE BEECH 1900D; THERE IS A ROTARY DIAL WHICH CTLS THE MODE IN WHICH THE XPONDER IS OPERATING. WHILE ON THE GND; IT IS IN STAND-BY MODE. ONCE CLRED FOR TKOF; IT IS PLACED IN THE TCASII/TARA MODE. BTWN STAND-BY AND TCASII MODES; THE DIAL PASSES THROUGH THE VFR MODE. IF THE DIAL IS PAUSED AT ALL IN THE VFR MODE; THE EXISTING SQUAWK CODE IS ERASED AND IS REPLACED WITH 1200. THIS IS WHAT WE BELIEVE INADVERTENTLY HAPPENED.