Narrative:

I was the captain on a beech 1900 performing a repositioning flight from washington dulles airport to manassas regional airport. After landing in manassas, I was informed that I needed to call approach control. When I called them, they told me that I had been issued a squawk code but we had squawked 1200 for the entire flight. The major problem 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 situation with the first officer, we confirmed with each other that we had performed the taxi checklist which includes a check of the navigation, communication, and transponder settings. We determined that the 1200 squawk code must have come from the transponder's VFR mode. On the TCASII/transponder installed in the beech 1900D, there is a rotary dial which controls the mode in which the transponder is operating. While on the ground, it is in stand-by mode. Once cleared for takeoff, it is placed in the TCASII/tara mode. Between 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.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.