PLT OF BE36 INADVERTENTLY SQUAWKED VFR CODE IN DC ADIZ WHEN 'VFR' SWITCH ON XPONDER ACCIDENTALLY BUMPED IN TURBULENT CONDITIONS.

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I FILED AN ADIZ/VFR FLT PLAN FROM HEF TO JYO DEPARTING AT APPROX XA00 WITH A RETURN TO MANASSAS AT APPROX XD30. THE TRIP TO JYO WAS FOR MAINT I HAD PRE-SCHEDULED. I DEPARTED JYO AT XD00 WITH AN ASSIGNED DISCRETE XPONDER CODE AND CONTACTED PCT DEP. I WAS GIVEN A SERIES OF HDGS WHICH TOOK ME TO THE E OF IAD. THERE WAS LIGHT TO MODERATE TURB AT 2000 FT. AFTER PASSING TO THE S OF IAD; THE CTLR ASKED IF I HAD MANASSAS IN SIGHT. I RESPONDED AFFIRMATIVELY AND WAS TOLD TO CONTACT THE MANASSAS TWR. IT PROVED DIFFICULT TO SWITCH TO THE TWR FREQ AND MAKE PWR ADJUSTMENTS DUE TO THE TURB AND MY HAND MAY HAVE BUMPED MY XPONDER WHICH IS LOCATED DIRECTLY ABOVE MY PWR CTLS. IN ANY EVENT; THE FREQ WAS CHANNELED AND THE PWR REDUCTIONS WERE MADE APPROX 8 NM FROM MANASSAS. THE XPONDER HAS A VFR BUTTON THAT WHEN PRESSED; AUTOMATICALLY SWITCHES THE XPONDER TO 1200. THE BUTTON IS VERY SENSITIVE AND IT HAS BEEN INADVERTENTLY ACTIVATED MORE THAN ONCE WHILE ON IFR FLT PLANS AND DURING FLT FOLLOWING. AFTER AN UNEVENTFUL LNDG AT MANASSAS; I WAS TOLD TO PROCEED TO THE FBO AND CONTACT THE TWR. I WAS TOLD TO CALL TRACON AT PCT. IN OUR CONVERSATION; HE EXPLAINED THAT I HAD COMMITTED A VIOLATION IN THE ADIZ. THE SUPVR EXPLAINED THAT MY XPONDER CODE HAD BEEN SWITCHED FROM THE ASSIGNED DISCRETE CODE TO 1200 PRIOR TO MY LNDG AT MANASSAS. I DID NOT RECALL MAKING AN INTENTIONAL SWITCH TO A 'VFR ONLY' CODE. I AM VERY AWARE THAT A DISCRETE CODE IS REQUIRED IN THE ADIZ. I CAN ONLY SURMISE THAT I MAY HAVE BUMPED MY XPONDER VFR BUTTON. I WAS IN CONTACT WITH PCT DEP AND THE MANASSAS TWR DURING THE ENTIRE DURATION OF THE FLT AND AT NO TIME WAS I TOLD THAT MY XPONDER CODE HAD BEEN CHANGED TO 1200. I FULLY REALIZE THE PLT'S RESPONSIBILITY; BUT A HEADS UP NOTIFICATION WOULD HAVE BEEN WELCOMED AND CORRECTIVE ACTION WOULD HAVE BEEN TAKEN. IN THE FUTURE; I WILL MONITOR THE XPONDER CODE AS PART OF MY NORMAL SCAN AND CHECK IT AFTER EVERY COM AND NAV FREQUENCY CHANGE AND ESPECIALLY AFTER PWR CHANGES IN TURB.