DC ADIZ PENETRATION WHEN A C172 PVT PLT STAYS IN A CLOSED PATTERN FOR 2 LNDGS; BUT FORGOT TO USE THE XPONDER CODE ASSIGNED IN HIS CLRNC FROM PCT CTLR FOR LOCAL FLT AT GAI; MD.

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I FILED A FLT PLAN TO STAY IN THE PATTERN AND PRACTICE TAKEOFFS AND LNDGS. THIS IS NECESSARY SINCE GAI IS IN THE WASHINGTON ADIZ. I ESTIMATED THAT TAKEOFF WOULD OCCUR AT XA15. MY LAST FLT HAD BEEN IN FEBRUARY AND I HAD A SINGLE LNDG. SO I NEEDED TO MAKE TWO LNDGS IN ORDER TO CARRY A PAX -- IN THIS CASE MY SAFETY PLT. I CALLED PCT CLRNC DELIVERY AND GOT MY CLRNC TO STAY IN THE PATTERN. I WAS GIVEN A XPONDER CODE AND TOLD TO CONTACT CLRNC DELIVERY AFTER I COMPLETED MY PATTERN WORK AND WAS BACK ON THE GND. I FORGOT TO CHANGE THE XPONDER AND WAS SHOWING 1200 DURING MY FIRST TAKEOFF AND LNDG. DURING THE SECOND CIRCUIT; THE GAI UNICOM ASKED IF ANY AIRPLANES WERE IN THE PATTERN. I RPTED MY CALL SIGN AND SAID I WAS IN THE PATTERN AND WAS TOLD TO CALL ATC WHEN I LANDED. REALIZING THAT SOMETHING WAS WRONG I CHKED THE XPONDER AND NOTICED; TO MY CHAGRIN THAT IT WAS SET ON 1200. I QUICKLY CHANGED IT TO THE ASSIGNED CODE; BUT BY THEN IT WAS TOO LATE. WHEN I LANDED I CALLED ATC; EXPLAINED WHAT HAPPENED; AND GAVE THEM MY PERSONAL INFO; ADDRESS; PLT NUMBER; ETC. CONTRIBUTING FACTORS: I WAS IN A HURRY TO GET MY LNDGS IN BEFORE MY SAFETY PLT ARRIVED. IN ADDITION; I WAS WAITING AT AN AREA WHERE TWO OTHER AIRPLANES WERE GETTING IFR CLRNCS. ONE DEPARTED AHEAD OF ME AND THE OTHER GOT HIS CLRNC AND ASKED FOR A 5 MIN DELAY IN TAKING OFF. I WAS TRYING TO TAKE OFF BEFORE HIM. I SHOULD HAVE TAKEN MY TIME AND DONE A FINAL CHK OF EVERYTHING. POSSIBLE SOLUTION: I USE A CHKLIST BUT SOME ITEMS ARE NOT INDIVIDUALLY LISTED AND FALL UNDER A GENERIC HDG; SUCH AS CHK AND SET INSTRUMENTS. I'VE DECIDED TO CONSTRUCT A CHKLIST WHERE I WILL DESIGNATE EACH ITEM THAT MUST BE CHKED BEFORE DEP AND USE A PEN TO PHYSICALLY CHK OFF ITEMS AS I GO DOWN THE LIST.