INSTRUCTOR AND STUDENT PLTS OF C172 PENETRATE DC ADIZ ON DUAL XCOUNTRY.

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WE WERE ON A NIGHT VFR FLT FROM THV TO JYO VIA EMI VOR. WE HAD FILED BOTH ON ADIZ AND A VFR FLT PLAN AND HAD OPENED OUR VFR FLT PLAN WITH WILLIAMSPORT FSS ON DEP FROM THV. AFTER EMI VOR; WE TURNED DIRECT TOWARDS JYO AND CALLED POTOMAC APCH AROUND 24-26 NM NE OF THE AML VOR TO GET OUR ADIZ XPONDER CODE. MY STUDENT WAS FLYING AND OPERATING THE RADIOS. I WAS MONITORING THE GPS TO SEE OUR DISTANCE FROM AML TO ENSURE WE WERE NOT PENETRATING THE ADIZ AND I INSTRUCTED MY STUDENT TO START HIS DSCNT AND ENSURE WE REMAINED BELOW THE SHELF OF THE CLASS B AIRSPACE (3000 FT MSL IN THAT AREA). AT FIRST; THERE WAS NO RESPONSE FROM ATC AND THEN WE WERE GIVEN A XPONDER CODE AND A VECTOR TO 270 DEGS. WE WERE THEN HANDED OFF TO ANOTHER CTLR WHO TOLD US TO CONTACT THEM VIA TELEPHONE ONCE WE WERE ON THE GND. UPON DOING SO; I WAS TOLD THAT WE HAD PENETRATED THE ADIZ BY ABOUT 3-4 NM N OF GAITHERSBURG; MD. I WAS SHOCKED AND THE CTLR ON THE PHONE ASKED ME WHY I HAD NOT CALLED POTOMAC UP OVER EMI VOR. I EXPLAINED THAT TWICE BEFORE I HAD TRIED THAT AND THEY COULD NOT FIND MY FLT PLAN. ACCORDING TO OTHER INSTRUCTORS I HAD TALKED TO; EMI VOR IS IN THE NY SECTOR AND SOMETIMES THERE IS A DISCONNECT WHICH CAUSES THE PROB. I WAS CONFIDENT I WAS CLR OF THE ADIZ; BUT AT NIGHT; IT IS DIFFICULT TO SEE THE USUAL VISUAL REFS. ALSO; THE TERMINAL CHART I WAS CONSULTING INDICATES THE 20 NM RING FROM AML AS THE LIMIT OF THE ADIZ; BUT THERE IS A SEGMENT THAT EXTENDS FURTHER TO THE E THAT IS NOT MARKED WITH A DISTANCE FROM A NAVAID; SO IT IS VERY DIFFICULT TO DETERMINE WHERE THAT IS. IT IS POSSIBLE THAT A DEATH IN THE FAMILY; 2 DAYS PRIOR; MAY HAVE BEEN ON MY MIND; BUT I WAS WELL RESTED AND HAD EATEN. WE HAD FILED ALL NECESSARY FLT PLANS AND I HAVE BEEN FLYING IN AND AROUND THE ADIZ SINCE ITS INCEPTION. I FEEL THAT I WAS DOING EVERYTHING PROPERLY; BUT WAS A BIT OFF COURSE; WHICH IS WHY ATC CLAIMS I HAD AN INCURSION IN THE AIRSPACE. WE WERE MONITORING THE FREQ AND I WAS VERY AWARE ON THE ADIZ AND FEEL IT IS VERY DIFFICULT TO DETERMINE ITS BOUNDARY. ESPECIALLY AT NIGHT IN THAT AREA.