C172 OPERATING IN THE DC ADIZ IS QUESTIONED REGARDING IDENT.

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ON FEB/SUN/04 I DEPARTED AN ARPT WITHIN THE WASHINGTON METROPOLITAN ADIZ ON AN INST TRAINING FLT; ACCOMPANIED BY A CERTIFICATED PVT PLT AND INST STUDENT OF MINE. IN COMPLIANCE WITH THE NOTAM GOVERNING THE WASHINGTON ADIZ; WE FILED AN ADIZ FLT PLAN; OBTAINED; ENTERED AND SQUAWKED A DISCRETE XPONDER CODE; AND ESTABLISHED AND MAINTAINED 2-WAY COMS WITH ATC. OUR INITIAL CALL TO POTOMAC APCH WAS; 'POTOMAC APCH; CESSNA XXX 1400 FT OFF W00; ENRTE TO THE DEALE PRACTICE AREA.' POTOMAC RESPONDED; 'CESSNA XXX POTOMAC APCH; ROGER. PROCEED VFR; FREQ CHANGE APPROVED AT PLT'S DISCRETION.' APPROX 10 MINS LATER; POTOMAC ASKED; 'IS THERE AN XXX ON FREQ?' I REPLIED THAT YES; WE WERE ON FREQ; AND HAD CHKED IN WITH HIM ABOUT 10 MINS PRIOR. HE REPLIED THAT HE WAS NOT AWARE OF THE REASON; BUT THAT WE WERE TO CONTACT TRACON BY TELEPHONE. AFTER ANOTHER 20 MINS OR SO OF FLT TRAINING; I HEARD THE CTLR PROVIDE TFC AVOIDANCE ADVISORIES TO 2 ACFT AHEAD OF US; AND SOUNDED AS IF HE WAS DESCRIBING OUR ACFT AS THE TFC. HE ADDED IN THE XMISSION THAT HE WAS NOT 'WORKING' THE ACFT IN QUESTION (US). SHORTLY THEREAFTER; HE SAID; 'ACFT SQUAWKING XXXX (US); POTOMAC.' I REPLIED THAT IT WAS CESSNA XXX AND THAT WE HAD ALREADY COMMUNICATED WITH THEM A COUPLE OF TIMES. AFTER A FEW MINS; HE ASKED US TO SQUAWK A NEW BEACON CODE; WHICH WE DID. AT THIS POINT; WE DECIDED TO RETURN TO W00; NOTIFIED POTOMAC; RECEIVED A FREQ CHANGE TO CTAF; LANDED; AND CALLED THE PHONE NUMBER PROVIDED INFLT. THE PHONE NUMBER CONNECTED ME TO POTOMAC TRACON; WHICH INITIALLY NOTIFIED ME THAT WE WERE OBSERVED BY NORTH EAST AIR DEFENSE SECTOR (NEADS) AFTER TKOF TO BE SQUAWKING 1200 IN THE ADIZ; RATHER THAN OUR ASSIGNED CODE OF XXXX. I DENIED THIS. THE NEXT DAY; I CALLED AGAIN TO FOLLOW UP AND WAS INFORMED THAT WE WERE NOT; IN FACT; OBSERVED SQUAWKING 1200; BUT THAT NEADS DID NOT OBSERVE OUR XPONDER AT ALL AFTER WE FINALLY TOOK OFF. BECAUSE OF THIS; NEADS TAGGED US AS A THREAT ACFT AND RPTED US AS SUCH; EVEN THOUGH WE WERE FLYING AWAY FROM WASHINGTON; DC; AND THEY OBSERVED OUR XPONDER THEREAFTER. THE FLT IN QUESTION WAS MORE THAN MY 500TH FLT SINCE THE WASHINGTON ADIZ WENT INTO EFFECT FEB/03. I FLY MANY FLTS PER DAY IN FULL COMPLIANCE WITH THE APPLICABLE NOTAM; AND; THEREFORE; AM CERTAINLY NO STRANGER TO THE REQUIRED PROCS AND EVERYDAY WORKINGS OF THE ADIZ. IN FACT; I AM RESPONSIBLE FOR TEACHING ADIZ PROCS TO MY STUDENTS AND OTHER PLTS WHO UTILIZE OUR ARPT. DUE TO THE INEVITABLE; OCCASIONAL RADAR MALFUNCTIONS; XPONDER FAILURES AND/OR OTHER OCCURRENCES; THERE HAVE BEEN SEVERAL FLTS WHERE MY XPONDER WAS NOT DETECTED IMMEDIATELY; OR AT ALL; BY ATC. THE DIFFERENCE ON THOSE OCCASIONS WAS THAT I WAS NOTIFIED BY ATC OF THE FACT; AT WHICH POINT A RECYCLING OF THE XPONDER; OR A REPOSITIONING OF THE ACFT TYPICALLY PRODUCED RADAR CONTACT. IF IT DID NOT; THEN I WAS ADVISED TO RETURN TO THE ARPT AND LAND; WHICH I DID. ON THIS OCCASION; ALTHOUGH WE HAD 2-WAY COMS; THERE WAS NO NOTIFICATION BY ATC OF ANY RADAR/XPONDER PROBS; WITHOUT WHICH WE WOULD BE UNAWARE OF ANY PROBS. TO THE CONTRARY; WE WERE TOLD TO PROCEED WITH OUR FLT UPON ESTABLISHING 2-WAY COMS. FURTHERMORE; I WAS ALSO NOTIFIED BY POTOMAC TRACON BY TELEPHONE THAT THE REASON WE WERE SWITCHED MID-FLT FROM A BEACON CODE OF XXXX TO YYYY WAS BECAUSE ANOTHER ACFT OUTSIDE OF THE ADIZ HAD ALSO BEEN ASSIGNED THE XXXX CODE. THIS; APPARENTLY; CONTRIBUTED TO THE CONFUSION AND INABILITY OF ATC AND/OR NEADS TO MAINTAIN POSITIVE IDENT OF OUR ACFT DURING THE FLT. IT IS MY BELIEF THAT 3 PRIMARY FACTORS CONTRIBUTED TO THIS PROB. 1) ALTHOUGH WE MAINTAINED 2-WAY COMS WITH ATC; WE WERE NOT NOTIFIED BY ATC DURING OUR FLT OF ANY ANOMALIES. 2) NEADS DETECTED A RADAR/XPONDER PROB NOT RELAYED TO ME; NOR APPARENTLY; TO POTOMAC TRACON; WHICH HAD CTL OVER THE AIRSPACE. 3) OUR ACFT WAS ASSIGNED A DISCRETE XPONDER CODE ALSO ASSIGNED TO ANOTHER ACFT. IN ORDER TO PREVENT A RECURRENCE OF THIS SIT; ALL FEDERAL AGENCIES INVOLVED WITH CTLING/PATROLLING THE WASHINGTON ADIZ MUST COMMUNICATE MOREEFFECTIVELY AND PROACTIVELY WITH PLTS AND WITH ONE ANOTHER. IN ADDITION; ATC MUST TAKE CARE NOT TO ASSIGN SPECIFIC DISCRETE XPONDER CODES TO MORE THAN 1 ACFT.