ONLY AFTER DEPARTING; REENTERING AND LNDG AT MTN WITHIN THE DC ADIZ ARE THE INSTRUCTOR PLT AND STUDENT PLT OF A PA28 ADVISED THAT THEIR XPONDER WAS NOT XMITTING THE DISCRETE CODE PROGRAMMED INTO IT.

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I WAS PROVIDING INSTRUCTION TO A COMMERCIAL PLT APPLICANT. UPON ARRIVING AT MTN; I WAS ADVISED BY THE STUDENT THAT HE HAD OBTAINED AN FSS BRIEFING AND FILED A FLT PLAN FOR THE GOLDA-MTN RETURN FLT. NO ADVERSE WX WAS RPTED DURING THE BRIEFING AND NO NOTAMS WERE BRIEFED. WE WOULD DEPART MTN AND THE ADIZ VIA GOLDA AND USING THE PRESCRIBED PROC #1 WITH A DISCRETE XPONDER CODE. WE DEPARTED MTN USING RWY 33; AND MADE A L DOWNWIND DEP TOWARDS GOLDA; AFTER CLRING R-4001A; CLBING TO 2000 FT MSL. JUST PRIOR TO DEPARTING THE MTN CLASS D AIRSPACE; THE TWR ASKED US TO VERIFY THAT WE WERE SQUAWKING XXXX. THE STUDENT CHKED; AS DID I; AND VERIFIED THAT THE XXXX CODE WAS DISPLAYED AND THAT THE UNIT WAS BEING INTERROGATED. THIS INFO WAS RELAYED TO MTN TWR. AT THAT POINT; TWR DIRECTED US TO MONITOR POTOMAC APCH. ONCE AT GOLDA (AND CLR OF THE ADIZ); WE DEPARTED TO THE SSE WHERE WE DID AIRWORK UNTIL IN THE VICINITY OF RJD. WE LANDED ON RWY 30 AT RJD AND TOOK A BREAK. WE DEPARTED RWY 30 AT RJD; CLBING TO 2500 FT MSL AND HDG TOWARDS GOLDA. ABOUT 5 MI S OF GOLDA; I CONTACTED POTOMAC APCH TO ADVISE THEM THAT WE WERE INBOUND TO MTN AND WANTED TO GET THE XPONDER CODE FOR OP IN THE ADIZ TO MTN. I WAS TOLD THAT THERE WAS NO FLT PLAN ON FILE. I TOLD THE CTLR THAT ONE HAD BEEN FILED EARLIER IN THE DAY; BUT HE SAID THAT HE DIDN'T HAVE IT AND THAT WE'D HAVE TO RE-FILE. I CALLED LEESBURG FSS AND ASKED THEM TO HELP. THEY FOUND THE FLT PLAN AND ADVISED THAT IT HAD BEEN FILED WITH ALTOONA FSS; THEREFORE; IT WAS XMITTED TO LEESBURG FSS AND NOT TO ZDC WHERE IT APPARENTLY SHOULD HAVE GONE. THE BRIEFER ASSISTED AND OBTAINED THE NECESSARY INFO TO FILE THE REQUISITE FLT PLAN. I RE-ESTABLISHED CONTACT WITH POTOMAC APCH AND THE CTLR ASSIGNED A DISCRETE XPONDER CODE; WHICH I ACKNOWLEDGED. THE CODE WAS SELECTED ON THE XPONDER AND PROPER FUNCTION AND OP WERE NOTED; THE INTERROGATION LIGHT WAS FLASHING. AS WE CAME WITHIN 10+ MI OF MTN; I TRIED TO ESTABLISH CONTACT WITH APCH. THE APCH CTLR RESPONDED TO MY CALL AND ASKED US WHERE WE WERE. I TOLD HIM AND HE SAW US BUT HE STILL DID NOT AUTH THE FREQ CHANGE TO MTN TWR. AT A POS WHERE WE WERE ABOUT 3 MI N OF THE ARPT; APCH AUTH THE FREQ CHANGE AND WE ESTABLISHED CONTACT WITH MTN TWR AND THEN DSNDED INTO THE CLASS D AIRSPACE. UPON LNDG; AT APPROX XC50 EDT; GND ADVISED US THAT THEY HAD A TELEPHONE NUMBER THAT NEEDED TO BE CALLED. WE RESPONDED THAT WE WOULD CONTACT THEM BY PHONE. THEY GAVE ME THE NUMBER OF POTOMAC APCH. I ESTABLISHED CONTACT WITH POTOMAC APCH AND SPOKE WITH SUPVR. HE TOLD ME THAT OUR FLT HAD BEEN OBSERVED ONLY AS A 'PRIMARY TARGET' (IE; THE XPONDER WAS NOT ON OR OPERABLE AND THE CODE WAS NOT BEING RECEIVED BY ATC). I TOLD HIM OF THE INITIAL MTN INQUIRY; THAT WE HAD BEEN IN CONTACT WITH ATC THROUGHOUT THE REQUIRED PORTIONS OF OUR FLT AND THERE WAS NO MENTION OF ANY XPONDER PROBS; AND THAT THE XPONDER WAS FUNCTIONING NORMALLY. CTLR ADVISED THAT A RPT WAS GOING TO BE FILED; ATC TAPES AND RADAR PLOTS RECORDED; AND THAT A REVIEW WOULD BE CONDUCTED BY THE LCL FSDO. HE TOLD ME THAT I COULD EXPECT TO BE CONTACTED BY THE FSDO WITHIN 2-3 WKS.