PA44 STUDENT AND INSTRUCTOR ALLEGE INTERMITTENT EQUIP MALFUNCTION; ADVISED BY PCT ON ESTABLISHED COM OF INCORRECT XPONDER CODE PENETRATING THE DC ADIZ.

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ACFT ENTERED ADIZ WITH XPONDER DISPLAYING 1200 AFTER XPONDER WAS SET TO XXXX BY PIC/PNF AS ADVISED BY POTOMAC APCH ON 125.05. SUSPECTED PROB: ELECTRONIC DISPLAY/XPONDER MALFUNCTION IN COCKPIT. CFI (PF) RECEIVING INSTRUCTION FOR INITIAL MULTI-ENG RATING FROM CFI/PIC (PNF). FILED 2 ADIZ VFR PLANS WITH LEESBURG FSS TO DEPART ADIZ JYO (LEESBURG; VA) DIRECT TO OKV (WINCHESTER; VA) AND RETURN. DEPARTED JYO. CONTACTED POTOMAC DEP; DIRECT OKV. EXITED ADIZ; SQUAWKED 1200. AIRWORK PERFORMED AS PREBRIEFED. GEAR EXTENDED SEVERAL TIMES DURING LESSON. UPON RETRACTION; GEAR EXHIBITED IN TRANSIT LIGHT AND LOW VOLTAGE BUS WARNING LIGHT FLASHED ON/OFF SEVERAL TIMES. PIC (A&P) TROUBLESHOT; UNABLE TO DUPLICATE INFLT. DEPARTED OKV VICINITY FOR RETURN JYO. APPROX XA00; OUTSIDE ADIZ; 300 DEG RADIAL AML; 25 DME; MADE 1 360 DEG TURN TO REMAIN CLR OF ADIZ AND CONTACTED ATC ON 125.05. RECEIVED CODE XXXX AND PIC ENTERED CODE WITH READBACK TO ATC. PIC (PNF) AND STUDENT (ALSO CFI) VISUALLY VERIFIED XXXX MODE C DISPLAYED ON XPONDER. ACFT ENTERED ADIZ AT XA05. APPROX 5 MINS LATER; ATC REQUESTED OUR POS AND HDG. PIC (PNF) ADVISED LOCATION AND HDG. ATC REQUESTED SQUAWK IDENT; PIC COMPLIED. ATC ADVISED XPONDER XMITTING CODE 1200 AND ADVISED 'SQUAWK XXXX QUICKLY.' PIC VERBALLY CONFIRMED XPONDER SET TO XXXX AND RECYCLED AS PRECAUTION. NO FURTHER COMMENT FROM ATC RECEIVED. PIC ADVISED JYO IN SIGHT. ATC APPROVED FREQ CHANGE TO CTAF. UPON RETURN TO FLT SCHOOL; WE WERE ADVISED TO CALL ATC. PIC INFORMED OF ATC COMPLAINT; BRIEF DISCUSSION OF XPONDER MALFUNCTION ENSUED. WROTE UP XPONDER DISCREPANCY PER 91.213; PLACARDED INOP; GNDED FOR MAINT. WROTE UP LOW BUS VOLTAGE WARNING LIGHT PER 91.213; TROUBLESHOT; NFF; CHK AT 100 HR. NEXT DAY; POTOMAC APCH QUALITY ASSURANCE ADVISED ATC TAPES SHOWED ACFT ENTERED ADIZ AT XA09; DISPLAYING CODE 1200; DESPITE COCKPIT DISPLAY OF XXXX.