PA28 PLT WAS SQUAWKING AN ASSIGNED DISCRETE XPONDER CODE WITHIN DC ADIZ; AND WAS TOLD TO SQUAWK 'VFR' BY THE PCT APCH CTLR WHEN NEARING HIS DEST ARPT; WITHIN DC ADIZ. WHEN HE COMPLIED WITH THE CTLR'S CLRNC; HE WAS VIOLATED FOR NOT SQUAWKING A DISCRETE XPONDER CODE WITHIN DC ADIZ.

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THE RPT DESCRIBES A PROB THAT OCCURRED ON A FLT FROM CRISFIELD; MD (W41) TO TIPTON/FORT MEADE (FME) IN MY PIPER CHEROKEE ON OCT/MON/03. I FILED AN ADIZ VFR FLT PLAN PRIOR TO DEPARTING W41; WHICH CALLED FOR ME TO ENTER THE ADIZ OVER EASTON; MD (ESN). WHEN I ARRIVED OVER ESN; I CONTACTED POTOMAC TRACON ON 126.75 TO OBTAIN A SQUAWK CODE. THE ATC TOLD ME TO SQUAWK XYZW; WHICH I DID. I THEN ENTERED THE ADIZ AND WAS UNDER CONTINUOUS 2-WAY CTL OF POTOMAC APCH DURING MY ENTIRE FLT FROM ESN TO FME. ON THE WAY; THE POTOMAC APCH ATC PROVIDED ALERTS TO THE PRESENCE OF OTHER ACFT. AT APPROX XA10 EDT I PASSED OVER THE DRAG STRIP FIX ON MY APCH TO FME; ABOUT 5 MI SE OF THE ARPT. AS I NEARED FME THE POTOMAC APCH ATC CONTACTED ME AND I UNDERSTOOD HIM TO SAY 'SQUAWK VFR FREQ CHANGE APPROVED.' I INTERPRETED THIS TO MEAN THAT I SHOULD SQUAWK 1200; THE VFR CODE AND CHANGE MY RECEIVER FREQ TO 123.05; THE FME UNICOM FREQ. SINCE I ALWAYS DO WHAT ATC TELLS ME TO DO; I CHANGED MY SQUAWK CODE FROM XYZW TO 1200; MY RADIO FREQ TO FME UNICOM AND THEN MADE AN UNEVENTFUL LNDG AT FME. AFTER LNDG AT FME; I WAS ADVISED BY THE ARPT MGR TO CALL POTOMAC TRACON WHICH I DID IMMEDIATELY. I WAS INFORMED THAT I WAS IN VIOLATION OF THE ADIZ NOTAM WHEN I SWITCHED THE SQUAWK CODE FROM XYZW TO 1200 PRIOR TO LNDG; AS I BELIEVED I WAS INSTRUCTED BY THE ATC. I HAVE FLOWN IN AND OUT OF THE ADIZ DOZENS OF TIMES AND HAVE NEVER HAD A VIOLATION. I HAVE ALWAYS FOLLOWED THE INSTRUCTIONS OF ATC CONCERNING SQUAWK CODES. ALWAYS IN THE PAST; ATC ADVISED ME TO STAY ON THE ASSIGNED SQUAWK CODE UNTIL LNDG ON THE RWY AND I HAVE FOLLOWED THESE INSTRUCTIONS. HOWEVER; THIS TIME WAS DIFFERENT BECAUSE I DIDN'T GET THIS INSTRUCTION. RATHER; I GOT WHAT I BELIEVED TO BE AN INSTRUCTION TO CHANGE TO THE VFR CODE. AS A CORRECTIVE ACTION; I RECOMMEND ATC ALWAYS EMPHASIZE THAT THE ASSIGNED SQUAWK CODE IS TO BE MAINTAINED DOWN TO THE GND AND THAT FURTHER ATC INSTRUCTIONS BE UNAMBIGUOUS AND PHRASED IN A WAY THAT WON'T BE MISCONSTRUED BY THE PLT. AS A PLT; I SHOULD ALWAYS DOUBLE-CHK TO VERIFY ANY INSTRUCTIONS THAT SEEM TO BE AMBIGUOUS BEFORE I TAKE ACTION. ALSO; IN THE FUTURE; I WILL ALWAYS STAY ON THE ASSIGNED SQUAWK CODE RIGHT DOWN TO THE GND NO MATTER WHAT I THINK I HEARD THE ATC SAY. FINALLY; I MIGHT WRITE THIS INCIDENT OFF AS A CASE OF ME MISUNDERSTANDING OR MISINTERPRETING WHAT THE ATC SAID DUE TO THE BUSY COCKPIT ACTIVITIES AND STRESSES OF MY APCH AND LNDG; EXCEPT THAT I TALKED TO THE PLT WHO LANDED RIGHT BEHIND ME AT FME AND HE SAID HE ALSO HEARD THE ATC GIVE ME THE 'SQUAWK VFR FREQ CHANGE APPROVED' INSTRUCTION; WHICH HE THOUGHT STRANGE IN LIGHT OF HIS UNDERSTANDING OF THE ADIZ NOTAM.