PLT OF PA28 RPTS HE WAS IMPROPERLY DIRECTED TO SQUAWK VFR CODE WITHIN DC ADIZ. CONTACTED BY PCT AND ADVISED OF POTENTIAL VIOLATION.

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XA:45 SUN; I WAS ENRTE FROM ANP TO BAY BRIDGE ARPT (W29) ON A VFR FLT PLAN. I WAS COMMUNICATING WITH PCT TRACON SQUAWKING A DISCRETE CODE. APPROX EIGHT MILES SW OF W29; BALTIMORE APCH ASKED IF I HAD W29 IN SIGHT. I INDICATED 'ARPT IN SIGHT' AND WAS INSTRUCTED FROM WHAT I RECALL TO SQUAWK VFR AND SWITCH TO ADVISORY FREQ. I BELIEVE MY READBACK WAS 'SQUAWKING VFR AND SWITCHING TO ADVISORY.' I RETUNED THE TRANSPONDER WITH 1200 AND CONTINUED COMMUNICATING ON 123.00. I ENTERED A L DOWNWIND; CONDUCTED A BEFORE LNDG CHK; AND NOTICED AN UNSAFE GEAR INDICATION. I DID NOT SEE A 'THREE-IN-THE-GREEN' GEAR INDICATION FOR THE LNDG GEAR. I IMMEDIATELY DEPARTED THE TFC PATTERN; CLBED TO 2000 FT MSL AND LOITERED APPROX FIVE NM S OF W29 TO SOLVE THE GEAR PROB. I ANNOUNCED MY ACTIONS ON 123.00. AFTER APPROX FIVE MINS; I SOLVED THE GEAR PROB; REENTERED THE TFC PATTERN; AND LANDED AT W29. AFTER LNDG AT W29; I CALLED PCT; I EXPLAINED TO THEM THAT I WAS ON THE GND AT W29. I REQUESTED A SQUAWK FREQ AND CLRNC TO DEPART W29 FOR ANP. I WAS TOLD TO SQUAWK DISCRETE AND TO CONTACT PCT TRACON IMMEDIATELY UPON DEP. THE CTLR I SPOKE WITH ASKED IF I SWITCHED MY SQUAWK TO 1200 ENRTE FROM ANP TO W29. I SAID 'YES; I DID THAT AS INSTRUCTED.' I ASKED IF I WAS RELEASED FOR A VFR DEP FROM W29 TO ANP. CTLR SAID I WAS CLRED FOR DEP. I FOLLOWED MY DEP INSTRUCTIONS EXPLICITLY AND WAS CLRED TO ANP ON COURSE. APPROX EIGHT NM E OF ANP PCT INSTRUCTED ME TO CALL THEM IMMEDIATELY UPON LNDG AT ANP. THE CTLR INDICATED THAT I MIGHT HAVE COMMITTED A VIOLATION. RECOMMENDATION: I BELIEVE A SOLUTION TO THIS PROB WITHIN THE WASHINGTON DC ADIZ INVOLVES A TWO-WAY COMMUNICATION CHK BTWN ATC AND THE PLT. WHEN APCH CTL RELEASES AN ACFT TO ADVISORY FREQ; THE VERBIAGE SHOULD SPECIFICALLY INSTRUCT THE PLT TO REMAIN ON THE DISCRETE SQUAWK UNTIL ON THE GND. THE PLT SHOULD ALSO READ BACK; 'SQUAWKING XXXX; SWITCHING TO ADVISORY.'