THE PLT OF AN MO2J DISCOVERED DURING THE SECOND PREFLT OF THE DAY THAT THE FIRST FLT HAD BEEN FLOWN WITH THE WRONG CODE IN THE XPONDER.

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IT APPEARS THAT I OPERATED AN ACFT USING THE WRONG XPONDER CODE. THE EVENT OCCURRED WITHIN THE 30 MI LIMIT OF DCA REQUIRING A WORKING XPONDER AND BELOW THE LIMITS OF THE WASHINGTON CLASS B AIRSPACE. I FLEW THE ACFT FROM APPROX XA30 TO XB50 ON AUG/XA/97 IN THE LCL TFC AREA AT W32 PERFORMING PRACTICE TKOFS AND LNDGS. AFTER ATTENDING A MEETING FOR 2 HRS AT THE ARPT; I CHKED THE ACFT BEFORE FLYING AGAIN. I FOUND THAT THE XPONDER WAS SET TO CODE XXXX INSTEAD OF 1200. I WAS THE LAST PERSON FLYING AND APPARENTLY HAD FLOWN THE ACFT IN THE TFC PATTERN WITH THE WRONG XPONDER CODE. THE ARPT IS LOCATED 4.5 MI FROM ADW AFB AND LESS THAN 10 MI FROM DCA. THE TFC PATTERN AT W32 IS BELOW AND NOT INCLUDED IN THE WASHINGTON CLASS B AIRSPACE. THE PROB WAS DISCOVERED DURING A CAREFUL AND COMPLETE PREFLT CHK FOR THE SECOND FLT OF THE DAY. APPARENTLY; THE FIRST PREFLT CHK OF THE DAY OMITTED CHKING THE XPONDER CODE EVEN THOUGH I WAS CAREFUL TO SET THE XPONDER TO 'ALT' (MODE C) BEFORE EACH TKOF AND TO TURN IT TO 'STANDBY' AFTER EACH LNDG. CONTRIBUTING FACTORS WOULD INCLUDE LIMITED FLT EXPERIENCE IN THE LAST 120 DAYS AND THE XPONDER NOT INCLUDED ON THE PREFLT CHKLIST. CORRECTIVE ACTIONS WILL INCLUDE ADDING THE XPONDER CODE TO THE 'BEFORE TKOF' CHKLIST FOR BOTH ACFT THAT I FLY. ALSO; I WILL DEVELOP THE HABIT OF TOUCHING THE XPONDER FACE PLATE TO INSURE THAT I READ THE CODE DISPLAYED. THIS EVENT DEMONSTRATES HOW EASY IT IS TO 'THINK YOU ARE CAREFUL' WHEN IN FACT; AN IMPORTANT STEP WAS OMITTED.