FLC OF AN LGT ACR ACFT INADVERTENTLY TAKE OFF WITH THE WRONG XPONDER CODE DUE TO NOT HEARING THEIR FILED CLRNC FROM CLRNC DELIVERY.

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ACR FLT DEPARTED DCA AT APPROX XY20L. IMMEDIATELY AFTER TKOF; CONTACT WITH DEP CTL WAS ESTABLISHED. THE CTLR REQUESTED A CHANGE OF THE XPONDER CODE FROM THAT SET PRIOR TO DEP. THE FLT PROGRESSED NORMALLY TOWARD LGA. DURING THE LATTER PORTION OF THE CLB; I LOOKED AT MY SCRATCH PAD AND NOTICED A CLRNC FROM THAT MORNING'S EARLIER TRIP TO LGA. ALTHOUGH THE COMPUTER STORED CLRNCS INVARIABLY REMAIN THE SAME; XPONDER CODES DO VARY FROM FLT TO FLT. AT THIS TIME; IT OCCURRED TO ME THAT THE CODE SET AT DEP ON THIS PARTICULAR FLT WAS THAT USED EARLIER IN THE DAY. I PONDERED THIS PECULIARITY FOR SEVERAL MINS; AND IT STRANGELY DAWNED ON ME THAT WE MAY NEVER HAD CONFIRMED THE XPONDER CODE WITH DCA CLRNC DELIVERY. WE DID CALL FOR OUR CLRNC AT APPROX XX50L; AND WAS TOLD TO STANDBY AS WE WERE #2 IN SEQUENCE. WE CONTINUED TO MONITOR CLRNC DELIVERY AND AT THE SAME TIME CONDUCTED OUR COCKPIT BUSINESS. AT DEP TIME OF XY00L; WE SWITCHED TO GND CTL; OBTAINED OUR PUSHBACK CLRNC; AND HENCE A NORMAL AND UNEVENTFUL SEQUENCE OF EVENTS TRANSPIRED FOR THIS FLT. THE ONLY PECULIARITY OF THIS DAY WAS A REQUEST TO CHANGE XPONDER CODES; WHICH IN ITSELF IS NOT UNUSUAL. HOWEVER; THE CODE SET IN WAS THAT GIVEN TO US 2 FLTS PRIOR THAT SAME DAY. MY ONLY CONCLUSION TO THIS MATTER IS THAT THE CLRNC WAS READ TO US; NOT HAVING WAITED FOR OUR ACKNOWLEDGEMENT; ANOTHER FLT POSSIBLY STEPPED ON OR INADVERTENTLY INTERFERED WITH COMS. THIS; COMBINED WITH OTHER PRE-DEP BUSINESS; MAY HAVE CREATED AN OVERSIGHT TO INSURE THAT THE CLRNC WAS PROPERLY OBTAINED. DURING BUSY TIMES AT DCA; 2 FREQS SHOULD BE USED TO INSURE THAT READBACKS ARE PROPER. CTLRS MUST SLOW DOWN SUFFICIENTLY TO INSURE THAT ALL FLTS ACKNOWLEDGE CLRNCS PROPERLY.