FLC OF AN LGT WERE GIVEN THE WRONG FLT PLAN CLRNC BY ATC PRIOR TO DEP DUE TO A CHANGE OF ACFT RESULTING IN THE USE OF THE WRONG XPONDER CODE AND CALL SIGN BY ATC. ATC SUBSEQUENTLY OBTAINED A CORRECTED FLT PLAN AND PASSED IT ON TO THE RPTR.

Narrative

DUE TO WX AND MAINT DELAYS; WE WERE APPROX 3 HRS LATE DEPARTING MEM. WE RECEIVED A PDC APPROX 1 HR AFTER OUR ORIGINALLY SCHEDULED DEP TIME. SUBSEQUENTLY; WE CHANGED TO A DIFFERENT ACFT DUE TO A MAINT PROB ON THE ORIGINAL ACFT. THIS CAUSED US AN ADDITIONAL DELAY OF APPROX 1 1/2 HRS. BELIEVING THAT OUR ORIGINAL CLRNC HAD 'TIMED OUT' BY THEN; WE CALLED MEM CLRNC TO GET A NEW CLRNC. MEM CLRNC ISSUED A CLRNC AS EXPECTED AND WE TAXIED FOR DEP. WE WERE CLRED FOR TKOF ON ARR AT RWY 27 AND DEPARTED NORMALLY. ONCE AIRBORNE; MEM TWR BEGAN CALLING US BY A DIFFERENT FLT NUMBER; CAUSING US CONCERN THAT WE HAD TAKEN OFF WITHOUT PROPER CLRNC. EACH OF THE 3 CREW MEMBERS WAS CERTAIN; HOWEVER; THAT MEM TWR HAD IN FACT CLRED OUR FLT NUMBER FOR TKOF; AND ONLY BEGAN CALLING US BY THE DIFFERENT FLT NUMBER ONCE WE WERE AIRBORNE. APPARENTLY; OUR CLRNC HAD IN FACT TIMED OUT; AND WHEN WE CALLED MEM CLRNC EARLIER; THE CTLR HAD MERELY READ US THE CONTENTS OF OUR ORIGINAL PDC; NOT REALIZING THAT IT HAD TIMED OUT. HAVING TIMED OUT; OUR XPONDER CODE HAD BEEN ASSIGNED TO ANOTHER FLT. SO; ONCE WE WERE AIRBORNE; ATC RADAR READ OUR XPONDER CODE AND IDENTED US AS THE OTHER FLT. AT THE SAME TIME; THERE WAS NO FLT PLAN INFO IN THE ATC COMPUTER FOR OUR FLT. IT TOOK ATC SEVERAL MINS TO UNDERSTAND WHAT HAD HAPPENED; AND TO INPUT A FLT PLAN FOR US. SUBSEQUENTLY; WE WERE CLRED TO OUR DEST; AND THE FLT PROCEEDED NORMALLY. IN THIS CASE; IT SEEMS LIKE WE FELL THROUGH ONE OF THE CRACKS IN THE ATC SYS. WHILE IT WAS DISCONCERTING; THE SYS FAILURE DID NOT ADVERSELY AFFECT FLT SAFETY DIRECTLY. IN THE FUTURE; I WILL WATCH FOR THESE CIRCUMSTANCES CAREFULLY; AND QUERY THE CTLRS TO BE SURE THAT OUR CLRNC HASN'T TIMED OUT WITHOUT ANYONE NOTICING.