Narrative:

En route to cvg we had been getting stepped down a couple thousand ft at a time. Level at FL240 on the sweed a arrival past the henderson VOR, ZID advised air carrier X descend and maintain FL200. I read back the clearance to ATC. I then set the mode control panel altitude to 20000 ft and announced '20000 ft set' as per company procedure. The captain responded, '20000 ft checked' as per company procedure. The captain then initiated a descent using the vertical speed wheel on the mode control panel. The rate was 1000 FPM. Descending through 23800 ft when an airplane showed up on TCASII directly below us showing minus 8. The captain immediately began to level off using the vertical speed wheel. I looked out the right windshield and observed what appeared to be a fokker jet passing below us and to the right. At this time the TCASII commanded a climb and we began a gradual climb. ZID then inquired as to what altitude we were at. We advised the controller he had cleared us to FL200. He told us to climb back to FL240, and that the FL200 was for another aircraft. Advised him I had read back 'air carrier X cleared to FL200.' he told us to listen up. The captain and I both heard the clearance. The captain heard me read back the clearance and confirmed the altitude was reset properly on the MCP. I do not know how this could have happened other than the ATC controller issued us a clearance he had intended for another aircraft. The WX in cvg was bad and things were backed up and busy. Supplemental information from acn 294505: air carrier X took the clearance of another aircraft and descended to FL235 in conflict with an aircraft at FL240.

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Original NASA ASRS Text

Title: ACR X UNAUTH DSCNT FROM ASSIGNED ALT TCASII RA HAD LTSS FROM ACR Y. SYS ERROR ALTDEV. EVASIVE ACTION TAKEN.

Narrative: ENRTE TO CVG WE HAD BEEN GETTING STEPPED DOWN A COUPLE THOUSAND FT AT A TIME. LEVEL AT FL240 ON THE SWEED A ARR PAST THE HENDERSON VOR, ZID ADVISED ACR X DSND AND MAINTAIN FL200. I READ BACK THE CLRNC TO ATC. I THEN SET THE MODE CTL PANEL ALT TO 20000 FT AND ANNOUNCED '20000 FT SET' AS PER COMPANY PROC. THE CAPT RESPONDED, '20000 FT CHKED' AS PER COMPANY PROC. THE CAPT THEN INITIATED A DSCNT USING THE VERT SPD WHEEL ON THE MODE CTL PANEL. THE RATE WAS 1000 FPM. DSNDING THROUGH 23800 FT WHEN AN AIRPLANE SHOWED UP ON TCASII DIRECTLY BELOW US SHOWING MINUS 8. THE CAPT IMMEDIATELY BEGAN TO LEVEL OFF USING THE VERT SPD WHEEL. I LOOKED OUT THE R WINDSHIELD AND OBSERVED WHAT APPEARED TO BE A FOKKER JET PASSING BELOW US AND TO THE R. AT THIS TIME THE TCASII COMMANDED A CLB AND WE BEGAN A GRADUAL CLB. ZID THEN INQUIRED AS TO WHAT ALT WE WERE AT. WE ADVISED THE CTLR HE HAD CLRED US TO FL200. HE TOLD US TO CLB BACK TO FL240, AND THAT THE FL200 WAS FOR ANOTHER ACFT. ADVISED HIM I HAD READ BACK 'ACR X CLRED TO FL200.' HE TOLD US TO LISTEN UP. THE CAPT AND I BOTH HEARD THE CLRNC. THE CAPT HEARD ME READ BACK THE CLRNC AND CONFIRMED THE ALT WAS RESET PROPERLY ON THE MCP. I DO NOT KNOW HOW THIS COULD HAVE HAPPENED OTHER THAN THE ATC CTLR ISSUED US A CLRNC HE HAD INTENDED FOR ANOTHER ACFT. THE WX IN CVG WAS BAD AND THINGS WERE BACKED UP AND BUSY. SUPPLEMENTAL INFO FROM ACN 294505: ACR X TOOK THE CLRNC OF ANOTHER ACFT AND DSNDED TO FL235 IN CONFLICT WITH AN ACFT AT FL240.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.