Narrative:

I was working 2 sectors combined (both FL230 and below) with 2 separate frequencys. Aircraft Y was on frequency overflt at FL210. Aircraft X checked on at FL240 and was descended to FL210 inbound to cvg (traffic was aircraft Y). Aircraft Z checked on (different frequency than aircraft X) at FL240 and was descended to FL190. 2 other aircraft were shipped to different facilities and another aircraft was descended. Traffic was issued to aircraft Y (aircraft X) and they replied. Traffic was then issued to aircraft X (aircraft Y) and there was no reply. Several seconds later they called saying that I had called them aircraft xx and that they were aircraft X and was the 'nineteen clearance' for them (I did not hear the altitude question part of their transmission). I transmitted, 'sorry, aircraft X, traffic 1 O'clock eastbound, one thousand below.' they replied, 'aircraft X roger, looking for traffic, down to one nine oh.' I did not hear them say down to FL190. I usually don't listen for a correct altitude readback when issuing traffic and no altitude was stated. I took my eyes off of the scope to check the ccld screen for traffic and to look for an aircraft that was inbound to cvg with a medical situation. This information had been physically coordinated to me right after I had issued traffic to aircraft X. Aircraft X descended to FL190 without a clearance, losing separation with aircraft Y. When I looked back at the scope, aircraft X was at FL199. I asked them to verify assigned altitude and they stated 'FL190.' during the 5 min period around this pilot deviation, I had caught 2 altitude readback errors. Again, I usually don't listen for correct altitude readbacks when issuing traffic when no altitude was stated. Supplemental information from acn 537481: I have no recollection of any ATC clearance deviations or inquiries, or of any TCASII TA's commanding an altitude deviation during this specific flight.

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Original NASA ASRS Text

Title: ZID OPERROR AND ACR PLTDEV CAUSED BY PERCEIVED CLRNC DSCNT AND MISSED READBACK AS A CARJ TAKES A DSCNT CLRNC OF A B190.

Narrative: I WAS WORKING 2 SECTORS COMBINED (BOTH FL230 AND BELOW) WITH 2 SEPARATE FREQS. ACFT Y WAS ON FREQ OVERFLT AT FL210. ACFT X CHKED ON AT FL240 AND WAS DSNDED TO FL210 INBOUND TO CVG (TFC WAS ACFT Y). ACFT Z CHKED ON (DIFFERENT FREQ THAN ACFT X) AT FL240 AND WAS DSNDED TO FL190. 2 OTHER ACFT WERE SHIPPED TO DIFFERENT FACILITIES AND ANOTHER ACFT WAS DSNDED. TFC WAS ISSUED TO ACFT Y (ACFT X) AND THEY REPLIED. TFC WAS THEN ISSUED TO ACFT X (ACFT Y) AND THERE WAS NO REPLY. SEVERAL SECONDS LATER THEY CALLED SAYING THAT I HAD CALLED THEM ACFT XX AND THAT THEY WERE ACFT X AND WAS THE 'NINETEEN CLRNC' FOR THEM (I DID NOT HEAR THE ALT QUESTION PART OF THEIR XMISSION). I XMITTED, 'SORRY, ACFT X, TFC 1 O'CLOCK EBOUND, ONE THOUSAND BELOW.' THEY REPLIED, 'ACFT X ROGER, LOOKING FOR TFC, DOWN TO ONE NINE OH.' I DID NOT HEAR THEM SAY DOWN TO FL190. I USUALLY DON'T LISTEN FOR A CORRECT ALT READBACK WHEN ISSUING TFC AND NO ALT WAS STATED. I TOOK MY EYES OFF OF THE SCOPE TO CHK THE CCLD SCREEN FOR TFC AND TO LOOK FOR AN ACFT THAT WAS INBOUND TO CVG WITH A MEDICAL SIT. THIS INFO HAD BEEN PHYSICALLY COORDINATED TO ME RIGHT AFTER I HAD ISSUED TFC TO ACFT X. ACFT X DSNDED TO FL190 WITHOUT A CLRNC, LOSING SEPARATION WITH ACFT Y. WHEN I LOOKED BACK AT THE SCOPE, ACFT X WAS AT FL199. I ASKED THEM TO VERIFY ASSIGNED ALT AND THEY STATED 'FL190.' DURING THE 5 MIN PERIOD AROUND THIS PLTDEV, I HAD CAUGHT 2 ALT READBACK ERRORS. AGAIN, I USUALLY DON'T LISTEN FOR CORRECT ALT READBACKS WHEN ISSUING TFC WHEN NO ALT WAS STATED. SUPPLEMENTAL INFO FROM ACN 537481: I HAVE NO RECOLLECTION OF ANY ATC CLRNC DEVS OR INQUIRIES, OR OF ANY TCASII TA'S COMMANDING AN ALTDEV DURING THIS SPECIFIC FLT.

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.