Narrative:

Air carrier X at FL200, was swbound. Cpr Y was sebound at FL210 landing mdw. Aircraft #3 was nwbound descending to FL220. I issued traffic to aircraft X and Y. I then looked at aircraft #3 who was landing at dbq and started working out my plan to get him down. I decided to take him under air carrier X and over cpr Y. After I had called traffic for aircraft X and Y, I had swapped their altitudes in my mind, thinking air carrier X was on top. I issued a clearance for cpr Y to FL170, heading 110 degrees. I then looked at #3 to see how the plan was looking now. I looked back at X and Y to see the conflict alert activate and separation already lost. I issued cpr Y FL210 but as I was giving the clearance, saw that his mode C already showed FL203. I didn't want to send him through air carrier X altitude twice, so I reissued a clearance to FL190 expedite through FL200. The direct cause of the error was my swapping the 2 aircraft's altitudes in my mind. The error could have been avoided by checking the radar 1 more time before issuing the clearance to FL170. Traffic was very light (4 aircraft on frequency), and I was probably too complacent about the traffic situation.

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Original NASA ASRS Text

Title: CPR Y DSCNT THROUGH OCCUPIED ALT HAD LTSS FROM ACR X. SYS ERROR.

Narrative: ACR X AT FL200, WAS SWBOUND. CPR Y WAS SEBOUND AT FL210 LNDG MDW. ACFT #3 WAS NWBOUND DSNDING TO FL220. I ISSUED TFC TO ACFT X AND Y. I THEN LOOKED AT ACFT #3 WHO WAS LNDG AT DBQ AND STARTED WORKING OUT MY PLAN TO GET HIM DOWN. I DECIDED TO TAKE HIM UNDER ACR X AND OVER CPR Y. AFTER I HAD CALLED TFC FOR ACFT X AND Y, I HAD SWAPPED THEIR ALTS IN MY MIND, THINKING ACR X WAS ON TOP. I ISSUED A CLRNC FOR CPR Y TO FL170, HDG 110 DEGS. I THEN LOOKED AT #3 TO SEE HOW THE PLAN WAS LOOKING NOW. I LOOKED BACK AT X AND Y TO SEE THE CONFLICT ALERT ACTIVATE AND SEPARATION ALREADY LOST. I ISSUED CPR Y FL210 BUT AS I WAS GIVING THE CLRNC, SAW THAT HIS MODE C ALREADY SHOWED FL203. I DIDN'T WANT TO SEND HIM THROUGH ACR X ALT TWICE, SO I REISSUED A CLRNC TO FL190 EXPEDITE THROUGH FL200. THE DIRECT CAUSE OF THE ERROR WAS MY SWAPPING THE 2 ACFT'S ALTS IN MY MIND. THE ERROR COULD HAVE BEEN AVOIDED BY CHKING THE RADAR 1 MORE TIME BEFORE ISSUING THE CLRNC TO FL170. TFC WAS VERY LIGHT (4 ACFT ON FREQ), AND I WAS PROBABLY TOO COMPLACENT ABOUT THE TFC SIT.

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.