Narrative:

A cmh inbound was wbound descending out of FL240 for 10000 ft. I then took the handoff on a cak inbound descending northbound out of FL210 for 15000 ft. I recognized that the 2 aircraft were in conflict in several mins, so I stopped air carrier X at FL180. He was now out of FL240. Cpa Y was now out of FL190 descending still to 15000 ft. I still had a couple of mins before they were in conflict. I then issued control instructions to several other aircraft. I then looked back to air carrier X and cpr Y to find that the conflict alert had activated. At this point the 2 aircraft were about 8-10 mi apart with altitudes of FL184 (air carrier X) and FL182 (cpr Y). I told cpr Y to expedite the descent through 17000 ft report leaving 17000 ft. Cpr Y was then turned 30 degrees right to insure they went well behind air carrier X. The resulting preliminary separation was 3.4 mi and 600 ft. Cpr Y did report leaving 17000 ft and reported the other aircraft in sight but by that time separation was lost. I attribute the error on my incorrect assumption that cpr Y's descent rate would remain constant and take it well below air carrier X's assigned altitude. It was not busy at the time, about 6 aircraft on frequency. T3 low altitude sectors were combined at 1 position. This event could easily have been prevented by stopping air carrier X descent at a higher altitude, and increasing cpr Y descent on initial contact or turning the aircraft to insure separation. Always be alert and never assume anything, especially at the end of a work shift.

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Original NASA ASRS Text

Title: OPERROR.

Narrative: A CMH INBOUND WAS WBOUND DSNDING OUT OF FL240 FOR 10000 FT. I THEN TOOK THE HDOF ON A CAK INBOUND DSNDING NBOUND OUT OF FL210 FOR 15000 FT. I RECOGNIZED THAT THE 2 ACFT WERE IN CONFLICT IN SEVERAL MINS, SO I STOPPED ACR X AT FL180. HE WAS NOW OUT OF FL240. CPA Y WAS NOW OUT OF FL190 DSNDING STILL TO 15000 FT. I STILL HAD A COUPLE OF MINS BEFORE THEY WERE IN CONFLICT. I THEN ISSUED CTL INSTRUCTIONS TO SEVERAL OTHER ACFT. I THEN LOOKED BACK TO ACR X AND CPR Y TO FIND THAT THE CONFLICT ALERT HAD ACTIVATED. AT THIS POINT THE 2 ACFT WERE ABOUT 8-10 MI APART WITH ALTS OF FL184 (ACR X) AND FL182 (CPR Y). I TOLD CPR Y TO EXPEDITE THE DSCNT THROUGH 17000 FT REPORT LEAVING 17000 FT. CPR Y WAS THEN TURNED 30 DEGS R TO INSURE THEY WENT WELL BEHIND ACR X. THE RESULTING PRELIMINARY SEPARATION WAS 3.4 MI AND 600 FT. CPR Y DID RPT LEAVING 17000 FT AND RPTED THE OTHER ACFT IN SIGHT BUT BY THAT TIME SEPARATION WAS LOST. I ATTRIBUTE THE ERROR ON MY INCORRECT ASSUMPTION THAT CPR Y'S DSCNT RATE WOULD REMAIN CONSTANT AND TAKE IT WELL BELOW ACR X'S ASSIGNED ALT. IT WAS NOT BUSY AT THE TIME, ABOUT 6 ACFT ON FREQ. T3 LOW ALT SECTORS WERE COMBINED AT 1 POS. THIS EVENT COULD EASILY HAVE BEEN PREVENTED BY STOPPING ACR X DSCNT AT A HIGHER ALT, AND INCREASING CPR Y DSCNT ON INITIAL CONTACT OR TURNING THE ACFT TO INSURE SEPARATION. ALWAYS BE ALERT AND NEVER ASSUME ANYTHING, ESPECIALLY AT THE END OF A WORK SHIFT.

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.