Narrative:

I was working 10 or more aircraft on sector 18, mostly overflts with a couple aircraft needing to descend for sdf entering the sector, and a couple of departures off of stl eastbound. Air carrier X checked on the frequency at FL370 approximately over stl VOR, about this same time. Air carrier Y was on the frequency southbound at FL350. I determined a converging target traffic call appeared unnecessary. They'd be close but shouldn't merge according to my route projections. I shipped air carrier Y to sector 19. I performed other duties, then got back to researching whether to start air carrier X down to FL330 below another FL350 southbound aircraft I was working. I was having trouble separating my data tags over stl VOR with the departures coming off, and after making 3 attempts to see pertinent traffic for air carrier X, descended him to FL330, and continued performing other duties on my d-side. (The right and D side were combined). Air carrier X asked the altitude of aircraft that passed below him, and when I looked at the scope the 2 tags were flashing (conflict alert) the targets were more than 2 mi apart diverging, air carrier X was leaving FL357 and I considered that even if the targets were off a turn or any assigned altitude were useless at this point. I immediately notified the supervisor in charge of my error. It perhaps wouldn't have happened if the positions weren't combined, or if FL350 and above were still being worked by fargo (a test was in effect whereby I was also working FL350 and above airspace that before at this time of day sector 9, fargo, would've been adding to the total number of aircraft on the frequency and very different traffic flows). I also felt extra tired after the 'quick turn around' of schedules. Worked evening shift and returning to work this morning. However, I've been doing this for 8 1/2 plus yrs. Supplemental information from acn 172141. Air carrier X instructed to descend from FL370 to FL330 by kansas city center. About 1 min later, as we were descending through FL360, I noticed air carrier Y pass directly beneath us on a southerly heading. Our position was approximately 20 NM west of enl, and we were on a easterly heading. I asked ATC about air carrier Y's us. The controller stated that it was 'his mistake'. No other explanation was offered. We continued our flight to louisville and landed. The traffic was never called to our attention by kansas city center and I did not observe the traffic until it was just starting to pass under us. I just caught a glimpse of it out my window (capts left clearview) and then observed it on its southerly heading out the first officer's clearview window.

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Original NASA ASRS Text

Title: AR X DESCENT THROUGH OCCUPIED ALT HAD LESS THAN STANDARD SEPARATION FROM ACR Y. SYSTEM ERROR.

Narrative: I WAS WORKING 10 OR MORE ACFT ON SECTOR 18, MOSTLY OVERFLTS WITH A COUPLE ACFT NEEDING TO DSND FOR SDF ENTERING THE SECTOR, AND A COUPLE OF DEPS OFF OF STL EBND. ACR X CHKED ON THE FREQ AT FL370 APPROX OVER STL VOR, ABOUT THIS SAME TIME. ACR Y WAS ON THE FREQ SBND AT FL350. I DETERMINED A CONVERGING TARGET TFC CALL APPEARED UNNECESSARY. THEY'D BE CLOSE BUT SHOULDN'T MERGE ACCORDING TO MY RTE PROJECTIONS. I SHIPPED ACR Y TO SECTOR 19. I PERFORMED OTHER DUTIES, THEN GOT BACK TO RESEARCHING WHETHER TO START ACR X DOWN TO FL330 BELOW ANOTHER FL350 SBND ACFT I WAS WORKING. I WAS HAVING TROUBLE SEPARATING MY DATA TAGS OVER STL VOR WITH THE DEPS COMING OFF, AND AFTER MAKING 3 ATTEMPTS TO SEE PERTINENT TFC FOR ACR X, DSNDED HIM TO FL330, AND CONTINUED PERFORMING OTHER DUTIES ON MY D-SIDE. (THE R AND D SIDE WERE COMBINED). ACR X ASKED THE ALT OF ACFT THAT PASSED BELOW HIM, AND WHEN I LOOKED AT THE SCOPE THE 2 TAGS WERE FLASHING (CONFLICT ALERT) THE TARGETS WERE MORE THAN 2 MI APART DIVERGING, ACR X WAS LEAVING FL357 AND I CONSIDERED THAT EVEN IF THE TARGETS WERE OFF A TURN OR ANY ASSIGNED ALT WERE USELESS AT THIS POINT. I IMMEDIATELY NOTIFIED THE SUPVR IN CHARGE OF MY ERROR. IT PERHAPS WOULDN'T HAVE HAPPENED IF THE POSITIONS WEREN'T COMBINED, OR IF FL350 AND ABOVE WERE STILL BEING WORKED BY FARGO (A TEST WAS IN EFFECT WHEREBY I WAS ALSO WORKING FL350 AND ABOVE AIRSPACE THAT BEFORE AT THIS TIME OF DAY SECTOR 9, FARGO, WOULD'VE BEEN ADDING TO THE TOTAL NUMBER OF ACFT ON THE FREQ AND VERY DIFFERENT TFC FLOWS). I ALSO FELT EXTRA TIRED AFTER THE 'QUICK TURN AROUND' OF SCHEDULES. WORKED EVENING SHIFT AND RETURNING TO WORK THIS MORNING. HOWEVER, I'VE BEEN DOING THIS FOR 8 1/2 PLUS YRS. SUPPLEMENTAL INFO FROM ACN 172141. ACR X INSTRUCTED TO DSND FROM FL370 TO FL330 BY KANSAS CITY CTR. ABOUT 1 MIN LATER, AS WE WERE DSNDING THROUGH FL360, I NOTICED ACR Y PASS DIRECTLY BENEATH US ON A SOUTHERLY HDG. OUR POS WAS APPROX 20 NM W OF ENL, AND WE WERE ON A EASTERLY HDG. I ASKED ATC ABOUT ACR Y'S US. THE CTLR STATED THAT IT WAS 'HIS MISTAKE'. NO OTHER EXPLANATION WAS OFFERED. WE CONTINUED OUR FLT TO LOUISVILLE AND LANDED. THE TFC WAS NEVER CALLED TO OUR ATTN BY KANSAS CITY CTR AND I DID NOT OBSERVE THE TFC UNTIL IT WAS JUST STARTING TO PASS UNDER US. I JUST CAUGHT A GLIMPSE OF IT OUT MY WINDOW (CAPTS L CLEARVIEW) AND THEN OBSERVED IT ON ITS SOUTHERLY HDG OUT THE F/O'S CLEARVIEW WINDOW.

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.