Narrative:

Atx X was inbound to ord at 8000' via the pmm 2 arrival. Air carrier Y, also inbound to ord via the pmm 2, had originally been cleared to 8000' in an anticipation of a slower speed for spacing behind the atx X. Air carrier Y was later reclred to 9000' to go over the top of atx X. Air carrier Z read back air carrier Y's clearance to 9000'. I did not catch this missed readback. 3 mins later I issued traffic to air carrier Y (whom I thought was out of 12500 for 9000') at 12 O'clock and 3 mi westbound at 8000'. Air carrier Y replied that he was looking and I shipped him to ord approach control. This error could have been prevented if air carrier Y had said he was descending to 8000' when I had issued the traffic. It also could have been prevented if I had caught the missed readback or if I had verified air carrier Y's altitude when I issued him the traffic. The ord controller may also have been able to prevent this error if he had verified air carrier Y's altitude descending to when he checked on. Supplemental information from acn 162970: air carrier Y was cleared to 8000'. Air carrier Z, the aircraft behind us, was a short time later cleared to 9000'. Air carrier Y was then given traffic at a lower altitude, an airspeed restriction to slow to 180 KTS and a change of frequency to approach control. The slowing of airspeed luckily delayed our descent to 8000'. Flight conditions were IMC and continued to be IMC till after the near miss. At approximately 8300' traffic was spotted parallel to our position in close on our starboard side at approximately our altitude. (Conditions at sighting were IMC with only 1 momentary sighting.)

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Original NASA ASRS Text

Title: ACR Y HAD LESS THAN STANDARD SEPARATION FROM ACR X. SYSTEM ERROR.

Narrative: ATX X WAS INBND TO ORD AT 8000' VIA THE PMM 2 ARR. ACR Y, ALSO INBND TO ORD VIA THE PMM 2, HAD ORIGINALLY BEEN CLRED TO 8000' IN AN ANTICIPATION OF A SLOWER SPD FOR SPACING BEHIND THE ATX X. ACR Y WAS LATER RECLRED TO 9000' TO GO OVER THE TOP OF ATX X. ACR Z READ BACK ACR Y'S CLRNC TO 9000'. I DID NOT CATCH THIS MISSED READBACK. 3 MINS LATER I ISSUED TFC TO ACR Y (WHOM I THOUGHT WAS OUT OF 12500 FOR 9000') AT 12 O'CLOCK AND 3 MI WBND AT 8000'. ACR Y REPLIED THAT HE WAS LOOKING AND I SHIPPED HIM TO ORD APCH CTL. THIS ERROR COULD HAVE BEEN PREVENTED IF ACR Y HAD SAID HE WAS DSNDING TO 8000' WHEN I HAD ISSUED THE TFC. IT ALSO COULD HAVE BEEN PREVENTED IF I HAD CAUGHT THE MISSED READBACK OR IF I HAD VERIFIED ACR Y'S ALT WHEN I ISSUED HIM THE TFC. THE ORD CTLR MAY ALSO HAVE BEEN ABLE TO PREVENT THIS ERROR IF HE HAD VERIFIED ACR Y'S ALT DSNDING TO WHEN HE CHKED ON. SUPPLEMENTAL INFO FROM ACN 162970: ACR Y WAS CLRED TO 8000'. ACR Z, THE ACFT BEHIND US, WAS A SHORT TIME LATER CLRED TO 9000'. ACR Y WAS THEN GIVEN TFC AT A LOWER ALT, AN AIRSPD RESTRICTION TO SLOW TO 180 KTS AND A CHANGE OF FREQ TO APCH CTL. THE SLOWING OF AIRSPD LUCKILY DELAYED OUR DSNT TO 8000'. FLT CONDITIONS WERE IMC AND CONTINUED TO BE IMC TILL AFTER THE NEAR MISS. AT APPROX 8300' TFC WAS SPOTTED PARALLEL TO OUR POS IN CLOSE ON OUR STARBOARD SIDE AT APPROX OUR ALT. (CONDITIONS AT SIGHTING WERE IMC WITH ONLY 1 MOMENTARY SIGHTING.)

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.