Narrative:

I was working east arrival position at C90, vectoring to runway 9L. Ord was landing runway 9L and runway 4R. WX conditions improved enough to permit approachs to runway 9R and I was instructed to transition my traffic flow from ILS 9L to ILS 9R. In doing so, I became slightly distraction and neglected to stagger altitudes on opposite direction base leg aircraft. This would not have been a problem since one of the aircraft had already been given a turn to intercept the localizer. The problem/error occurred with that aircraft missing his instructions and I missing the readback error by another aircraft. Supplemental information from acn 400896: on arrival into ord we were assigned runway 4R ILS approach. While being vectored first officer runway 9R we were given a 360 degree heading and a change of runway to 9L. We were assigned a 120 degree heading for vectors back to runway 9R. This clearance was accepted by another aircraft. We remained on a 360 degree heading and waited for a clear frequency to advise ATC another aircraft had accepted our clearance and we were heading 360 degrees at 5000 ft. We then noted a yellow TCASII target, same altitude, opposite direction, at 12 O'clock position. We were still unable to communicate with ATC because of frequency congestion. ATC then issued a descent to 4000 ft and a turn to 270 degrees for us. We simultaneously received a TA then an RA. In responding to ATC clearance to 4000 ft, I also advised ATC we had received an RA. ATC responded with an immediate turn to 270 degrees and dscnd to 4000 ft. I then advised ATC we were complying and had the opposite direction traffic in sight. The RA was terminated and we were vectored to runway 9R ILS. ILS approach and landing were normal. In summary -- frequency congestion, 3 approach frequency changes, 4 different runway assignments, and a readback error by another aircraft at the world's busiest airport were contributing factors to this conflict. As a footnote: 2 weeks earlier on the same leg we received 4 runway assignments on the same approach sequence from ord approach control. This is a re-send of an earlier report.

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Original NASA ASRS Text

Title: DURING A CHANGE IN RWYS AT ORD, THE CTLR ALLOWED OPPOSITE DIRECTION TFC, A B727 AND AN MD80 TO CONFLICT, RESULTING IN AN OPERROR.

Narrative: I WAS WORKING E ARR POS AT C90, VECTORING TO RWY 9L. ORD WAS LNDG RWY 9L AND RWY 4R. WX CONDITIONS IMPROVED ENOUGH TO PERMIT APCHS TO RWY 9R AND I WAS INSTRUCTED TO TRANSITION MY TFC FLOW FROM ILS 9L TO ILS 9R. IN DOING SO, I BECAME SLIGHTLY DISTR AND NEGLECTED TO STAGGER ALTS ON OPPOSITE DIRECTION BASE LEG ACFT. THIS WOULD NOT HAVE BEEN A PROB SINCE ONE OF THE ACFT HAD ALREADY BEEN GIVEN A TURN TO INTERCEPT THE LOC. THE PROB/ERROR OCCURRED WITH THAT ACFT MISSING HIS INSTRUCTIONS AND I MISSING THE READBACK ERROR BY ANOTHER ACFT. SUPPLEMENTAL INFO FROM ACN 400896: ON ARR INTO ORD WE WERE ASSIGNED RWY 4R ILS APCH. WHILE BEING VECTORED FO RWY 9R WE WERE GIVEN A 360 DEG HDG AND A CHANGE OF RWY TO 9L. WE WERE ASSIGNED A 120 DEG HDG FOR VECTORS BACK TO RWY 9R. THIS CLRNC WAS ACCEPTED BY ANOTHER ACFT. WE REMAINED ON A 360 DEG HDG AND WAITED FOR A CLR FREQ TO ADVISE ATC ANOTHER ACFT HAD ACCEPTED OUR CLRNC AND WE WERE HDG 360 DEGS AT 5000 FT. WE THEN NOTED A YELLOW TCASII TARGET, SAME ALT, OPPOSITE DIRECTION, AT 12 O'CLOCK POS. WE WERE STILL UNABLE TO COMMUNICATE WITH ATC BECAUSE OF FREQ CONGESTION. ATC THEN ISSUED A DSCNT TO 4000 FT AND A TURN TO 270 DEGS FOR US. WE SIMULTANEOUSLY RECEIVED A TA THEN AN RA. IN RESPONDING TO ATC CLRNC TO 4000 FT, I ALSO ADVISED ATC WE HAD RECEIVED AN RA. ATC RESPONDED WITH AN IMMEDIATE TURN TO 270 DEGS AND DSCND TO 4000 FT. I THEN ADVISED ATC WE WERE COMPLYING AND HAD THE OPPOSITE DIRECTION TFC IN SIGHT. THE RA WAS TERMINATED AND WE WERE VECTORED TO RWY 9R ILS. ILS APCH AND LNDG WERE NORMAL. IN SUMMARY -- FREQ CONGESTION, 3 APCH FREQ CHANGES, 4 DIFFERENT RWY ASSIGNMENTS, AND A READBACK ERROR BY ANOTHER ACFT AT THE WORLD'S BUSIEST ARPT WERE CONTRIBUTING FACTORS TO THIS CONFLICT. AS A FOOTNOTE: 2 WKS EARLIER ON THE SAME LEG WE RECEIVED 4 RWY ASSIGNMENTS ON THE SAME APCH SEQUENCE FROM ORD APCH CTL. THIS IS A RE-SEND OF AN EARLIER RPT.

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.