Narrative:

Air carrier X being vectored by chicago approach control (frequency 119.0). I was leveling aircraft at 2700 ft MSL and was given a turn from an assigned heading of 360 degrees to 310 degrees to intercept runway 22R localizer. While turning, we received a TCASII TA. It was indicating an approaching aircraft from my side (left side). I was looking out the window to try and get a visual on the approaching traffic but was looking somewhat westward toward evening on a hazy day and could not acquire the traffic. TCASII changed the alert from a TA to an RA with red square indicating approaching traffic was 200 ft below our altitude. TCASII commanded a climb at a rate of over 3000 FPM. I climbed at 3200 FPM and noticed an aircraft that appeared to be a swept-wing business type jet, cpr Y, pass under my aircraft. After TCASII RA ended, the copilot called chicago approach to report the traffic and advise approach control of our climb. We had climbed approximately 1000 ft. Approach control acknowledged that our present altitude was '1000 ft above the traffic.' we stated we were returning to our assigned altitude of 2700 ft MSL. We intercepted runway 22R localizer and GS. I really believe in TCASII. TCASII helped prevent us from getting any closer to the traffic before evasive action would have been taken. We might have missed extremely close with the traffic had it not been for TCASII. Supplemental information from acn 244546: I reported to approach control that we were responding to a TCASII RA and we were told that the target was 1000 ft below us, which it was after completion of the RA following the TCASII event.

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Original NASA ASRS Text

Title: ACR X TCASII TA/RA EVASIVE ACTION TAKEN CLB HAD LTSS SEPARATION FROM CPR Y. SYS ERROR.

Narrative: ACR X BEING VECTORED BY CHICAGO APCH CTL (FREQ 119.0). I WAS LEVELING ACFT AT 2700 FT MSL AND WAS GIVEN A TURN FROM AN ASSIGNED HDG OF 360 DEGS TO 310 DEGS TO INTERCEPT RWY 22R LOC. WHILE TURNING, WE RECEIVED A TCASII TA. IT WAS INDICATING AN APCHING ACFT FROM MY SIDE (L SIDE). I WAS LOOKING OUT THE WINDOW TO TRY AND GET A VISUAL ON THE APCHING TFC BUT WAS LOOKING SOMEWHAT WESTWARD TOWARD EVENING ON A HAZY DAY AND COULD NOT ACQUIRE THE TFC. TCASII CHANGED THE ALERT FROM A TA TO AN RA WITH RED SQUARE INDICATING APCHING TFC WAS 200 FT BELOW OUR ALT. TCASII COMMANDED A CLB AT A RATE OF OVER 3000 FPM. I CLBED AT 3200 FPM AND NOTICED AN ACFT THAT APPEARED TO BE A SWEPT-WING BUSINESS TYPE JET, CPR Y, PASS UNDER MY ACFT. AFTER TCASII RA ENDED, THE COPLT CALLED CHICAGO APCH TO RPT THE TFC AND ADVISE APCH CTL OF OUR CLB. WE HAD CLBED APPROX 1000 FT. APCH CTL ACKNOWLEDGED THAT OUR PRESENT ALT WAS '1000 FT ABOVE THE TFC.' WE STATED WE WERE RETURNING TO OUR ASSIGNED ALT OF 2700 FT MSL. WE INTERCEPTED RWY 22R LOC AND GS. I REALLY BELIEVE IN TCASII. TCASII HELPED PREVENT US FROM GETTING ANY CLOSER TO THE TFC BEFORE EVASIVE ACTION WOULD HAVE BEEN TAKEN. WE MIGHT HAVE MISSED EXTREMELY CLOSE WITH THE TFC HAD IT NOT BEEN FOR TCASII. SUPPLEMENTAL INFO FROM ACN 244546: I RPTED TO APCH CTL THAT WE WERE RESPONDING TO A TCASII RA AND WE WERE TOLD THAT THE TARGET WAS 1000 FT BELOW US, WHICH IT WAS AFTER COMPLETION OF THE RA FOLLOWING THE TCASII EVENT.

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.