Narrative:

We were on an assigned heading to intercept mdw runway 31C localizer, assigned altitude was 5000 ft (at pilot's discretion). We were level at 5000 ft. I saw gary airport below us on the right side 3-5 NM through broken clouds. We received a TA and the TCASII showed 2 targets at 1 O'clock, 300 ft below and climbing. Subsequently got a climb RA and captain initiated climb to 6000 ft while I advised ATC. ATC said target was level at 4000 ft, but subsequently he determined traffic was at 5000 ft. I don't know why the controller got no conflict alert, but I think TCASII saved us from a midair collision or a very near miss. Never saw other aircraft due to IMC. Don't know if there were really 2 aircraft of just 1.

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Original NASA ASRS Text

Title: NMAC INDICATED ON TCASII.

Narrative: WE WERE ON AN ASSIGNED HDG TO INTERCEPT MDW RWY 31C LOC, ASSIGNED ALT WAS 5000 FT (AT PLT'S DISCRETION). WE WERE LEVEL AT 5000 FT. I SAW GARY ARPT BELOW US ON THE R SIDE 3-5 NM THROUGH BROKEN CLOUDS. WE RECEIVED A TA AND THE TCASII SHOWED 2 TARGETS AT 1 O'CLOCK, 300 FT BELOW AND CLBING. SUBSEQUENTLY GOT A CLB RA AND CAPT INITIATED CLB TO 6000 FT WHILE I ADVISED ATC. ATC SAID TARGET WAS LEVEL AT 4000 FT, BUT SUBSEQUENTLY HE DETERMINED TFC WAS AT 5000 FT. I DON'T KNOW WHY THE CTLR GOT NO CONFLICT ALERT, BUT I THINK TCASII SAVED US FROM A MIDAIR COLLISION OR A VERY NEAR MISS. NEVER SAW OTHER ACFT DUE TO IMC. DON'T KNOW IF THERE WERE REALLY 2 ACFT OF JUST 1.

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.