Narrative:

Ord approach issued a heading 285 degrees to join runway 31C localizer (we had been cleared to 5000 ft earlier). We were IMC at 5000 ft when TCASII showed 2 targets at 1 O'clock position, 300 ft below and climbing. I initiated a climb as TCASII squawked to 'climb, climb.' we climbed to approximately 6000 ft and my first officer advised ATC of our RA. Controller stated our conflict was supposed to be at 4000 ft, conflicting aircraft stated he reported 5000 ft. Nevertheless, we were where we were supposed to be. How could radar not have been showing a conflict alert? We never saw the conflicting traffic as we were always in IMC. TCASII saved us from a midair, no doubt about it, but why did the system break down to that level of safety? It seems as though our TCA's are saturated with traffic and controllers are simply maximum'ed out with too much talk, too much traffic, and a mix of traffic (little and slow with air transport category) that doesn't mix very well.

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Original NASA ASRS Text

Title: SAME ASSIGNED ALT. THE TCASII WARNING WAS CORRECT, THERE WAS ANOTHER ACFT AT THE SAME ALT. THE FLC EVADES.

Narrative: ORD APCH ISSUED A HDG 285 DEGS TO JOIN RWY 31C LOC (WE HAD BEEN CLRED TO 5000 FT EARLIER). WE WERE IMC AT 5000 FT WHEN TCASII SHOWED 2 TARGETS AT 1 O'CLOCK POS, 300 FT BELOW AND CLBING. I INITIATED A CLB AS TCASII SQUAWKED TO 'CLB, CLB.' WE CLBED TO APPROX 6000 FT AND MY FO ADVISED ATC OF OUR RA. CTLR STATED OUR CONFLICT WAS SUPPOSED TO BE AT 4000 FT, CONFLICTING ACFT STATED HE RPTED 5000 FT. NEVERTHELESS, WE WERE WHERE WE WERE SUPPOSED TO BE. HOW COULD RADAR NOT HAVE BEEN SHOWING A CONFLICT ALERT? WE NEVER SAW THE CONFLICTING TFC AS WE WERE ALWAYS IN IMC. TCASII SAVED US FROM A MIDAIR, NO DOUBT ABOUT IT, BUT WHY DID THE SYS BREAK DOWN TO THAT LEVEL OF SAFETY? IT SEEMS AS THOUGH OUR TCA'S ARE SATURATED WITH TFC AND CTLRS ARE SIMPLY MAX'ED OUT WITH TOO MUCH TALK, TOO MUCH TFC, AND A MIX OF TFC (LITTLE AND SLOW WITH AIR TRANSPORT CATEGORY) THAT DOESN'T MIX VERY WELL.

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.