Narrative:

On approach into ord at an altitude of 11000' (assigned), we received a traffic advisory. TCAS ii indicated traffic 700' below our altitude, displayed as a yellow target on our scope. The traffic advisory was followed by a RA. The target turned red on our scope and was indicating 600' below our aircraft at the time we received the command. TCAS ii commanded us to climb (approximately 1500-2000 FPM). The captain followed the command, and I notified ATC that we were climbed to avoid traffic per TCAS ii command. We climbed to 11500' when TCAS ii advised us to monitor climb, at which point the traffic conflict had been resolved. Since the traffic had been avoided, we returned to our assigned altitude of 11000'. Callback conversation with reporter revealed the following: 174615: there were approximately 15-20 advisory targets on the display, many within 10 mi of our aircraft, but well below our altitude. There was considerable overlap of data making it difficult to read altitude on individual targets. I asked the controller if he had observed the conflicting traffic. He responded that he had, but that it was operating VFR with legal sep (500'). The controller expressed his displeasure with my action in response to the TCAS ii RA, indicating that the traffic was VFR with legal VFR sep and my action (to climb) could have created an additional conflict with traffic above. I stated that we are trained to immediately respond to a TCAS ii RA, unless the conflicting traffic is in sight and can be avoided visually. I was told later that the controller has filed an altitude deviation report, an action that is not uncommon in a situation of this type.

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Original NASA ASRS Text

Title: ACR MLG EXPERIENCES ALT EXCURSION DUE TCAS RA.

Narrative: ON APCH INTO ORD AT AN ALT OF 11000' (ASSIGNED), WE RECEIVED A TFC ADVISORY. TCAS II INDICATED TFC 700' BELOW OUR ALT, DISPLAYED AS A YELLOW TARGET ON OUR SCOPE. THE TFC ADVISORY WAS FOLLOWED BY A RA. THE TARGET TURNED RED ON OUR SCOPE AND WAS INDICATING 600' BELOW OUR ACFT AT THE TIME WE RECEIVED THE COMMAND. TCAS II COMMANDED US TO CLB (APPROX 1500-2000 FPM). THE CAPT FOLLOWED THE COMMAND, AND I NOTIFIED ATC THAT WE WERE CLBED TO AVOID TFC PER TCAS II COMMAND. WE CLBED TO 11500' WHEN TCAS II ADVISED US TO MONITOR CLB, AT WHICH POINT THE TFC CONFLICT HAD BEEN RESOLVED. SINCE THE TFC HAD BEEN AVOIDED, WE RETURNED TO OUR ASSIGNED ALT OF 11000'. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: 174615: THERE WERE APPROX 15-20 ADVISORY TARGETS ON THE DISPLAY, MANY WITHIN 10 MI OF OUR ACFT, BUT WELL BELOW OUR ALT. THERE WAS CONSIDERABLE OVERLAP OF DATA MAKING IT DIFFICULT TO READ ALT ON INDIVIDUAL TARGETS. I ASKED THE CTLR IF HE HAD OBSERVED THE CONFLICTING TFC. HE RESPONDED THAT HE HAD, BUT THAT IT WAS OPERATING VFR WITH LEGAL SEP (500'). THE CTLR EXPRESSED HIS DISPLEASURE WITH MY ACTION IN RESPONSE TO THE TCAS II RA, INDICATING THAT THE TFC WAS VFR WITH LEGAL VFR SEP AND MY ACTION (TO CLB) COULD HAVE CREATED AN ADDITIONAL CONFLICT WITH TFC ABOVE. I STATED THAT WE ARE TRAINED TO IMMEDIATELY RESPOND TO A TCAS II RA, UNLESS THE CONFLICTING TFC IS IN SIGHT AND CAN BE AVOIDED VISUALLY. I WAS TOLD LATER THAT THE CTLR HAS FILED AN ALT DEVIATION RPT, AN ACTION THAT IS NOT UNCOMMON IN A SITUATION OF THIS TYPE.

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.