B757 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 13167; Flight Crew Type 10234
Communication Breakdown
ATC Issue All Types; Conflict Airborne Conflict
Air Traffic Control Issued Advisory / Alert; Flight Crew Took Evasive Action
Procedure
Normal flight until descent on arrival passing approximately 9;200 for 9;000 feet. TCAS RA followed by TCAS avoidance maneuver. Transmitted event to ATC. Controller stated that it was at 8;500 feet and nothing else concerning the threat aircraft. Received a level off TCAS and we did so at around 9;300-9;400 feet. TCAS clear-of-conflict then descended to previously cleared altitude. We heard 2 or 3 other aircraft execute TCAS maneuvers during the time on the approach frequency. The Controller never warned any of the descending aircraft of the traffic conflict.
B757 First Officer reported his aircraft as well as others on approach to ORD received TCAS RAs for traffic which was not reported by ATC.