Date: 201204
Local time of day: 1201-1800
Location: ORD.Airport, IL
Aircraft: EMB ERJ 145 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Climb
Airspace: Class B ORD
ATC / advisory: TRACON C90
Second aircraft: A320
Reporter organization: Air Carrier
Reporter function: Departure; Approach
Reporter qualification: Air Traffic Control Developmental
Human factors: Communication Breakdown
Second reporter function: First Officer
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft RA; Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: I was working south departure while ORD arrivals were going to Runway 4R and 10. I RADAR identified an E145 departure off ORD heading 140 climbing to 5;000 FT. I turned the aircraft south to tunnel under the arrivals that were at 70. A supervisor was standing behind to tell me about a go-around I would be getting off of Runway 4R; when I heard an aircraft say their callsign 'The E145'. That wasn't his callsign; but I noticed right away that the E145 had traffic right in front of it descending to their altitude. The traffic was an A320. It was an arrival for Runway 4R on a westbound heading; descending to I don't know what. The A320 should have not been lower than 7;000 FT. By the time I noticed the A320; he was already at 5;200 FT. I immediately called traffic to the E145 and told him to turn. I believe both aircraft were already in the process of responding to a RA. Once the altitudes were corrected and traffic was no longer a factor; the E145 continued without further incident. I didn't know this until today; but I was told this is a continuous problem for aircraft being vectored for Runway 4R. Some aircraft respond to the runway assignment as a clearance to descend to 4;000 FT. I'm not one for extra phraseology; but maybe restate the altitude assignment when assigning Runway 4R.


Second narrative: We checked in with ORD departure with our heading (180) and altitude (5;000 FT). The Controller acknowledged and no further instructions were given. I saw the conflict aircraft on TCAS and noticed it was descending below 6;000 FT and getting closer. When the aircraft was about 3 miles horizontal and 500 FT vertical from us on the TCAS I queried ATC with no response. I queried again and right then we received a 'Descend' RA. I pushed the nose down and started to make a left turn to pass behind the conflict aircraft (moving from left to right). We announced to ATC that we were in a 'TCAS descent.' After receiving the 'clear of conflict' message we returned to our original assigned heading and 5;000 FT altitude and advised. At this time we did acquire the conflict aircraft visually. The Controller asked if we had received an RA and we replied 'yes.'


Synopsis: C90 Controller and EMB-145 F/O described a conflict event when an arrival aircraft descended below issued altitude toward a departure aircraft being tunneled underneath; the C90 Controller reporter noting a Runway 4 assignment may have been confused with a 4;000 altitude clearance.