Incident Overview
On June 1, 2009, at 1216 central daylight time, an operational error occurred at Chicago Terminal Radar Approach Control (TRACON) involving two American Airlines flights. American Airlines flight 879 (AAL879), a Boeing MD-82, and American Airlines flight 93 (AAL93), a Boeing 767, passed within 0.35 nautical miles at 3,400 feet while on approach to O'Hare International Airport (ORD) in Chicago, Illinois. AAL93, a scheduled passenger flight from Dublin, Ireland, carried 12 crew and 202 passengers. AAL879, a scheduled flight between St. Louis and ORD, carried 5 crew and 105 passengers. No damage was reported to either aircraft, and no injuries occurred. Both flights continued to ORD and landed without further incident.
Sequence of Events
AAL93 was in communication with the Center Arrivals controller, on an extended straight-in approach to runway 27L. Initially cleared for an Instrument Landing System (ILS) approach, it was later cleared for a visual approach after the incident. AAL879 was in communication with the West Arrivals controller and was being vectored from a left downwind to the final approach course for runway 28.
At 1713:36, the West controller advised AAL879 of preceding traffic at 10 o'clock and three miles, and instructed a heading of 350. The crew acknowledged. At 1715:35, the West controller instructed an uninvolved aircraft to descend to 4,000 feet, with a pilot responding "blocked." The instruction was repeated and acknowledged. At 1715:47, AAL879 asked, "AAL cleared [for] the visual?" The West controller responded, "AAL879 is cleared for the visual approach to 28, and the tower's… uh 120… are you turning uh left to 250 and descend to 2500." At 1716:00, AAL879 acknowledged. At 1716:30, the West controller again cleared AAL879 for a visual approach to 28 and instructed contact with the tower.
Meanwhile, the Center Arrival controller was handling AAL93. At 1715:44, the pilot of AAL93 transmitted, "American we're going to be reacting to a super 80." The Center controller did not understand and asked for clarification. AAL93 responded, "American uh 93 we got a super 88 crossing our flight path right now on 27L." The Center controller replied, "American 93 heavy roger he's doing a visual to 28. Maintain visual separation with him, but if you need to turn right you can." At 1716:03, the pilot said, "We're going to have to – he's uh on our centerline." At 1716:13, the controller asked if the airport was in sight; the pilot replied affirmatively. The controller issued a visual approach clearance for 27L. At 1716:39, AAL93 was instructed to contact the tower.
Controller Statements
The West Arrival controller stated he had been on position for about 45 minutes. The airport was in a west configuration with visual approaches to runways 28, 27L, and 27R. He was required to clear successive arrivals to follow preceding traffic to runway 28. After pointing out traffic to AAL879, he did not issue the visual approach clearance. He could not recall distractions. When AAL879 asked about the clearance, he thought it was a confirmation. He realized something was wrong when he saw the aircraft continuing north past the runway 28 final approach course. He then instructed a turn to heading 250 and descent to 2,500 feet.
The Center Arrival controller stated he did not hear the first transmission from AAL93. He responded after the second call, noting the conflicting aircraft. He told AAL93 that the other aircraft was on a visual for the parallel runway and that AAL93 could turn right. He thought a safety alert was unnecessary because AAL93 had reported the traffic in sight. He did not report the event, assuming others saw it and the supervisor would handle it.
The front line manager was monitoring from the supervisory radar position. She noticed AAL879 was not turning toward the airport. The data block indicated AAL879 had been cleared for a visual approach, so she assumed an overshoot. She went to the West Arrival controller's position, heard the controller issuing a heading back, and saw the aircraft diverging. She took no further action, believing the situation was resolved.