What happened
During an approach for the ILS Runway 9, two DC-9 aircraft were operating in instrument meteorological conditions (IMC) under the direction of a Final TRACON controller. At the time of the event, the controller was managing two sectors and 23 other aircraft.
Continental Flight 122 was positioned southwest of the airport, flying a heading of 060 degrees at 3,000 feet MSL while approaching the localizer. TWA Flight 806 was located approximately 8 miles northwest of Flight 122 and had been cleared to intercept the localizer.
The controller instructed Flight 122 to maintain 2,000 feet until established on the localizer and asked if the crew intended to fly through the localizer. The crew of Flight 122 responded that they were, noting they had not been previously instructed to intercept. The controller then issued an instruction to maintain 3,500 feet and "turn left 360," intending for the aircraft to turn to a heading of 360 degrees.
The crew of Flight 122 interpreted this as an instruction for a 360-degree circling turn and began a right turn. While performing the maneuver, the crew requested a heading to roll out on. The controller instructed them to fly heading 230. Upon realizing that the two aircraft were converging, the controller instructed Flight 122 to descend to 2,000 feet and issued an instruction to "Continental 806" to maintain 3,500 feet.
Because the controller used the incorrect company designator, the crew of TWA Flight 806 did not respond to the instruction. Radar data indicated that the two aircraft passed each other at approximately 3,100 feet with a separation of only .079 miles.