What happened
At the Chicago O'Hare Air Traffic Control Tower (ATCT), British Airways (BA) Flight 296 was cleared for takeoff on runway 32R, with instructions to execute a right turn to a heading of 070 degrees. Approximately 49 seconds later, United Airlines (UA) Flight 243 was cleared for takeoff on runway 4L, with instructions to perform a left turn to a heading of 320 degrees.
During the departure sequence, the North Local Controller at the ATCT issued multiple altitude and heading changes to UA Flight 243. These instructions continued until the flight paths of the two heavy airplanes eventually diverged.
While the minimum lateral separation required between two heavy aircraft within the Chicago terminal area is four miles, the pilot of UA Flight 243 estimated that the actual minimum separation achieved between the two aircraft was only one mile horizontally and 300 feet vertically.
At the time of the event, the ATCT was staffed by a full complement including two local controllers, a relieving North Local Controller, a local control monitor, an area supervisor, and an area manager. Although the traffic volume was described as fairly busy and the configuration was considered complex, the operational environment was not deemed abnormal for the O'Hare ATCT.