Incident Overview
On a departure from Chicago-O'Hare International Airport (ORD), American Airlines Flight 637 was climbing through 7,300 feet MSL when it passed American Airlines Flight 393 with approximately 500 feet of vertical separation and one-quarter mile of horizontal separation. The proximity event occurred after an air traffic assistant working the clearance delivery position at ORD had assigned the same transponder code to Flight 637 and United Airlines Flight 441.
Air Traffic Control Actions
A trainee controller working the West Departure radar control position subsequently mistook the radar target of United Flight 441 for that of American Flight 637. Based on this misidentification, the trainee issued climb instructions to Flight 637. The instructor supervising the trainee did not recognize the error in time to correct the action before the separation between the two American flights was compromised.
Outcome
The incident did not result in a collision, but the loss of standard separation occurred. The report does not specify whether any injuries or damage occurred. The error chain began with the duplicate transponder code assignment and continued through the misidentification by the trainee and the failure of the instructor to intervene promptly.