Incident Overview
On a routine descent into Chicago O'Hare International Airport, the flight crew of United Airlines Flight 1517 received a Traffic Advisory (T/A) from the Traffic Alert and Collision Avoidance System (TCAS). Shortly thereafter, a Resolution Advisory (R/A) followed. The crew reported their altitude at approximately 11,500 feet when the T/A was received, and the first radar return of the conflicting aircraft was at 11,200 feet.
Evasive Maneuver and Injuries
In response to the R/A, the captain executed an abrupt pull-up to avoid a collision with the other aircraft. During this maneuver, two flight attendants sustained injuries. The nature of their injuries was not disclosed in the investigation.
Conflicting Aircraft Details
The pilot of the other aircraft reported that while passing through 10,000 feet, he noticed his Mode C transponder was not selected. He subsequently changed the selector position. The investigation did not specify the type or registration of either aircraft.
Separation Analysis
Investigation of the event revealed that the closest separation between the two aircraft was 0.1 nautical miles horizontally and 50 feet vertically. No further details regarding the cause of the conflict were provided.