The Traffic Collision Avoidance System (or TCAS) is a computer system installed on board many large aircraft, designed to prevent mid-air collisions.

The TCAS display can be integrated in the ND (Navigation Display). This is the case in the Airbus family of aircraft. It can also be integrated in the IVSI (Instantaneous Vertical Speed Indicator; which indicates the vertical speed of the aircraft, if it's descending or climbing). This is the case in Boeing aircraft.

The TCAS displays other aircraft equipped with TCAS in the vicinity of the aircraft (the range is selected by the pilot and can vary from 2.5 to about 30 miles). If collision with another aircraft appears imminent, an audio and visual warning will occur, indicating the incoming aircraft, and the audio signal indicates the action to be taken by the pilot. Examples can be either climb climb climb or dive dive dive. Of course the TCAS in the other aircraft will signal the opposite so a collision can be avoided.

TCAS is not entirely foolproof. In a mid-air crash in 2002, both planes were fitted with TCAS, but a conflict between the computer and the instructions from air-traffic control led to a crash anyway. See Bashkirian Airlines Flight 2937.

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