A traffic collision avoidance system, or traffic alert and collision avoidance system (TCAS), is a system used to allow aircraft to communicate their positions to avoid midair collisions.
TCAS operates independently of air traffic control on the ground. The International Civil Aviation Organization mandates that TCAS be used on all aircraft that have a maximum take-off mass greater than 5,700 kg or that are authorized to carry more than 19 passengers.
The system consists of a TCAS computer unit, which surveys the airspace, tracks intruders, measures altitude, detects threats, and issues resolution advisories. Antennas on the top and bottom of the aircraft enable the transponders to send and receive information. The information is presented to pilots on a traffic display and resolution advisory display.
Each aircraft equipped with a TCAS interrogates other aircraft in the vicinity about their position via transponders, and other aircraft respond. The interrogation-and-response cycle can take place several times per second. The TCAS system creates a three-dimensional map of the aircraft based on their range, altitude, and bearing and determines whether there is a threat of a collision.
If the system identifies potential for a collision, it directs the pilots to take evasive action, which may include changing altitude or modifying climb or sink rates. The system directs each aircraft to take opposite, coordinated actions.
In modern glass cockpit aircraft, the TCAS display can be integrated with the Navigation Display or the Electronic Horizontal Situation Indicator. In older glass cockpit aircraft or those that use mechanical instruments, a TCAS display may take the place of the mechanical Vertical Speed Indicator.