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Traffic Collision Avoidance Systems warn pilots about other, nearby aircraft that may be a collision risk

A traffic collision avoidance system, or traffic alert and collision avoidance system, (both abbreviated as TCAS and pronounced tee-kas) is a system designed to reduce mid-air collisions between aircraft.

TCAS units use transponder signals to gather information about nearby aircraft. If two aircraft are on a collision course, their TCAS units communicate with other to negotiate an avoidance plan, e.g. one aircraft climbs and the other descends. The unit then alerts the crew with a spoken warning or instruction such as "climb, climb!". Crews are required to follow the TCAS instruction immediately, even if ATC has given other instructions.

ICAO mandates TCAS for all aircraft that have a maximum take-off mass (MTOM) of over 5,700 kg (12,600 lb) or are authorized to carry more than 19 passengers.