The Traffic Collision Avoidance System (or TCAS) is a computer system installed on board many large aircraft, designed to prevent mid-air collisions. Pilots refer to it jokingly as the Fish Finder because of the similarity to the sonar devices that fishers use.
The TCAS display can be integrated in the ND (Navigation Display). This is the case in the Airbus and Boeing family of aircraft. It can also be integrated in the IVSI (Instantaneous Vertical Speed Indicator) if fitted, which indicates the vertical speed of the aircraft, if it's descending or climbing).
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, called a Resolution Advisory or RA, 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. FAA and other authorities' rules state that in the case of a conflict between TCAS RA and air-traffic control (ATC) instructions, the pilot is supposed to obey the TCAS RA, because if one aircraft follows TCAS RA and the other follows ATC instructions, a collision can occur.
In a mid-air crash in 2002, both planes were fitted with TCAS, but one of the aircraft did not follow the regulations and followed the ATC instructions rather than the TCAS RA. See Bashkirian Airlines Flight 2937.
