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Figure 11.17: TCAS protection volume and traffic/resolution advisories
Traffic Collision Avoidance System (TCAS)17 is an airborne aircraft collision avoidance system designed to reduce the incidence of Mid-Air Collision (MAC) between aircraft. It acts as the last safety back-up layer. Based on SSR transponder signals, it monitors the airspace around an aircraft for other aircraft equipped with a corresponding active transponder (independently of air traffic control) and advises instructions to pilots in case of the presence of other aircraft which may present a threat.
A TCAS installation consists of the following components: Telecommunication systems (antennas, transponder, etc.), TCAS computer unit, and cockpit presentation. In modern aircraft, the TCAS cockpit display may be integrated in the Navigation Display (ND).
TCAS issues the following types of advisories: Traffic advisory (TA) and Resolution advisory (RA). Traffic advisory is a situational awareness advisory, i.e., pilots must be aware of conflicting aircraft to either maintain separation in visual rules or coordinate with ATC to avoid the thread in instrumental rules. The RA is the last safety layer. It is advised when a mid-air collision is to occur within less than 25 to 35 seconds (depending on the TCAS generation). In this case, pilots are expected to respond immediately and the controller is no longer responsible for separation of the aircraft involved in the RA until the conflict has been resolved. Typically the RA will involve coordinated instructions to the two aircraft involved, e.g., flight level up and flight level down.
17. also refereed to as Aircraft Collision Avoidance System (ACAS).