Traffic Collision Avoidance System
From Freepedia
Image:TCAS Indicator.jpg The Traffic Alert and Collision Avoidance System (or TCAS) is an implementation of the Airborne Collision Avoidance System mandated by ICAO to be fitted to all aircraft over 5700 kg or authorised to carry more than 19 passengers, designed to reduce mid-air collisions.
The TCAS display can either be integrated in the ND (Navigation Display), such as in the Airbus and Boeing family of aircraft, or, if fitted, in the IVSI (Instantaneous Vertical Speed Indicator), which indicates the speed with which the aircraft is descending or climbing.
The TCAS displays any other TCAS- or Mode C Transponder-equipped aircraft within a range selected by the pilot, which can vary from 2.5 to about 30 miles. If another aircraft appears to be a potential collision threat, a Traffic Advisory (TA) is issued. The TA warns the pilot that another aircraft is in near vicinity, announcing "traffic, traffic", but does not offer any suggested course of action. However, if the situation worsens and collision with another aircraft appears imminent, an audio and visual warning, called a Resolution Advisory (RA) will occur, indicating the incoming aircraft, and audibly signalling the action to be taken by the pilot. The suggestive action may be "positive", suggesting the pilot changes course by announcing "descend, descend" or "climb, climb". By contrast a "preventive" RA may be issued which simply warns the pilots not to deviate from their present course, announcing, for example, "monitor vertical speed". Of course, the TCAS system in the other aircraft will offer an alternate instruction so a collision can be avoided. More modern TCAS II systems automatically communicate and synchronise with each other to ensure an appropriate "deconflicted" response. When a threat has passed, the system announces "clear of conflict".
Although the system sometimes causes false alarms, pilots are under strict instructions to regard all TCAS messages as genuine alerts requiring an immediate response. The FAA and most other country's authorities' rules state that in the case of a conflict between TCAS RA and air traffic control (ATC) instructions, the TCAS RA always takes precedence — because if one aircraft follows TCAS RA and the other follows ATC instructions, a collision can occur, such as the July 1, 2002 collision over Ueberlingen, Germany of a Bashkirian Airlines Tupolev Tu-154M and a DHL Boeing 757-200 freighter. 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.
