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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. In Glass Cockpit aircraft the TCAS display may be integrated in the ND (Navigation Display). In older glass cockpit aircraft and those with mechanical instrumentation, the mechanical IVSI (Instantaneous Vertical Speed Indicator - which indicates the speed with which the aircraft is descending or climbing) is replaced by an electronic instrument which incorporates the TCAS display. 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 signaling 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. TCAS II systems coordinate their resolution advisories before issuing commands to the pilots. This ensures that both systems will not issue the same command. It is desirable to have one aircraft go up and the other go down as the TCASs will always increase separation between aircraft. When a threat has passed, the system announces ''"clear of conflict"''. HISTORY Although research into airborne collision avoidance systems had been ongoing since the early 1950s, the impetus that spurred the Federal Aviation Administration into action was several mid-air collisions involving airliners and loss of life. Some of these tragic accidents include the Grand Canyon Midair Collision in 1956 and PSA Flight 182 , a Boeing 727 which collided with a Cessna 182 in 1978 . TCAS II Description Development Although the system sometimes causes false alarms, pilots are now 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 Überlingen, Germany of a Bashkirian Airlines Tupolev Tu-154 M and a DHL Boeing 757 -200 freighter. Both airplanes were fitted with TCAS, but the 757 followed TCAS while the Tupolev followed the controller. See '' Bashkirian Airlines Flight 2937 ''. REGULATORY SITUATION AROUND THE WORLD
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