iscover
(Left) Any aircraft within the Collins TCAS-II scan, but not a potential hazard, is displayed as a blue diamond outline. Also its relative altitude is shown as a plus or minus symbol followed by two digits indicating hundreds of feet above or below. Should an aircraft come within 6nm laterally or 1200ft vertically, the diamond becomes solid. If TCAS decides the intruder will come too close in the next 35 to 45 seconds the diamond changes to a yellow circle with an arrow showing climb or descent. At this stage the pilot is also given an audible “traffic, traffic” warning. (Right) When the system decides the offending aircraft will become dangerously close in the next 20 to 30 seconds, it enters the RA or resolution advisory stage. The symbol on the VSI becomes a red square and the pilot is given an audible instruction to avoid collision. This could be "climb, climb” or "reduce climb” etc. (Collins Avionics)
Airspace Management
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Who Is In Control?
Since early 1994 in the US, commercial aircraft with 30 or more seats have been required to carry Traffic Alert and Collision Avoidance Systems (TCAS). Across the Atlantic, however, the UK CAA has not made the system mandatory, nor has any other country. But its use is slowly becoming more widespread largely because international flights operating into the US are obliged to carry and use TCAS.
The TCAS system works by electronically creating a bubble around the aircraft. This bubble extends 30nm ahead, 10nm either side and 3000ft vertically of the aircraft. The bubble is scanned once per second by a transmitter which triggers the transponders on any aircraft inside the bubble. A receiver processes and analyses aircraft positions and decides if there is a potential conflict. This is only possible if the intruding aircraft’s transponder is transmitting altitude. The TCAS system includes a datalink so that when two TCAS equipped aircraft are flying too close together, the advice to the pilots is not contradictory.
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