TCAS and Resolution Advisories (RAs) explained
The TCAS (Traffic Collision Avoidance System) installed in modern airline aircraft is a small black box which sends interrogation signals to nearby aircraft. The transponders (another small black box, compulsory in Australia in Class E airspace) in these aircraft then send back response signals. The system interprets these signals to determine the location, speed, and course of each aircraft, then uses data to avoid a potential collision.
In addition to displaying on a small screen the locations of nearby aircraft, the units in Australian airline aircraft also provide a voice message called a “Resolution Advisory” when needed. This system determines the course of each aircraft, and if it is climbing, descending or flying straight and level, then issues a Resolution Advisory instructing the pilots to execute an evasive manoeuvre necessary to avoid other aircraft (such as “climb” or “descend”).
Note the Resolution Advisory is designed with a protected zone around the aircraft with a radius of 500’ and a height of 200’. This is defined as a “near mid-air collision” (NMAC) area.
The Mitre Report says “‘TCAS should reduce NMAC probability by at least 90 to 98 percent,’ depending on whether one or both aircraft in an encounter are equipped with TCAS.”In the American system it states “Though NMACs, especially those involving commercial passenger aircraft, are already extremely rare, McLaughlin notes that ‘TCAS is intended to reduce their probability even further.’”
(From the Mitre Corporation site www.caasd.org/proj/tcas/)