Over the years, air traffic has continued to increase. The development of modern air traffic control systems has made it possible to cope with this increase, whilst maintaining the necessary levels of flight safety. However, the risk of airborne collision remains. This is why, as early as the fifties, the concept and initial development of an airborne collision avoidance system, acting as a last resort, was being considered. A series of mid air collisions occurred in the United States, initiating the further stages of the system’s development. In parallel to the development of TCAS equipment, ICAO (International Civil Aviation Organisation) has developed, since the beginning of the eighties, standards for Airborne Collision Avoidance Systems (ACAS).
ACAS II is a system used for detecting and tracking aircraft in the vicinity of your own aircraft, and is applicable to all aircraft with a MTOW over 5,700kg. By interrogating their transponders it analyses the replies to determine range, bearing, and if reporting altitude, the relative altitude of the intruder. Should the ACAS II processor determine that a possible collision hazard exists, it issues visual and audio advisories to the crew for appropriate vertical avoidance manoeuvres. ACAS is unable to detect any intruding aircraft without an operating ATCRBS transponder (operating in Mode A and C) or a Mode S transponder.
There are two types of cockpit displays for ACAS II, the Resolution Advisory (RA) display and the Traffic Advisory (TA) display. The RA display is incorporated into the vertical speed indicator (VSI). By illuminating red and green arcs around the dial it displays the required rate, or limitation of climb or descent, to avoid a possible collision.
The TA display shows the intruding aircraft's relative position and altitude with a trend arrow to indicate if it is climbing or descending at greater than 500 feet per minute. This TA display may be provided on the weather radar indicator, on a dedicated ACAS display or a TANSI display. The TA display identifies the relative threat of each intruder by using various symbols and colors. Complementing the displays, ACAS II provides appropriate synthesized voice announcements. ATC procedures and the "see and avoid concept" will continue to be the primary means of ensuring aircraft separation. However, if communication is lost with ATC, ACAS II adds a significant backup for collision avoidance.