Aircraft collision avoidance systems
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Modern aircraft can use several types of collision avoidance systems to prevent unintentional contact with other aircraft, obstacles, or the ground:
- Airborne Radar can detect the relative location of other aircraft, and has been in military use since World War II, when it was introduced to help night fighters (such as the de Havilland Mosquito and Messerschmitt Bf 110) locate bombers. While larger civil aircraft carry weather radar, sensitive anti-collision radar is rare in non-military aircraft.
- a Traffic Collision Avoidance System (TCAS), which actively interrogates the transponders of other aircraft and negotiates collision-avoidance tactics with them in case of a threat. TCAS systems are relatively expensive, and tend to appear only on larger aircraft. They are effective in avoiding collisions only with other aircraft that are equipped with functioning transponders with altitude reporting.
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a Portable Collision Avoidance System (PCAS) is a less expensive, passive version of TCAS designed for general aviation use. PCAS systems do not actively interrogate the transponders of other aircraft, but listen passively to responses from other interrogations. PCAS is subject to the same limitations as TCAS, but also is ineffective if there is not at least one system (such as air traffic control or a TCAS-equipped aircraft) interrogating another aircraft's transponder.
Small PCAS device for use in light aircraft.
- a Ground proximity warning system (GPWS), or Ground collision warning system (GCWS), which uses a radar altimeter to detect proximity to the ground or unusual descent rates. GPWS is common on civil airliners and larger general aviation aircraft.
- a Terrain awareness and warning system (TAWS) uses a digital terrain map, together with position information from a navigation system such as GPS, to predict whether the aircraft's current flight path could put it in conflict with obstacles such as mountains or high towers, that would not be detected by GPWS (which uses the ground elevation directly beneath the aircraft).
- Synthetic vision provides pilots with a computer-generated simulation of their outside environment for use in low or zero-visibility situations.
Large, sophisticated aircraft may use several or all of these systems, while a small general aviation aircraft might have only PCAS, or no collision-avoidance system at all (other than the pilot's situational awareness).
