TCAS
Traffic Alert and Collision Avoidance System · Also: ACAS, ACAS II, collision avoidance system
Onboard system that interrogates the transponders of nearby aircraft and, if it detects a collision risk, tells the pilots to climb or descend.
What it is
It works independently of air traffic control. It interrogates the transponders of surrounding aircraft, works out their range, altitude and trajectory, and shows them on the navigation display. If another aircraft gets too close, it first issues a traffic advisory (TA), with the aural alert ‘Traffic, traffic’, so the crew can look out for it. If the risk persists, it gives a resolution advisory (RA): a vertical command such as ‘Climb, climb’ or ‘Descend, descend’.
When both aircraft carry TCAS, the units coordinate with each other so that the manoeuvres are complementary: one climbs while the other descends. The rule is clear: pilots must follow a resolution advisory immediately, even if it contradicts an instruction from the controller, and report it as soon as possible.
ICAO requires its generic equivalent, ACAS II, on turbine-engined aeroplanes above 5,700 kg or authorised to carry more than 19 passengers. It is a safety net: it does not replace the separation provided by air traffic control, but steps in if that fails. It can only see aircraft with an operating transponder.
Related terms
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This page was last updated on October 9, 2026
APA
Vuelopedia. (2026, October 9). TCAS. Retrieved October 17, 2026, from https://vuelopedia.org/en/glossary/tcas
MLA
“TCAS.” Vuelopedia, Fundación Poderia, 9 Oct. 2026, vuelopedia.org/en/glossary/tcas. Accessed 17 Oct. 2026.
Chicago
Vuelopedia. “TCAS.” Fundación Poderia. Last modified October 9, 2026. https://vuelopedia.org/en/glossary/tcas.
Harvard
Vuelopedia (2026) TCAS. Available at: https://vuelopedia.org/en/glossary/tcas (Accessed: 17 October 2026).
BibTeX
@misc{vuelopedia_en_glossary_tcas,
author = {{Vuelopedia}},
title = {{TCAS}},
publisher = {Fundación Poderia},
year = {2026},
date = {2026-10-09},
url = {https://vuelopedia.org/en/glossary/tcas},
howpublished = {\url{https://vuelopedia.org/en/glossary/tcas}},
urldate = {2026-10-17},
note = {Accessed 2026-10-17}
}Wikipedia
{{cite web |url=https://vuelopedia.org/en/glossary/tcas |title=TCAS |website=Vuelopedia |publisher=Fundación Poderia |date=2026-10-09 |access-date=2026-10-17 |language=en}}