What Is TCAS?

TCAS (traffic collision avoidance system) is an onboard system that detects nearby aircraft and tells the crew how to avoid a collision. It works independently of air traffic control.

How it works

TCAS sends interrogation signals to nearby aircraft’s transponders and listens to their replies. From the replies it tracks the other aircraft’s range, bearing and altitude, and predicts whether they could come too close.

Two kinds of alert

• Traffic advisory (TA): a warning that another aircraft is nearby. Pilots look for it but do not manoeuvre.
• Resolution advisory (RA): an instruction, such as “climb” or “descend”, that pilots must follow immediately. Two aircraft equipped with TCAS coordinate so their instructions are opposite.

Why it matters

After a mid-air collision over Germany in 2002, in which one crew followed air traffic control instead of TCAS, the rule was clarified: if the instructions conflict, pilots follow the TCAS resolution advisory.

TCAS and what you see on a tracker

TCAS needs transponders, so every airliner carries one that works. That is also the reason airliners appear on trackers: the same transponder that supports TCAS also sends ADS-B. See Mode S transponders.

General information only, not legal or safety advice. Details change; check with your airline and the relevant authority. See our editorial policy.