Definition
An airborne system, independent of ground-based air traffic control, that interrogates the transponders of nearby aircraft to detect potential mid-air collision threats. It tracks intruder aircraft, evaluates closure rates and projected flight paths, and issues two levels of alert to the flight crew: a Traffic Advisory (TA), which calls attention to nearby traffic, and a Resolution Advisory (RA), which directs a vertical maneuver (climb or descend) to maintain safe separation.
Plain English
TCAS is a system on board the aircraft that watches for other aircraft nearby and warns the pilots if one is getting too close. If a collision risk is real, it tells the pilots whether to climb or descend to stay clear.
Context Anchor
You encounter TCAS on cockpit traffic displays and as voice alerts, especially in larger aircraft operating near other traffic.
Why Pilots Care
It supplies an independent last layer of protection against mid-air collision when visual separation or ATC instructions are insufficient.
Intuition Check
Do not assume TCAS can see every aircraft or automatically avoid a collision. It warns about aircraft it can detect, and the pilot or crew must respond correctly.
Example Sentence 1
The crew received a TCAS Resolution Advisory to descend and immediately followed it, then advised ATC of the deviation.
Example Sentence 2
When the resolution advisory commanded a climb, the pilot followed it immediately to pass above the conflicting traffic.