Definition
An airborne collision avoidance system that interrogates the transponders of nearby aircraft, tracks their range, bearing, and altitude, and issues two levels of alert to the pilot: a Traffic Advisory (TA) indicating nearby traffic that may become a threat, and a Resolution Advisory (RA) directing a vertical maneuver (climb or descend) to avoid a collision. TCAS II is mandated for most turbine-powered transport aircraft operating in U.S. airspace.
Plain English
An onboard system that watches for other aircraft using their transponder signals and warns the pilot if one is getting too close. If a collision is likely, it tells the pilot to climb or descend to stay clear.
Context Anchor
Seen in instrument flying when discussing cockpit traffic displays and spoken alerts such as “Traffic, traffic” or “Climb, climb.”
Derivation
TCAS stands for Traffic Alert and Collision Avoidance System. The 'II' indicates the second-generation version, which adds vertical resolution advisories. Earlier TCAS I only provided traffic advisories without recommending an avoidance maneuver.
Why Pilots Care
Provides an independent last layer of protection against mid-air collisions even when air traffic control instructions or visual scanning are insufficient.
Grounding Statement
If another aircraft gets too close, TCAS II first helps you notice the threat and then, if needed, gives a vertical escape instruction.
Intuition Check
TCAS II is not an autopilot and it does not turn the airplane. It warns the pilot and gives climb, descent, or level-off guidance to avoid nearby traffic.
Example Sentence 1
As the two aircraft converged at the same altitude, TCAS II issued a Resolution Advisory instructing the crew to climb.
Example Sentence 2
When the TCAS II directed an immediate climb, the pilot followed the command to restore safe separation.