Definition
An onboard airborne collision avoidance system that interrogates the transponders of nearby aircraft, tracks their range, bearing, and altitude, and warns the pilot of potential conflicts. TCAS issues two levels of alert: a Traffic Advisory (TA), which notifies the crew of nearby traffic that may become a threat, and a Resolution Advisory (RA), which directs the pilot to climb, descend, or adjust vertical speed to maintain safe separation. TCAS operates independently of ground-based air traffic control.
Plain English
Equipment in the cockpit that watches for other aircraft nearby and tells the pilot if any are getting too close. If a collision risk develops, it tells the pilot exactly what to do — climb, descend, or change vertical speed — to stay clear.
Context Anchor
Pilots encounter TCAS on cockpit traffic displays and through spoken alerts such as traffic warnings or climb and descend instructions.
Derivation
The name describes the function directly: it gives a 'traffic alert' (a warning about nearby aircraft) and supports 'collision avoidance' (steering clear of them). 'System' indicates the integrated equipment that makes this happen.
Why Pilots Care
TCAS reduces the risk of mid-air collisions when visual separation or ATC separation is insufficient.
Intuition Check
Do not read “traffic” here as cars or general congestion. In TCAS, traffic means other aircraft near your aircraft.
Example Sentence 1
When TCAS issued a Resolution Advisory to climb, the captain disconnected the autopilot and followed the guidance immediately, then notified ATC.
Example Sentence 2
During the approach briefing the crew reviewed how TCAS would coordinate with ATC instructions.