Definition
An onboard aircraft system that monitors the airspace around the aircraft for other transponder-equipped traffic, alerts the crew to potential conflicts, and, in more capable versions, issues vertical maneuver instructions (climb or descend) to avoid a midair collision. TCAS operates independently of air traffic control and works by interrogating the transponders of nearby aircraft to determine their range, bearing, and altitude.
Plain English
Equipment in the aircraft that watches for other aircraft nearby and warns the pilots if one is getting too close. Higher-end versions also tell the pilots whether to climb or descend to stay clear.
Context Anchor
Pilots encounter TCAS on cockpit traffic displays, during preflight equipment checks, in airline and turbine-aircraft operations, and in maintenance discussions about avionics antennas, wiring, and displays.
Why Pilots Care
It supplies the last layer of protection against mid-air collision when visual lookout or ATC separation has not prevented a conflict.
Intuition Check
TCAS is not an autopilot, and it does not see every aircraft in the sky. It warns about aircraft it can detect through compatible electronic equipment, and the pilot still remains responsible for safe operation.
Example Sentence 1
The TCAS alerted the crew to converging traffic and issued a climb advisory, which they followed immediately.
Example Sentence 2
The crew followed the TCAS resolution advisory and climbed two hundred feet to increase separation.