TCAS
Initialism of traffic (alert and) collision avoidance system, a system for warning pilots of impending mid-air collisions and telling them what (if any) evasive action to take.
TCAS: the radar that keeps aircraft from hitting each other
TCAS is an airborne system that uses radar and transponder signals to detect other aircraft nearby and warn pilots of collision risk. It operates independently of air traffic control, giving crews a last-resort safety layer when separation breaks down. The system interrogates Mode C and Mode S transponders in surrounding aircraft, calculates their range, bearing, and altitude, then decides whether to issue an alert.
There are two warning levels. A Traffic Advisory (TA) alerts the crew to a potential threat without specifying action, typically issued when an intruder will come within 1,200 feet vertically or a few miles horizontally. A Resolution Advisory (RA) goes further, commanding specific vertical maneuvers, climb or descend at a particular rate, to increase separation. The system generates these instructions by modeling closure rates and predicted trajectories; it does not tell pilots to turn or change heading.
Two generations, fundamentally different
TCAS I, the simpler version, issues only traffic advisories and is typically fitted to smaller commercial aircraft and general aviation. TCAS II, the full system, provides resolution advisories and is mandated on large transport category aircraft. TCAS II versions have evolved: Version 7 added improved logic to reduce unwanted RAs and coordinate with other nearby TCAS-equipped aircraft through coordinated vertical maneuvers, cutting false alarm rates substantially compared to earlier versions.
The system's effectiveness depends on transponder coverage. Aircraft without a functioning Mode C or Mode S transponder are invisible to TCAS. This creates a blind spot that also affects primary radar separation methods; the reason modern NextGen and CPDLC systems integrate GPS-based position reporting is to work around this gap and improve situational awareness when radar coverage is spotty or nonexistent.
Maintenance technicians must verify that the TCAS antenna, typically a blade or planar element mounted on the fuselage, has clear line of sight and correct electrical continuity. The avionics box itself runs self-tests continuously and flags faults in the flight deck. A failed TCAS does not ground a large aircraft under current rules, but operators are required to notify ATC that the system is not available, allowing controllers to apply wider minimum separation standards to that flight.