TCAS — Traffic Alert and Collision Avoidance System

Two Levels of Warning

TCAS — Traffic Alert and Collision Avoidance System — is an onboard system that independently tracks nearby transponder-equipped aircraft and warns the crew before a collision risk becomes critical, entirely separate from and in addition to whatever separation air traffic control is providing. It escalates through two distinct stages. A Traffic Advisory (TA) is the earlier, informational stage: the system announces "Traffic, traffic" and highlights the other aircraft — typically in yellow — on the cockpit display, showing its relative position and altitude, but issues no instruction to maneuver; it exists purely to help the crew visually acquire the traffic and prepare. A Resolution Advisory (RA) is the escalation: when the geometry closes to a genuinely dangerous point, TCAS issues an explicit, verbal directive — "Climb, climb!" or "Descend!" — that crews are trained and, in most jurisdictions, required to follow immediately, overriding a conflicting ATC instruction if one is given at the same moment.

How It Talks to the Other Aircraft

TCAS II — the version mandated on most commercial aircraft — requires a Mode S transponder, which is what lets two TCAS-equipped aircraft closing on each other coordinate their responses directly rather than independently picking the same evasive maneuver. When one aircraft's TCAS commands a climb, that coordination link lets the other aircraft's system know a climb has already been claimed, so it commands a descent instead — the entire point being that the two aircraft never end up moving toward each other rather than apart. This transponder-based interrogation is fundamentally different from ADS-B, which broadcasts an aircraft's own position passively rather than interrogating others; standard TCAS II does not currently rely on ADS-B data at all, though future collision-avoidance systems are expected to incorporate it.

Why Following an RA Matters

Because an RA is generated from real-time geometry between two specific aircraft, it reflects information that ATC — working from a wider, less instantaneous radar picture — may not have in exactly the same form at exactly that moment. Aviation accident investigations have repeatedly found that following the TCAS RA rather than a simultaneous, conflicting ATC instruction is the safer choice, which is why RA compliance is treated as close to non-negotiable in modern crew training and procedure — a lesson learned in part from past accidents where a crew followed ATC instead of TCAS.