TCAS (Traffic Collision Avoidance System) is the last line of defence against a mid-air collision. It listens to nearby transponders, builds a 3D picture of surrounding traffic, and — when geometry says a collision is imminent — barks an instruction at the pilots: CLIMB CLIMB or DESCEND DESCEND. Here is what each warning means, why crews must obey it even over ATC, and how the Überlingen accident in 2002 rewrote the rules.
📡 Watch live traffic on the radarEvery airliner carries a TCAS computer connected to a Mode S transponder. The transponder interrogates every other Mode S/C transponder within roughly 40 nautical miles, several times per second. From each reply it extracts the other aircraft's altitude (from Mode C) and computes its range from the reply delay. By tracking those returns over time, TCAS predicts whether any other aircraft is on a converging track that will close inside a danger threshold. If so, it issues a warning.
A TA fires roughly 40 seconds before the closest point of approach (CPA) — the moment two aircraft would be nearest one another if both held course. The audio call is "TRAFFIC, TRAFFIC". A yellow circle appears on the traffic display showing the threat's bearing, range, and relative altitude. The crew is expected to visually acquire the traffic and prepare to act, but no manoeuvre is required yet. Roughly three-quarters of TAs never escalate — the geometry resolves itself.
An RA fires roughly 25 seconds before CPA. The threat symbol turns red and the synthesised voice issues a precise instruction:
The vertical speed indicator on the primary flight display shows a red zone for prohibited rates and a green band for the commanded rate. The pilot flying has ~5 seconds to start the manoeuvre and ~10 seconds total to reach the commanded rate. RA compliance is a mandatory pilot reaction trained to muscle memory.
When both aircraft carry TCAS II, their Mode S transponders negotiate a complementary pair of RAs. If aircraft A is told to climb, aircraft B will be told to descend — never the same direction. The two airborne computers exchange Coordination Messages on 1030/1090 MHz roughly once per second during the encounter, each one announcing its chosen sense and reserving the opposite for the other. If one aircraft has only Mode C (no Mode S), TCAS still issues an RA but cannot coordinate, so the unequipped traffic is treated as unpredictable.
On 1 July 2002, a Bashkirian Airlines Tupolev Tu-154 and a DHL Boeing 757 were on converging tracks at 36,000 ft over Überlingen, Germany. Both crews had TCAS RAs at the same moment: the 757 was told to descend, the Tu-154 to climb. The lone controller on duty at Skyguide Zurich, unaware of the RAs, simultaneously instructed the Tu-154 to descend. The Russian crew followed ATC instead of TCAS, descended into the 757's path, and the two aircraft collided. All 71 people on both aircraft died.
The accident rewrote the global procedure. ICAO doctrine since 2003 is explicit: "In the event of an RA, pilots shall respond immediately by following the RA as indicated, even if there is a conflict between the RA and an air-traffic-control instruction to manoeuvre." ATC clearances are suspended for the duration of an RA. Once the encounter clears ("CLEAR OF CONFLICT"), the pilot resumes the original clearance and tells ATC.
TCAS only sees aircraft with a transponder replying to its interrogations. It will not warn the crew of:
The system is also famously prone to RA cascades in dense terminal areas, which is part of why ACAS X was designed — it uses a probabilistic model of intent rather than purely geometric thresholds.
TCAS predates ADS-B by about 15 years and was built around active Mode S/C interrogation, not passive ADS-B reception. They are complementary: TCAS provides the time-critical collision-avoidance loop on the flight deck, while ADS-B provides the wider situational picture used by ATC and by the public flight trackers on the ground. ACAS X is the convergence point — newer installations can fuse ADS-B In data with the existing Mode S interrogations for richer threat tracking.
The full final report on the Überlingen collision is published by the German BFU as report AX001-1-2/02 and is the most thorough TCAS-vs-ATC case study in the public record.