TCAS: The System That Keeps Planes Apart

A Safety Net That Works on Its Own

Air traffic control keeps planes safely apart. But aviation never relies on just one layer of safety. So every airliner carries its own collision-avoidance system, usually called TCAS.

It doesn't depend on air traffic control, satellites or anyone on the ground. It's a last, independent safety net — always switched on and always watching.

How It Works

TCAS sends out radio signals to other planes nearby, and their equipment answers. From those replies, it works out:

• where each nearby plane is
• how high it is
• whether it's getting closer

It does this constantly, second by second, for every plane around.

Step 1: A Heads-up

If another plane might come close, TCAS gives the pilots an early heads-up. This can come up to about 48 seconds before the planes would be at their closest. A calm voice says "Traffic, traffic" and the other plane shows on a screen. Most of the time, nothing more is needed.

Step 2: A Clear Instruction — Agreed by Both Planes

If the planes would come too close, TCAS gives a clear instruction, such as "Climb, climb" or "Descend, descend". This usually comes around 15 to 35 seconds before the closest point.

Here's the clever part: the two planes' systems talk to each other and agree opposite moves. If one is told to climb, the other is told to descend. They can never both be told to go the same way.

Even If Air Traffic Control Makes a Mistake

What if a controller gave two planes conflicting instructions? TCAS covers that too.

The rules say pilots must follow a TCAS instruction immediately — even if it conflicts with what air traffic control said. TCAS sees both planes directly, in real time, so it always takes priority. Afterwards, the pilots tell air traffic control what happened.

Practised Regularly

Pilots practise responding to TCAS in the simulator. The response is quick, smooth and well-rehearsed. If it ever happened on your flight, you might feel a firm climb or descent for a few seconds. Then the plane returns to normal flight.

Mandatory on Airliners

TCAS has been required on airliners for decades. In Europe, its current version has been mandatory since December 2015.

The Takeaway

Even in the rare case that something goes wrong on the ground, every airliner has its own guardian on board. TCAS talks to the other plane, agrees a plan and tells both crews exactly what to do.

Sources

• SKYbrary, Airborne Collision Avoidance System (ACAS) — how TCAS works, advisory timings, coordinated manoeuvres, and version 7.1 mandatory in Europe since December 2015: https://skybrary.aero/articles/airborne-collision-avoidance-system-acas
• UK CAA Regulatory Library, SERA.11014 — ACAS resolution advisory (RA) — pilots must follow an RA immediately, even if it conflicts with an ATC instruction: https://regulatorylibrary.caa.co.uk/923-2012/Content/Regs/02740_SERA11014_ACAS_resolution_advisory_RA.htm
• EASA, Safety Information Bulletin 2013-11R1 — ACAS II — crews must respond immediately to a resolution advisory: https://ad.easa.europa.eu/blob/EASA_SIB_2013_11_R1.pdf/SIB_2013-11R1_1