Following the recent and tragic collision between a helicopter and passenger aircraft in the USA, there have been a lot of questions about TCAS. So here is a summary on the system – not intended as any sort of commentary on the accident, just to clarify what TCAS is and how it works.

What it TCAS?

TCAS stands for Traffic Collision Avoidance System (TCAS) and it is a safety system designed to prevent mid-air collisions between aircraft. It works off aircraft transponders which “interrogate” each other and then calculate the slant range, altitude, relative bearing etc of the traffic around them. From this they can work out the ‘Closest Point of Approach’, and give alerts to the crew if another traffic is going to “intrude” within a safe boundary.

TCAS II was designed to operate in traffic densities of up to 0.3 aircraft per square nautical mile, i.e., 24 aircraft within a 5 nm radius, which was the highest traffic density expected when this was brought in.

It is mandatory for all civil fixed-wing, turbine-powered aircraft having with an MTOW exceeding 5,700 kg, or a maximum approved passenger seating configuration of more that 19 seats to be equipped with TCAS II, Version 7.0.

Why was it brought in?

If you want the full history, then check out this document published by the FAA.

The need for more protection against midair collisions really came into focus in 1956 though, because of the Grand Canyon mid-air collision which occurred between a United Airlines DC7 and a Trans World Airlines Super Constellation. 128 passengers were killed, making it the first commercial airline incident to exceed one hundred fatalities… and highlighting the need for more mitigations against this!

How does it prevent them?

Traffic is depicted on the nav displays, using white symbols and arrows to show if traffic is climbing or descending, and the difference in altitude between the aircraft. Other aircraft show as white diamonds (or an arrow shape) which becomes solid when the traffic is proximate I.e. comes within a certain distance.

Depending on the CPA, the system will issue two types of alert – a Traffic Advisory (TA) or a Resolution Advisory (RA).

A TA is just to help pilots visually identify traffic that might become an issue. Pilots don’t manoeuvre based off this but instead look for the aircraft, adjust their rate of climb or descent, possibly query ATC on whether there might be conflict. A TA traffic shows in amber and “TRAFFIC, TRAFFIC” sounds.

An RA on the other hand gives a command and it is mandatory for pilots to follow this. It takes precedence over any clearances from ATC and is critical crew follow what their system directs them to do because the RA the other aircraft gets will be coordinated with theirs via Mode S datalink. RA traffic turns red.

This is really important, especially regarding ATC clearances because people make mistakes, but the system shouldn’t! There have also been cases of fake ATC issuing clearances intended to cause conflict – back in early 2024, there were at least 10 reports of fake controllers operating on Mogadishu FIR frequencies. They gave false instructions including climbs and descents, and could have caused conflict between opposite traffic. So always follow TCAS not ATC clearances when they contradict each other!

An RA can give a variety of commands from “CLIMB, CLIMB NOW” (or descend) to “LEVEL OFF” or “MAINTAIN VERTICAL SPEED”.

What does the pilot need to do?

The general memory action for a TCAS RA is to disconnect the autopilot (sometimes the auto throttle) and to visually follow the guidance. On older systems this generally means flying “into the green” and “out of the red” area on the VSI. It is expected that you reach the target climb or descent within 4 seconds. The manoeuvre needs to be done expeditiously, but not violently – it should be controlled and smooth (so passengers don’t notice).

The system can also issue reversions I.e. requiring the pilot to go from a climb to a descent or vice versa if the other traffic is not achieving what is required to avoid conflict. On receiving a TCAS RA, the pilot monitoring makes the call “[Callsign] TCAS RA” and when it is resolved, informs ATC “[Callsign] Clear of Conflict”. ASRs and other reports would be filed.

There are more modern systems available nowadays too. Airbus have automated TCAS on the A380 and A350 which still alerts the crew in the same way, but the autopilot can fly the manoeuvre without requiring pilot intervention (unless of course it doesn’t work then they must take over manually). When a TA occurs, the aircraft can also reduce ROC/ROD approaching its target altitude – something known as the TCAP function.

These help reduce pilot error in following TCAS orders, and can reduce the number of RAs. Fairly shockingly, a Eurocontrol study showed that on average only 33.7% of the “climb” and “descend” RAs were correctly followed, 44.2% did not reach the expected target, and 22.1% were not flown correctly.

If you want more on this, take a look at Airbus’ ‘Safe Handling of TCAS Alerts’ doc here.

But it doesn’t always work?

No, it doesn’t. Sometimes this is because crew fail to correctly follow the TCAS alert (see above on Eurocontrol’s stats). But sometimes it is because the system is inhibited… and this is actually intentional.

TCAS RAs are inhibited from around 1000 feet radio altitude in the descent (to around 900 feet if a go around is flown). Below these heights, even with RA selected, only a TA will be given. TA voice alerts are inhibited below around 400 feet in the descent to about 600 feet on the go-around. All increase RAs are inhibited from below 1450’ and up to 1650’, and all descend RAs are inhibited below 1000’ and up to 1200’.

Why inhibit it at all?

Well, to avoid nuisance alerts (at busy airports, you could potentially get them from aircraft waiting at the holding point while you’re trying to land) and to avoid aircraft descending when very close to the ground! TCAS alerts are also sometimes inhibited by other systems, for example if an EGPWS alert is occurring, or wind shear because these take priority (another aircraft can get out of your way, a mountain can’t!)

Boeing 787 inhibits during landing (there are similar ones for take-off as well)

TCAS is the final barrier

TCAS should be a final barrier to prevent conflict, but it is not something that should be relied on entirely. Good situation awareness regarding other traffic, separation by ATC and good communication is needed first and foremost. This is especially important within the airport vicinity because there is more traffic and the alerts can be inhibited.

In the USA, often visual approaches are given where it becomes necessary for crew to maintain their own visual separation with other traffic. This is not always possible and can be challenging at night. Bear in mind the closure rate between aircraft can be exceptionally high leaving minimal time for crew to react. So again, situation awareness and ATC support (and control) is needed.

We do not know what happened in the US accident and should not speculate, but we can (and must) be aware of the limitations of the TCAS system and of the other barriers and mitigations which we should be applying. Ultimately, TCAS has prevented countless accidents over the years, but it has not removed any of the responsibility on pilots and ATC to continue managing safety and preventing conflict too.