TCAS is one of the most overlooked and under-appreciated systems in today’s global aviation industry. It has contributed a lot to the massively improved safety levels that have developed over time. It has proven itself over and over again, but is poorly understood at times. There is a lot going on within this system, and it can be very confusing due to all the technicalities and 234,987 abbreviations in your average TCAS documentation! But don’t worry, today we’ll be breaking it all down, and go over everything step by step. So what is TCAS, and how does it work?

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What is TCAS?

TCAS stands for Traffic Collision Avoidance System. It’s a type of Airborne Collision Avoidance System (ACAS) that helps pilots identify and avoid other traffic, without the need for Air Traffic Control (ATC).

TCAS constantly talks to other aircraft equipped with TCAS, and can even protect us from aircraft that only have a conventional transponder. Its mission is clear: provide adequate separation from other traffic to avoid a collision.

ACAS can be split up in ACAS I, II, and III. For most of this article we’ll be talking about TCAS II, as it’s the most common one. TCAS II falls under ACAS II, which can provide us with solutions to incoming traffic.

What is TCAS?

TCAS is completely independent of ground or air based radar. It detects the presence of nearby aircraft by interrogating other aircraft’s transponders.

What does ‘interrogating’ mean? Well, it’s really just a fancy word for the system asking other aircraft: ‘HEY, who are you’? TCAS then gets an answer to this question, plus the track, range, and altitude of that traffic. The bearings are determined by external onboard antennas. It then uses all of this information to show the pilots what is going on exactly with all the nearby traffic. This looks something like this:

What is TCAS?

However, TCAS is not just a system that gives pilots more situational awareness. It can also go a step further and actually alert the pilots what actions are required to avoid specific traffic, if the risk of collision crosses a certain threshold. Let’s zoom in on these first.

Types of TCAS Alerts

TCAS can generate 2 types of alerts to pilots: Traffic Advisories (TA) and Resolution Advisories (RA). The way TCAS displays traffic like these is displayed below, but let’s have a closer look at what these type of alerts are exactly.

What is TCAS?

The first two symbols is how TCAS shows other traffic to the pilots, based on their potential threat level (low and higher).

The Traffic Advisory

The first type of alert is the TA. A TA is an alert that basically shouts ‘have a look at this guy please!’ It is not seen as a threat by the system (yet), and this alert purely serves as information to assist the pilot in the search for the relevant aircraft. In addition to this, it also serves to prepare for a further increase in potential risk.

TA’s help pilots with situational awareness. In an ideal world it would never be required, as a TA in IFR airspace is already a pretty big deal. In addition to this, if the perceived risks increases further, the next alert might get generated:

The Resolution Advisory

The RA is the alert that follows after a TA and is calibrated for an increased amount of risk. If the other nearby aircraft crosses this risk boundary, the RA alert gets generated. The goal of an RA is to either maintain or increase vertical separation with the other traffic (also called the ‘intruder’).

If the intruder and our own aircraft both have TCAS II equipped, the 2 systems will work together and coordinate their RA’s with each other. This means that if aircraft A’s TCAS II tells the pilot to descend, then aircraft B’s TCAS II will tell the pilot to climb, and vice versa. This is called a coordinated RA. Pretty clever stuff! This principle has already saved many lives.

There’s an important distinction here though. There are 2 different types of RA’s, the Preventative RA (PRA) and the Corrective RA (CRA). What’s the difference? Let’s have a look.

A PRA simply calls for an avoidance of the current vertical flight plan. In other words, it could tell us to maintain vertical speed. In this case, TCAS has calculated both flight paths and has determined that as long as nothing changes with our flight path, we’re all good!

The PRA could be any of the following commands:

  • MAINTAIN VERTICAL SPEED MAINTAIN (Continue existing Vertical Speed)
  • MAINTAIN VERTICAL SPEED CROSSING MAINTAIN (maintain Vertical Speed while crossing the intruders flight altitude)
  • MONITOR VERTICAL SPEED (ensure that Vertical Speed is outside the red band indicated on the Vertical Speed Indicator)
  • CLEAR OF CONFLICT (no further action required)

A CRA is a call to action, a change in the current vertical flight path. This could be any of the following commands:

  • CLIMB CLIMB (start a rate of climb to the required amount indicated in green on the vertical speed indicator, usually between 1500 and 2000 feet per minute)
  • DESCEND DESCEND (start a rate of descent to the required amount indicated in green on the vertical speed indicator, usually between 1500 and 2000 feet per minute)
  • CLIMB CROSSING CLIMB (start a rate of climb to the required amount, indicated in green on the vertical speed indicator to climb through intruder’s flight altitude)
  • DESCEND CROSSING DESCEND (start a rate of descent to the required amount, indicated in green on the vertical speed indicator to climb through intruder’s flight altitude)
  • LEVEL OFF (reduce the vertical speed to achieve level flight)
  • CLIMB CLIMB NOW (received after a DESCEND command, to get the pilot to reverse his initial action and start climbing to avoid a manoeuvring intruder)
  • DESCEND DESCEND NOW (received after a CLIMB command, to get the pilot to reverse his initial action and start descending to avoid a manoeuvring intruder)
  • INCREASE CLIMB (received after an initial CLIMB command, to indicate a higher rate of climb is required to avoid the intruder, usually about 2500 to 3000 feet per minute)
  • INCREASE DESCENT (received after an initial DESCEND command, to indicate a higher rate of descent is required to avoid the intruder, usually about 2500 to 3000 feet per minute)

That’s quite the list, isn’t it? It’s always good to remind ourselves of what is actually required for each TCAS command. If you’re flying a modern aircraft with integrated avionics, this will all be indicated on the VSI anyway, but there are still TCAS systems out there that are completely separate from primary flight displays.

How does TCAS work?

So how does TCAS actually work? Let’s cover it step by step.

STEP 1: TCAS constantly listens for something called spontaneous transmissions, also called ‘squitters’. These are little signals generated once every second by a Mode S transponder. Inside this tiny signal is the address of the aircraft it belongs to.

What is TCAS?

STEP 2: After this is received and decoded, TCAS sends a Mode S interrogation to the other aircraft.

What is TCAS?

STEP 3: The other aircraft replies with the information we discussed earlier (range, bearing, altitude).

What is TCAS?

STEP 4: All this information gets processed and if the other aircraft is equipped with TCAS as well, this becomes a two way street where both aircraft transponders are essentially in constant conversation!

What is TCAS?

What’s important to understand is that there a few different ways TCAS determines the threat severity (i.e the amount of collision risk).

The way it does it is by constantly thinking ahead in time and predicting various versions of “what will happen if?” The concept it uses for this is called Closest Point of Approach or CPA. CPA is what TCAS considers the closest allowable distance or shortest amount of time between two aircraft at any point in time.

Imagine a cylinder around our aircraft that’s projected forwards in time. The volume of the cylinder is the CPA. TCAS uses two different variables to find out if the CPA can be safeguarded or not: the time it takes for an intruder aircraft to ‘hit’ the CPA (the technical name for this is TAU as seen in the table below), and a minimum distance as well, whichever comes first! So when do we get a TA or RA? Have a look at the table here:

What is TCAS?

As you can see, the allowable times before hitting the CPA go up as altitude goes up as well, so it’s not a static figure. You can’t just say ‘oh we’ll get a TA within this distance’. It’s mostly time based, and therefore speed and closure rate based. As the table shows, it’s also different for heights on Radalt compared to altitudes on Barometric altitude.

To achieve all of this, it depends on a variety of sensors and systems to acquire the required data and then integrates it all into a nice overview for the pilots.

The required sensors for TCAS are:

What is TCAS?

Does TCAS override ATC?

Yes, TCAS has a higher authority than ATC. This is laid out in the master of all global aviation rulebooks: PANS OPS. What do we do when an RA conflicts with an ATC instruction?

PANS OPS section III-3-3-1 tells us all we need to know in this case. Let’s split it up by TA’s and RA’s.

Actions in the event of an RA

In the event of an RA, pilots shall:

1) respond immediately by following the RA as indicated, unless doing so would jeopardise the safety of the aircraft;

2) follow the RA even if there is a conflict between the RA and an air traffic control (ATC) instruction to manoeuvre;

3) not manoeuvre in the opposite sense to an RA;

4) as soon as possible, as permitted by flight crew workload, notify the appropriate ATC unit of any RA which requires a deviation from the current ATC instruction or clearance;

5) promptly comply with any modified RAs;

6) limit the alterations of the flight path to the minimum extent necessary to comply with the RAs;

7) promptly return to the terms of the ATC instruction or clearance when the conflict is resolved;

8) notify ATC when returning to the current clearance.

Seems pretty definitive right? TCAS definitely has a higher authority than ATC! What’s also important to keep in mind is how pilots should respond to RA’s. PANS OPS mentions:

“RAs requiring a change in vertical speed, initiation of a response in the proper direction within five seconds of the RA being displayed. Proper division of responsibilities between the pilot flying and the pilot not flying.

The pilot flying should respond to the RA with positive control inputs, when required, while the pilot not flying is providing updates on the traffic location, checking the traffic display and monitoring the response to the RA.”

So most of the equipment criteria and calibrations have all been based on the fact that pilots should react within 5 seconds to an RA, something to think about!

Actions in the event of a TA

PANS OPS also goes into what to do in case of a TA (or what not to do..):

“Pilots shall not manoeuvre their aircraft in response to traffic advisories (TAs) only;

TAs are intended to alert pilots to the possibility of a resolution advisory (RA), to enhance situational awareness, and to assist in visual acquisition of conflicting traffic. However, visually acquired traffic may not be the same traffic causing a TA. Visual perception of an encounter may be misleading, particularly at night.

The above restriction in the use of TAs is due to the limited bearing accuracy and to the difficulty in interpreting altitude rate from displayed traffic information.”

So to summarise: Do exactly what the RA tells you to do, but do not change the flight path based on a TA only!

TCAS Modes

There are 3 main TCAS modes: Standby, TA/RA, and TA ONLY.

What is TCAS?

Standby is usually only used during ground taxi. The main functions of TCAS are unavailable in this mode.

TA ONLY is a mode that tells TCAS not to generate RA’s. In addition to this, it also tells other TCAS systems of aircraft flying around us that we cannot comply with an RA. All RA’s are converted into TA’s and no vertical speed advisories will be displayed on the VSI. The TA time warning threshold is now set to 20 seconds or less.

So if we have a single engine failure for instance, and cannot climb at the rate we normally would, we should set TCAS to TA ONLY. This way, other TCAS systems know that a coordinated RA is not an option.

TA/RA is the mode that gives us full protection!

What are the Limitations of TCAS?

TCAS, like any system, comes with quite a few limitations. The most important ones to know are:

  • TCAS relies on other aircraft being equipped with a transponder, or TCAS II in the case of coordinated RA’s.

  • Any form of ACAS will not track aircraft without transponders, failed transponders, or transponders equipped with only mode A, something to bear in mind!

  • The maximum tracking range of ACAS is 14 nm around the aircraft, with a display of this traffic within 12 nm. Any traffic outside this range will simply not be displayed. ACAS can usually track a maximum of 60 aircraft at any given time, and display up to 30 of them.

  • In addition to these limitations, TCAS will fail if input from the Radalt, Barometric Altimeter, or main transponder is lost. In some aircraft, a loss of other air data systems or heading reference systems can also result in a loss of TCAS.

  • Aircraft below 380’ above ground level (AGL) will not be displayed, unless this capability is specifically equipped and approved (mainly modern helicopters).

  • TCAS is not able to provide RA’s using lateral movements, only vertical.

  • Stall warnings, ground proximity warnings and wind shear warnings always take priority over TCAS. This means that a TA or RA might be suppressed depending on the circumstances we’re in.

  • TCAS alerts can be inhibited as well, so the use of this function will completely silence TA’s and RA’s.

TCAS Incidents

Even in just the last few years, there have been a lot of TCAS related incidents where without TCAS, the outcome could’ve been a lot uglier. There are over 50 very recent incidents where TCAS saved the day. Don’t believe us? Have a look at this brilliantly summarised article from Skybrary.

As you can see, there are various ways TCAS still saves pilots. Whether it’s due to mistakes in the see and avoid technique, not following ATC instructions, or ATC mistakes. This has been an amazing safety net that does not receive enough credit.


TCAS, or any form of ACAS, is an amazing system. It has contributed to safer global aviation, but is often not fully understood. Hopefully this article has helped with some of the core mechanisms and principles. If you have any thoughts on all of this, please feel free to give feedback and we can keep the discussion going!

Categories: Technical

Jop Dingemans

AW169 HEMS Commander | Founder of Pilots Who Ask Why | Aerospace Engineer | Former Flight Instructor


Anonymous · August 26, 2023 at 4:53 PM

Excellent article!Very easy to understand and complete!

Erik Brouwer · November 10, 2022 at 10:11 AM

Weer top gedaan Job! Ik heb je artikelen al bij talloze studenten aanbevolen.

    Jop Dingemans · November 10, 2022 at 10:28 AM

    Thanks Erik!

      George Williams · November 10, 2022 at 4:02 PM

      Good stuff Jop. Again good clear language and nice diagrams. Us lesser mortals have to be satisfied with just TAS of course.

      I would contest one of your points though. The TCAS units do not share their relative bearings. That’s all worked out by the antenna – hence it’s the least accurate part of the system and hence TCAS resolution is currently only in the vertical axis. With ADS-B, position sharing locations for sharing for mapping purposes is probably better but TCASII very much only cares about time of closure and vertical separation through Mode C/S.

      The bearing inaccuracy is similar in TAS systems and has led to some incidents of looking the wrong way.

      Jop Dingemans · November 10, 2022 at 4:44 PM

      Thanks for your valuable feedback George, I’ll amend this in the article asap!

Vikas · November 9, 2022 at 6:14 AM

Thank you for the article…

Anonymous · November 8, 2022 at 11:57 AM

Good Article

David D Wallace · November 8, 2022 at 2:23 AM

Outstanding and very informative article! Thank you buddy!

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