TCAS: Preventing Mid-Air Collisions

Avoiding Mid Air Collisions

For over 20 years, TCAS has been standing guard, protecting the skies from mid-air collisions. Countless catastrophes have been avoided thanks to the technology. Find out what TCAS is and how it works!

This AeroSavvy feature is written as general information for non-pilots. If you are a pilot seeking training information, consult your aircraft operating manual or TCAS/ACAS manufacturer information guide. 

Quick history leading to TCAS development

In 1956, a mid-air collision between a United DC-7 and a TWA Super Constellation over the Grand Canyon killed all 128 occupants of both airliners. At the time, it was the deadliest accident in commercial aviation history. Research was initiated to develop an airborne collision avoidance system. Read more about this tragic accident.

By the 1960’s and early 70’s, prototype collision avoidance systems were available but gave numerous unnecessary alarms when tested in busy terminal areas.

PSA Flight 182
PSA Flight 182

On September 25th, 1978, Pacific Southwest Airlines (PSA) flight 182 collided with a four seat Cessna 172 near San Diego Airport. Total fatalities: 144. After this accident, the FAA initiated development of what is now known as TCAS (pronounced Tee-Cass) – Traffic Alert and Collision Avoidance System. The system is also known in some countries as ACAS (pronounced Ā-Cass) – Airborne Collision Avoidance System.

The final straw came in 1986 when an Aeroméxico DC-9 collided with a four seat Piper Archer over Cerritos, California causing 82 fatalities (15 on ground). Soon after, the FAA mandated that airliners in U.S. airspace be equipped with TCAS. Other countries followed.

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How does TCAS work?

An aircraft’s TCAS constantly queries (talks to) transponders of other aircraft in the vicinity. Thanks to fancy antennas and a little math, TCAS determines the bearing, distance, and altitude of these other aircraft. TCAS then watches and protects an area around the aircraft. If the system sees an “intruder” enter the protected area, it alerts the flight crew. TCAS gives pilots enough notice to smoothly and safely avoid the intruder.

TCAS Protection Areas
TCAS provides alerts when intruders enter the Caution and Warning areas.

What’s a transponder?

Transponder and TCAS control

Airplanes flying in busy airspace are required to have a transponder. When an air traffic control radar “hits” an aircraft, the radar electronically asks (queries) “Who are you?” The transponder replies with a four digit code and the aircraft’s altitude. This information then appears on the controller’s radar display. TCAS queries aircraft transponders in a similar way.

TCAS Traffic Display

The TCAS traffic display shows pilots the location and altitudes of other airplanes in the vicinity. There are several types of displays manufactured. The one shown here is similar to what you would find installed on an older aircraft. Newer airplanes have the traffic display integrated on large screens with other information. Traffic symbols and colors are standardized on all equipment.

TCAS Traffic Display Key

  The Traffic Advisory – TA

The most advanced version of TCAS (called TCAS-II, used by airliners world-wide) provides two types of alerts. The Traffic Advisory, or TA, gives the flight crew a heads up that there is an aircraft nearby that could potentially cause a problem. A TA is triggered when the intruder aircraft is about 20-48 seconds away. The actual time varies with aircraft speed, altitude and maneuvering.

The intruder causing the Traffic Advisory is shown on the Traffic Display as a yellow dot along with altitude and climb/descent information. The pilots also get an audible warning. A loud voice on cockpit speakers will state: “Traffic, Traffic!” It really gets your attention when you hear it.

TCAS traffic display integrated with an Electronic Horizontal Situation Indicator (EHSI)

Upon hearing the Traffic Advisory, the crew will look at the Traffic Display to see where the traffic is located. If flying in clear skies, the crew will look out the windows and attempt to spot the intruder. Even if the crew sees the traffic, they must still follow commands from air traffic control. It’s very likely that the other aircraft won’t cause a problem. Sometimes, safe maneuvering, like an aircraft leveling off at a safe altitude below, will trigger a Traffic Advisory. In a busy month of flying I’ll get one or two TAs. Crews flying in really busy areas like New York or LA might see a few more.

TCAS DisplayTCAS Trivia!
Pilots love to come up with nicknames. The TCAS Traffic Display has a few good ones:

  • Fish Finder
  • Discovery Channel
  • Metal Detector
  • The Box

  The Resolution Advisory – RA

The second TCAS alert is the Resolution Advisory, or RA. When TCAS determines that the intruder aircraft is 15-35 seconds away, it uses the altitude, direction and speed of both aircraft to calculate and issue a resolution. The pilot of the aircraft with TCAS will be advised to climb, descend, or maintain altitude.

If the intruder aircraft is equipped with only a transponder and no TCAS, its pilot may be unaware of the potential problem unless they see the other aircraft out the windows.

TCAS communication with a transponder
TCAS queries the transponder of the intruder and determines if a Traffic or Resolution Advisory is necessary.

TCAS really shines when both aircraft are equipped with it. The two TCAS systems communicate with each other and negotiate a course of action for both aircraft. A typical resolution might have one system advising its crew to climb while the other advises its crew to descend.

TCAC Communicating with a transponder
When both aircraft have TCAS, the two systems coordinate the maneuver before issuing instructions.

When TCAS issues a Resolution Advisory, the crew will receive a loud command over the speaker. Some examples:

“Climb, climb!”
“Descend, descend!”
“Increase climb, increase climb!”
“Level off, level off!”

TCAS Superhero
TCAS has reduced mid-air collisions.

TCAS can only suggest altitude changes to avoid traffic. It won’t suggest a turn. Changing altitude is an effective way to avoid a mid-air collision. It keeps things simple.

When a crew hears a Resolution Advisory, there is no hesitation. The autopilot is immediately turned off and the flying pilot smoothly climbs or descends based on the instructions. During a TCAS maneuver, it’s important that the crew ignore all instructions from an air traffic controller.

TCAS provides enough advance notice (15-35 seconds) that the RA maneuver does not need to be abrupt. When done properly, passengers should not realize the maneuver took place. No coffee is spilled when an RA maneuver is performed properly.

How much should the pilot climb or descend during an RA?

When a pilot hears a Resolution Advisory, he or she will look at the RA display. Different aircraft have different styles of RA displays, but they all provide the exact same information. The Resolution Advisory display shows the rate of climb or descent required to avoid the conflict. The RA display is often integrated with the aircraft’s vertical speed indicator.

The Vertical Speed Indicator: 

Vertical speed indicator
Vertical speed indicating level flight.

The instrument at the heart of a TCAS maneuver is the vertical speed indicator. The VSI shown here is similar to the type found in older cockpits. Newer aircraft have vertical speed information integrated with other instruments on CRT or LCD displays.

The VSI displays how fast the aircraft is climbing or descending in thousands of feet per minute (fpm). When the pointer is at zero, the aircraft is level. An airliner typically climbs at about 2000 fpm (pointer at the 2 near the top of the display).

Sometimes air traffic control will request that we climb or descend at a specific rate: “AeroSavvy flight 132, descend at 1000 feet per minute.” In this case we descend so the pointer is on the 1 at the bottom of the display. Easy!

During a TCAS Resolution Advisory, red and green highlights illuminate on the vertical speed indicator showing the exact climb or descent rate required to safely avoid the intruding traffic.

TCAS Resolution Advisory indications on a vertical speed indicator

Combined VSI EADI RA
TCAS guidance on an artificial horizon and vertical speed tape.

Newer aircraft may have Resolution Advisory guidance displayed on the artificial horizon as a red rectangle. The “vertical speed tape” on the right of the display also shows the red no-fly area along with the green target area. These types of displays are very intuitive.

Once the intruder is no longer a threat, TCAS will announce: “Clear of conflict.” All guidance information will disappear indicating to the crew that they can safely return to normal flight.

TCAS Aviation Superhero
TCAS Superhero character created by Haley Hoke

Here’s a video that brings it all together! Watch the TCAS Traffic Display as an intruder aircraft descends to our altitude while flying towards us. The intruder first triggers a Traffic Advisory so we can start looking for him. When the intruder triggers a Resolution Advisory, the red and green guidance bars appear, followed by the crew climbing the aircraft to safety.

TCAS Soundboard!

The FAA and ICAO determine what audio callouts a TCAS system must make. Individual hardware manufacturers decide what type of “voice” and gender their system will have. Some TCAS systems are “male,” others are “female.” Here are a few of the required callouts in both genders. Click the “►” symbols to play the sounds.

Audio may not be available in all browsers. If you can’t see the audio controls below, try a different browser.

TCAS soundboard
Female Alerts Male Alerts
Traffic, Traffic Traffic, Traffic
Climb, Climb Climb, Climb
Climb, Crossing Climb Climb, Crossing Climb
Increase Climb Increase Climb
Monitor Vertical Speed Monitor Vertical Speed
Descend, Descend Descend, Descend
Increase Descent Increase Descent
Crossing Descend Crossing Descend
Clear of Conflict Clear of Conflict

TCAS Test on a B767 LDS System

Here’s a TCAS preflight test on a Boeing 767-300F with the Rockwell Collins Large Display System (LDS).

Santa’s State of the Art TCAS

Santa Claus Sleigh TCAS AeroSavvy
If an aircraft flies in busy airspace, it needs to have TCAS!

Further Reading


  1. What a comprehensive explanation! Love the way your posts have multimedia elements. 🙂 TCAS is awesome. Perhaps a follow up piece in TCAD, ADS-B,and other such collision avoidance tools would make a good compliment to this post?

    • Hi Pim,

      TCAS uses complex algorithms to calculate time. The TA, RA, and collision areas are three dimensional, but their exact sizes are based on time, not distance or altitude. You could have an aircraft 2500 above you and descending like gangbusters that will trigger a Resolution Advisory if it’s within the 15-35 second time frame.

      Hope that explains it.

      Thanks for reading!!

  2. Ken you’ve done it again. Great multi media effort to explain a critical part of the aviation system to those of us in the back of the plane.

  3. Thank you for providing these informations regarding the safety of aircraft. I hope you may do the same with some another topic which will be very useful for the the people working in aviation sector.

  4. Hello Ken,

    When explaining Transponder your flesh is pointing at a black dot. Is it the On/Off button for the Transponder?

    I don’t know if you have heard or watched the air crash investigation for these two planes : Bashkirian Airlines Flight 2937 (Tupolev Tu-154M) and DHL Flight 611 (Boeing 757). If not it’s an extra ordinary, very rare and confusing situation which led these two planes crashing into each other while both using TCAS !

    Excellent article as always. Thank you again

    • Hi Ehsan,

      The black dot near the arrow on the transponder is a test button.

      The DHL accident was very sad. Caused by one crew following the TCAS commands and the other crew ignoring them. Airline crews are all taught about this accident so that it is not repeated.

      Thanks for reading!

      • I was just going to mention this crash as well. The documentary said following TCAS over human controllers if they contradict each other is taught in the Western world but not so much in the East and the they were hoping for that rule to be implemented globally. It’s been a few years since the Überlingen incident now, has this been implemented internationally since?

        I wasn’t aware that TCAS systems negotiate with each other, that’s so fascinating. Are controllers in busy airspaces aware of what the TCAS instructions are? If two planes are on a collision course and the human controller decides that aircraft A should climb and B should descend to avoid the issue, will they know if TCAS recommends for it to be done the other way around? Or is the human controller just meant to prevent TCAS springing into action in the first place?

        • All pilots, worldwide, that fly TCAS equipped aircraft are trained to ignore ATC during a TCAS event. ATC has no idea what’s going on with TCAS. When TCAS issues a Resolution Advisory, we follow the guidance and tell ATC: “TCAS RA.” That lets ATC know that the humans (pilots and/or ATC) have screwed up and TCAS is attempting to resolve the problem.

          Why not follow ATC? Air traffic control radar is a great tool. But when two aircraft get really close together, radar is not accurate enough to determine a collision avoidance strategy. TCAS takes into account multiple aircraft, closure rates, and climb/descent rates.

          After the event, we tell ATC that the event is over. At that point ATC will issue updated clearances to the aircraft involved.

          Thanks for reading!

  5. Thank you for posting this! It is very informative and very well explained. I would like to ask if it possible for an aircraft move itself to avoid collision when another plane is nearby and the pilots are not doing anything to avoid MAC?

  6. Hello Ken,

    Really informational article!! Got to know so much about what happens in the cockpit when we are sitting sipping our coffee 🙂 . You guys are awesome!!

  7. This is extremely educational! I am writing about the TCAS system for a college paper and this really broke things down in such a great way. The visuals for this are also very helpful.

  8. Obviously this is a close-to-ground issue only, but has this changed at all now that more and more people have small personal drones? I.e. what prevents someone from standing near the airport and flying a drone / a plane colliding with it?

    • Hi Annie,

      Many of the nicer drones have software that prevents them from being operated in areas where they don’t belong (like airports). Violating the rules can result in heavy fines and penalties for the operator. It’s definitely a serious issue!

  9. Thanks very much Ken, for your great and informative work, including a fair amount of humor.

    I was actually taking a closer look at some flightradar24 info when I noticed your website again.
    Today is perfect for spending time learning more, as we just had a cold front crossing Switzerland, yesterday, 24°C and now, 8.5 !

    I will certainly forward your link to some friends which are also interested in aviation 🙂

    Btw do you know these two guy’s comic books (Netherlands) where one of them, it says, “Say something” on the cover, with a disoriented pilot near an airport and ATC going nuts.. very funny.
    Or, tower asks a pilot: “HB-xyz What’s your heading ?” and the pilot, going in a turn, reports, 182, 200, 280…

    Have a great day


  10. Thanks alot
    I’m an aerospace engineer and you couldn’t imagine how worthy is such information for me.

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