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« The primary purpose of ATC worldwide is to prevent collisions [...] Many aircraft also have collision avoidance systems, which provide additional safety by warning pilots when other aircraft get too close. » (source)

Why is ATC tasked to prevent collisions, when for this specific purpose aircraft already have "collision avoidance systems" on board, which even take precedence over ATC in case of conflict ?

This may look like an apparent contradiction to laymen.

I presume that there are several reasons for this, including:

  • Because not all aircraft are TCAS-equipped.
  • Because ATC ensures separation minima (although TCAS could be extended to enforce separation).

Which other reasons are there ?

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  • $\begingroup$ TCAS will give quick results while ATC results will be delayed. I don't see any logic in external system taking the preference when reliable aircraft internal system is available. $\endgroup$ – Mad Max Jan 2 at 11:16
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    $\begingroup$ How is this question substantially different from the other one you just asked? $\endgroup$ – Michael Hall Jan 2 at 18:16
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TCAS is sort of a fallback, last-resort system used in case all else fails. TCAS works within the scope of what happens within the next minute or so, and will require the involved aircraft to perform fairly extreme maneuvers to avoid a collision. TCAS has one single purpose: to ensure that two (or more) aircraft that have already gotten much too close will not collide. TCAS responds to a single, imminent danger that should not normally exist.

In contrast, ATC is a hugely complicated system that keeps planes moving safely and efficiently through the complex network that is our air infrastructure. Air traffic is carefully planned by ATC up to several hours in advance, and clearances and instructions issued by ATC ensure that planes are kept well clear of each other, so that close encounters do not happen at all.

As an analogy to road traffic, you can think of ATC as a set of traffic lights controlling an intersection, by taking input from a number of sensors and showing red/amber/green lights. TCAS on the other hand, is more comparable to the emergency auto-brake systems that many modern cars have, that will stop the car if, for example, a pedestrian suddenly crosses the road in front of the car.

So why does TCAS take precedence? Because in order for the TCAS to have been triggered at all, something else has already gone wrong - the main system has somehow failed, so the fallback system steps into effect. Looking at our road analogy, suppose you have driven into in intersection because you have a green light. If a car suddenly appears in front of you, you want the emergency brake to kick in, even if the light is indeed green and you should be allowed to go.

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While ATC's primary goal is to prevent collisions, it is also responsible for the efficient movement of aircraft through the airspace. That requires a strategic approach to managing air traffic to get aircraft from point A to point B safely and in a timely manner. The primary tool ATC uses is the flight clearance.

If air crews follow their clearances accurately and VFR traffic maitains adequate visual separation, there won't be any problems with separation being met.

TCAS can't issue a flight clearance. It doesn't know anything more than the traffic it can track via Mode C or Mode S interrogations. It's a tactical system that provides a backup when ATC or the crew makes a mistake and separation is lost. In this case, the TCAS is the automatic backup to resolve the loss of separation and avoid the collision.

The reason a TCAS RA must be followed, taking precedence over ATC, is that when there is a loss of separation immediate action must be taken.

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