There’s a new sort of TCAS in town, but the question is will you need it?

What is ACAS Xa?

In case you don’t remember, TCAS II 7.1(7.0 was about since the 2000s) really came into action around 2008, before being mandated in European airspace (and other airspace worldwide).

7.1 has the “level off, level off” RA call we now know (instead of the earlier “adjust vertical speed” that was occasionally confusing because that adjust could be 2000fpm, 1000fpm or 0fpm). It also has improved reversal logic and is just a whole bit better all around at preventing aircraft collisions.

Until now…

That’s right, ACAS Xa is a new technology funded and developed by the FAA, which will be heading into airplanes in the not too distant future. That’s ‘X’ not ‘ten’ by the way. There is also Xo which is especially designed for closely spaced parallel approaches, Xp which works off passive ADS-B to track (not interrogate) and some other letters in between for helicopter and UAS aircraft too.

But we are only really interested right now in Xa

What’s the difference between TCAS 7.1 and ACAS Xa?

In terms of what the pilot will see, nothing. In terms of the logic working behind it all? Some.

TCAS II interrogates the other aircraft’s transponder to work out where it is and where it will be in a projected time period. If it thinks there is a conflict threat, the two transponders coordinate their advisories, worked out by using pre-set rules and some maths on closest approach time and projected miss distances.

ACAS Xa ‘actively interrogates’ intruders but does not have hard coded rules or the same programmed sensitivity levels, instead it uses ‘a logic based on optimised probabilistic airspace models involving statistical representations and safety/operational considerations dynamically programmed’… yeah, no, I don’t really understand it either, so let’s just say ‘it’s more clever’

More clever (hopefully) means:

  • Less unnecessary advisories
  • Adaptable to future airspace separation standards
  • Collision avoidance with other aircraft
  • Ability to use range of modern surveillance sources for more accurate responses
  • Higher safety levels
  • Doesn’t need much change to pilot training or airplane ‘infrastructure’

What are EASA saying?

They have issued NPA 2023/04 which is (don’t worry!) just a notice of a proposed amendment. It is looking at how to introduce ACAS Xa ops into Single European Sky airspace, and considers the PBN specs required for oceanic ops. As with all EASA notices, the changes are highlighted in blue for “ease” of reading.

The real big considers are on what certification standards for installation might be required, and on how it will work in airspace that does not meet PBN standards – some oceanic airspace, some SIDs/STARs etc. Again, none of which marks a huge change for operators.

What the NPA does state (which night be of interest) is that:

“The safety benefit from the upgrade from TCAS II version 7.1 to ACAS Xa was not proven to justify a mandate. Therefore, at this time, it is proposed that operators should be able to either opt for equipage with ACAS Xa or continue using TCAS II version 7.1; the continued operation of aircraft equipped with either TCAS II version 7.1 or ACAS Xa is being considered acceptable.”

In other words, it will be voluntary whether you want to upgrade to ACAS Xa, not mandatory (as TCAS 7.1 has become).

The NPA is 94 pages long and there is a whole lot more to it than that, but for now, this seems the real ‘need to know’ bits.


Link to relevant resources here:


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