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Does TCAS take priority over controllers?


Daniel Smyth
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Anything you have to do to ensure the safe operation of the aircraft overrides any ATC instruction, real-world or VATSIM.  In either case you should be prepared to explain why you could not comply with the controller's assignment or instruction.  Complying with a TCAS RA is a good reason.  And obviously, with much less at stake on VATSIM, the overall level of seriousness and scrutiny of such instances is lowered accordingly. 

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Cheers,

-R.

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  • 3 months later...

Yes. The TCAS-device with the higher serial number (=newer device) will take control and decide what it will do and let the other TCAS know about so it can do the opposite maneuver.

https://skybrary.aero/index.php/Airborne_Collision_Avoidance_System_(ACAS)

https://www.skybrary.aero/index.php/TCAS_II_version_7.1

https://www.skybrary.aero/index.php/Toolkit:TCAS

Edited by Andreas Fuchs
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That's subject to how it's implemented sim-side though.

AFAIK, VATSIM does not simulate this kind of TCAS interaction, so it might not actually work out exactly like it would IRL - the TCAS on either side does not have a way of telling the TCAS on the other side its serial number and what it intends to do.

The rules by which TCAS implementations make those decisions are fairly well-defined though, so even without this communication, the advisories on both sides should normally match.

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1 hour ago, Tobias Dammers said:

The rules by which TCAS implementations make those decisions are fairly well-defined though, so even without this communication, the advisories on both sides should normally match.

Ok. How in the world should one TCAS know instantly what the other is doing or is going to? Correct, it cannot. It can only react to the maneuver (changed vector) of the enemy-TCAS and this is not the primary way how it should work. This is some kind of degraded mode, when a TCAS has to react to an enemy that has only a regular transponder with altitude information, but no TCAS.

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2 hours ago, klaus legrand said:

Does the TCAS behave like in real life? I mean if on my side TCAS says "DESCEND" will the other pilot be told "CLIMB" ?

No, there is no communication between TCAS systems on VATSIM, or any other online flying network that I'm aware of.

Developer: vPilot, VRC, vSTARS, vERAM, VAT-Spy

Senior Controller, Boston Virtual ARTCC

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Then I misunderstood the question. Anyway, what kind of plane are you using? My addon aircraft all have TCAS-functions and they normally start yelling when planes get too close to me, the basic functions - including RAs - do work.

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Dear Daniel, following a TCAS RA is not a mistake, whether in VATISM or in real life, but it is a procedure and a safety practice. According to PANS-OPS Doc 8168, Volume I, Part III, Section 3, Chapter 3, 3.2 c) 4)). Read the document in this link

Yours,

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Ismail El Moussati
Northern Africa Division Founder l VATSIM Network Supervisor
Royal Air Maroc Virtual CEO
Virtual AirTraffic Simulation Network

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On 10/29/2020 at 7:52 PM, Andreas Fuchs said:

Then I misunderstood the question. Anyway, what kind of plane are you using? My addon aircraft all have TCAS-functions and they normally start yelling when planes get too close to me, the basic functions - including RAs - do work.

Me too Andreas ! I was just wondering if the other airplane would have the appropriate RA as in the real world , as for me yesterday i crossed a WIZZ air flight over Austria, my RA was telling me "descend" i just hope the other aircraft had "climb" advisory.

Something maybe to try and check online 🙂

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4 hours ago, klaus legrand said:

Me too Andreas ! I was just wondering if the other airplane would have the appropriate RA as in the real world , as for me yesterday i crossed a WIZZ air flight over Austria, my RA was telling me "descend" i just hope the other aircraft had "climb" advisory.

Something maybe to try and check online 🙂

As has been discussed earlier, the VATSIM protocol has no facilities that would allow transponders to talk to each other, and even if it did, it would be quite difficult to hook this up with the various TCAS systems in all sims and aircraft models. Hence, TCAS on VATSIM behaves like a real-world TCAS encountering traffic equipped with a Mode-C, non-TCAS transponder - it will figure out a best guess at a deconfliction path, but it won't know what the other side will do, and it won't be able to tell it either. The way TCAS rules are written, this should still lead to successful deconfliction in most cases, but it's certainly not guaranteed.

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Hi Klaus
usually the situation where a TCAS RA is triggered is rather clear as for example one aircraft is descending or climbing into the level of another aircraft. So from that the TCAS, if it is half-descent, can provide you with a good RA even not "talking" to the one on the other aircraft. And if he/she has also a half-descent TCAS there will also be a corresponding RA which makes sense. The part of communicating TCAS is mainly important if there is a same-level head-on for example and aircraft are at the exact same altitude - here it is by chance but even then usually a slight difference in altitude is present and I would expect the slightly higher one to get a "climb!" and the slighly lower one a "descent!" even without negotiation.
Never heard of tests being conducted with different aircraft online - so feel free to grab a volunteer to test it and provide us with the insight - would be interesting to know. 🙂
Cheers
Michael

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