TCAS selection to TA only

[Professor]

To All Airbus drivers:

Do not you think that TCAS sw should be in TA&RA position all the times even in case of engine failure? I am aware of the degraded performance of aircaft in case of engine failure in which case the ECAM will request TCAS sw to be in TA only position. But I find it wise to leave it in TA/RA position despite the degrade a/c performance. At least you will have the RA which might save your life.

Pls comment.

[Haas_320]

I had the same question, the reasoning behind it is that your Tcas will transmit its inability to manouvre to the other system causing the deviation commanded by the RA to the other ACFT to be correspondingly larger, So as long as the other ACFT doesn't have a fan out this should be safer.

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Mens sanem non urinat in ventum

[HPSOV]

I agree with you that it should be left in TA/RA.
I fly a real aircraft (Boeing) and in our engine failure checklist is calls for the switch to be placed in TA only.
But I think its important to know about an RA, even if you cant comply you can try and turn away or something!

[Spearing Britney]

Hmmm. But if you get 'climb, climb now' with one donk and a full load you ain't gonna be doing it are you? Put it in TA only and you can still see where the traffic is so you can turn (bit dodge in London TMA) if you wish but there is the added benefit of the stronger RA in the other A/C (assuming TCAS fit) which still has full manoeverability.
Makes sense to me anyway...

[Haas_320]

I am very much in agreement with the TA selection in case of engine failure, the book says no manouvering on TA only, but we all know what happens when push comes to shove, If you have any decent conditions you might actualy be able to see and avoid based on a TA

[Reimers]

I think this discussion is a bit academic.
In most cases one will propably cry "mayday" and in the usual places all effort will be made by ATC to avoid all situations in which an RA might be generated.
Our TCAS training states that most deviations commanded will be quite little regarding the altitude gain/loss, and the V/S asked for will not exceed one engine out performance.
However, when certifying the new system, I think they had to show the system will not ask for something you can't do, which they could not, so they need to ask you to switch off that part.
Fumbling with the TCAS selector should in my opinion occur only when everything else is settled and ATC will have vectored everybody els off by then.

[Capt Pit Bull]

Its not academic at all.

If you know that you may not be able to fly an RA, its very important to turn your TCAS to TA Only.

The reason for this is TCAS coordination. You need your TCAS to be able to let the other aircraft know that it is free to do any manoeuvre.

Bear in mind that the other aircraft may be altitude or Performance inhibited from carrying out particular RA's.

E.G. TCAS may have decided that the best solution is for one of you to CLIMB. If the other Aircraft can't DESCEND, then one solution is for it to be issued a Preventative RA to stop it climbing and hitting you, whilst you get the CLIMB.

I.E. The fact that you are getting a corrective RA does not gauruntee that the other aircraft is getting one as well. If you don't climb when directed to, then you will be in trouble.

Whereas, if you had selected TA only, the other aircraft would have assumed you were going to continue on your present path and would have Climbed.

There are many aircraft that can't manage CLIMB or INCREASE CLIMB with an engine out.

By way of comparison, what do you do for Parallel Runways Operations? You turn TCAS to TA Only, right? As opposed to briefing 'If we have an RA we'll ignore it'. The reason is the same - you don't want to take part in a coordinated encounter unless you are going to follow the RA.

Basically, TA Only is like a priority button - it makes other aircraft get out of your way. And you still do get a TA, so you can try and get a visual spot and turn instead, so you are no worse off than pre TCAS.

Fundamentally, if its in the checklist (and it is in ours) then you should do it.

It really boils down to whether you are operating in an environment where the other aircraft will have TCAS. If you are, then turning yours to TA only because of an emergency doesn't compromise your safety at all.

In my opinion, there is even a powerful arguement for putting it to TA only during an emergency descent, strange as that may seem! But thats another story.

CPB

[HighSpeed]

Capt PB,

You're absolutely spot on...

HS

[max lenz]

It seems I have lost track of latest TCAS features; please tell me how and on which frequency conflicting aircraft are chatting with each other.
If they really can do so then chaos must be complete because there are still planes around with bare transponders mode C and older type TCAS without chat-function all requiring different treatment by the TCAS computer.
I agree this tratment list is a "nice to know" because the computer has to do the main work when time is a factor but when having this list in hand of YOUR TCAS system it makes you confident it really exists...

If you are using CDG departures to north please reduce your rate of climb when approaching FL 100 because we are flying steady at FL 110 on approach and cannot help in avoiding "nuisance alerts" - I allways do this when I am on departure and you are on the approach.

[ortotrotel]

Are we forgetting that climbing or descending is not the only way to avoid colliding with another aircraft? If you're descending because of a depressurization, you're not necessarily on one engine, and you always retain the option to maneuvre laterally.

Simply arresting the ROD briefly might be enough to escape the danger. You don't HAVE to CLIMB.

As for calling 'mayday'...didn't you squawk 7700? Your hands and minds are full of other business in an emergencty descent, assuming it's not your own window that has let go...

[Capt Pit Bull]

ortorotel,

Too right. TCAS cannot gauruntee to solve every encounter possibility, hence the purpose of the Traffic Display - to get your eye on the intruder, so you can turn based on a visual spot (or if an avoiding turn is offerered by ATC). A turn is definitely worth considering, as long as it is not based solely on the Traffic Display.

max lenz.

The normal SSR frequencies - 1030 and 1090 MHz.

I don't follow your comment about 'choas must be complete'. Do you mean the idea that all the aircraft have to keep track of one another?

Very brief explanation - TCAS uses the mode S address (which, crucially, is unique) of the intruder to maintain a 'Roll Call' list of those aircraft it is worried about.

Its datalinking transmission are (a) directional (b) at the minimum power level required to establish contact (c) done only as often as is required in relation to the level of threat, and (d) contain the mode S address of the aircraft being contacted, so are ignored by every other aircraft.

They alternate this with a general scan of the area using 'mode c all call' which is ignored by mode S aircraft.

If you would like more info, let me know.

I don't actually believe that a pilot needs to know all these details, so long as they are prepared to follow the manufacturers operating instructions. But if a pilot wants to know why the checklist is the way it is, (or even to second guess it!) then clearly more research is required.

Also I'd like to second your point about reducing climb rate when approaching cleared level. Particularly, if you get a TA as you approach your clearance. This means you are in (or very nearly in) the triggering parameters horizontally, so moderating your vertical speed is a good idea and saves a load of paperwork! It also completely above board as long as you are not breaking your clearance (i.e. maintain at least 500 fpm).

CPB

[Eff Oh]

I agree with Captain Pit Bull 100%
If you cannot follow the TCAS demands, what is the point in knowing about it? You can maneouvre visually if you get a TA in that case! Your TCAS TA/RA also lets the other aircraft know that your system is operational. Telling it porkies can only DEGRADE safety. Our QRH (B757)also states in an engine out case;
TRANSPONDER MODE SELECTOR............TA
[Prevents climb commands which can exceed single engine performance capability.]
In my opinion it is a stupid idea to set it to TA/RA with an engine out! Well... At least you will know what you SHOULD have done, as you smash into the side of an other aircraft in a busy TMA!! :) Hope to God it aint mine!!!! :)
But hey thats my opinion, and what do I know?? :) :) :)
Safe flying.
Eff Oh. :)

[Tor]

A question:

Haas_320 wrote: "is that your Tcas will transmit its inability to manouvre to the other system causing the deviation commanded by the RA to the other ACFT to be correspondingly larger".

Spearing Britney wrote: "there is the added benefit of the stronger RA in the other A/C"

Capt Pit Bull wrote "You need your TCAS to be able to let the other aircraft know that it is free to do any manoeuvre."

etc, etc.

As max lenz, I would also like to know how this works. How does the TCAS transmit system degradations?

[WOK]

By being selected to TA...........

[Tor]

..so all A/Cs with TCAS in TA "transmits" that they have an "engine failure"?

(Not trying to sound sarcastic, I know nothing about it!)

[Spearing Britney]

Tor,

As far as I know transponders 'talk' (callsign, altitude, track, speed,destination and even capability of A/c) on the 1030 and 1090MHz frequencies of the Secondary Surveillance radar - Mode S allows a significant rate of data transfer (5 time 10 to the 33 meassages) that TCAS systes can utilise. There are something like 16million A/C addresses available and

Ground stations are constantly interrogating transponders when over land and mode s systems have a squitter functoin to randomly reply over oceanic areas so other A/c can still 'see' each other.
(ps Tor do you like your new pick up or do I have the wrong guy?)

Reimers, I see your point about being academic and at the level its ended up maybe but as for selecting TA, not academic at all. OK it may be low onthe priority list but anyhting we can do to break the chain that leads to accidents helps...

[ft]

When a TCAS system issues an RA, it emits an inhibit command to the conflicting a/c over the mode-S link, causing the second TCAS to issue a preventive RA rather than another corrective one, which could potentially nullify the one issued by the first system.

If they both detected the conflict simultaneously and issued a corrective RA each, the TCAS system with the highest Mode-S address will have priority and the RA for the system with the lower address will be cancelled and changed to a preventive.

If you are in TA&RA mode while unable to climb, this can clearly lead to a situation where another TCAS system 'nhibits or cancels the corrective RA that could have saved the day and instead issues a preventive one that would have saved the day as well - if you had been able to climb as your TCAS told you.

Oh, on the technical side. Distance is determined by the reply times as per usual ATCRBS operation, altitude by the altitude encoding in the replies and bearing by using a directional antenna.

Does anyone have handy the details of what information mode-S is capable of transmitting?

With all this possible with plain ole´ mode-S, imagine what will be doable with ADS-B. Wow.

Cheers,
/ft

[Tor]

Thank you for the brilliant explanations!

(Spearing Britney - wrong guy, sorry).