OK. The reason I said use the search function is because I and many others have spent probably days answering TCAS related questions on this board.
And tbh, I'd leave it at that if the subject wasn't so incredibly safety critical. Though I am forced to wonder about the mandated theoretical training that aircrew are suposed to have received. I get the impression that it still isn't happening for a significant proportion of the profession.
So, here goes:
Lets start with response to a TA.
1. Don't make up your own
avoiding manouevre because you've received a TA. Some very serious incidents have occured where crews have got themselves (unduly) concerned about separation, manouevred as a result, and ended up causing a reduction in separation rather than an increase. In a TA, you have no legal basis for breaking your ATC clearance.
2. HOWEVER... this is often interpreted as meaning that you should do nothing to change your flight path. This is incorrect. You can - and probably should - manouevre in response to a TA, but only insofar as it is permissable by your ATC clearance.
Rationale: Statistically, most RA's these days are 1,000 foot level offs. If you get an RA, there is a good chance thats its because you are approaching your cleared level with an inappropriate vertical speed, OR, you are being sensible, but the other aircraft has a high rate. Everyone OUGHT to be following the guidance of not exceeding 1500 fpm within 2,000' of the cleared level, but my contacts in the ATC world tell me (with a hint of dismay I might add) that there are still way too many aircraft that either don't know or choose not to follow this guidance.
Hence, in most RA's, there is at least one aircraft that hears "TRAFFIC TRAFFIC" when they have an excessive vertical speed as they approach their cleared level. If they have any sense, the crew will use part of their 10-15 seconds to punch VS and spin the pitchwheel to a more sensible value, in addition to doing anything else.
At this point someone will probably accuse me of promoting reckless behaviour because their SOP doesn't mention this. My response: The fact that you are having a TA doesn't mean give yourself a lobotomy. If you find yourself with a big vertical speed and a TA, and you REALLY want to frighten yourself, just keep that high VS going. You're on course for an altitude crossing RA - and if the other aircraft is non TCAS, and god forbid on a different freq, or has TCAS but doesn't follow it (yes, its still hapening) you could be looking at a last minute reversal ....at best...
Whereas a prompt reduction in VS will defuse this into something less stressful, like a corrective RA that is not altitude crossing, or better still a preventative RA, or best of all no RA at all, thereby saving yourself a bunch of paperwork.
Of course you are still legally required to maintain a minimum 500 fpm towards you cleared level, so don't level off in response to a TA, but reducing your rate is a very very wise thing to do. Better still, reduce it before you even get a TA in the first place...
Onwards then, to consider turning in response to an TCAS RA.
1. You should never initiate a turn based on information from TCAS. This is sometimes taken to mean that you should not turn during a TCAS encounter. This is most emphatically not the case; you are perfectly entitled to turn as long as you are not doing it solely because of TCAS.
2. It is often sugested that the aircraft you are looking at may not be the one that TCAS is worried about. This is very important to appreciate, but people often miss the point.
You see, in the earlier days of TCAS the guidance used to be that if you had visually identified the intruder and did not feel the RA was needed, you didn't have to follow it. At our airline, our in house guidance was follow the RA anyway, because .... the aircraft you are looking at may not be the one TCAS is worried about.
But can you see how this is not the same as looking at an aircraft and deciding a turn will help?
The first case involves looking at an aircraft and deciding it
isn't a threat. As such, if its the wrong aircraft, not following the RA is going to be dangerous.
The second case involves looking at the aircraft and deciding its a threat. If you're looking at it, and its on a constant relative bearing, and its getting bigger, really,
who cares if its the one TCAS is generating an RA on or not? If its a threat, then a turn
may well help (more on that later). So we follow the RA (because thats definitely a good idea, whether there's one aircraft or two), and the decision about whether to turn or not goes into the melting pot of knowledge and judgement.
3. In addition to visual spotting, you could well be offered avoiding action by ATC. Current (in the UK at least, can't speak for the rest of the world) training for ATCOs is: Don't attempt to change the flight path of either aircraft once you become aware a TCAS RA has occured. Up untill the magic words "TCAS RA" are heard though, the ATCO remains responsible for attempting to provide separation, however they are strongly advised NOT to offer last minute vertical avoiding action, (statistically we pilots don't tend to get "TCAS RA" mentioned until well into the RA). As such giving you a turn ought to be more likely.
In this case, it is an ATC instruction and as such you are obliged to comply, however of course once an RA starts (and it may have started before you receive the avoiding action from ATC) then YOU, not the ATCO, are solely responsible for collision avoidance. The turn you've been offered now becomes advisory. I would suggest though if you've got any sense, you'll follow it.
4. But what about aircraft performance? It is often mooted that TCAS performance inhibitions are invalidated by turning, and that therefore to turn during an RA is to ensure that your aircraft will plummet like a dead sparrow etc etc...
This viewpoint is not without truth. Yes, your aircraft
might have marginal performance to complete the RA and turning
might compromise that. However, for most aircraft in all RA's, and all aircraft in most RA's, the RA will be easily achievable and a turn will be possible without difficulty.
Notwithstanding that, if your performance is marginal, you
must sacrifice the turn in order to meet the RA. The RA is your main defence, the turn is incase the other bloke decides to be an idiot (or is non TCAS).
As an aside, even if you are NOT turning, your performance may be marginal, and you may not be able to fully comply with your RA even if the wings are level. I draw the line at rehashing that arguement (its only a few weeks old, use the search facility). If you can't comply fully with the RA, get as close as you can, whatever you do
don't manouevre in the opposite sense.
5. To turn or not to turn....
OK, so we've established that you could be in an RA, with possibly an advisory turn from ATC, and / or your own visual perception that a turn might help.
Do you, or do you not, make the turn?
No simple answer for this. I'm not going to attempt to list them all, but there are clearly risk factors with either course of action, I know this'll wind up those who feel that every situation must have a 'correct' response and that everything else must be wrong. I'm not going into details here, ultimately its a judgement call.
One thing I will say is this though. Don't even
think of looking at the traffic display to decide if you should turn during an RA. Really, its a bad idea at ANY time, and you shouldn't be doing it, but during an RA most TCAS installations surpress 'other traffic' blips so its a doubly bad idea.
6. If you are on a collision course, is a turn likely to help?
It depends. Whether a turn will actually help or not is a function of geometry. Obviously if you turn a long time before the collision point, you will change your flight path and miss the other aircraft.
Whereas, if you don't turn, and TCAS has not worked for whatever reason (other crew kept manouevring opposite, or didn't have TCAS etc), then there remains the possibility that even though you
think you are going to collide, that your vertical judgement is wrong. You might be lucky, and miss one another by a few feet vertically.
In this case, if you have just started to turn, you are going to make matters worse. There is a period of time, as you roll the bank on, where you are making your aircraft bigger vertically, so you are more likely to overlap the intruder and hit him.
I recently read a study that suggested that as the time to impact decreases to less than 10 seconds you very rapidly reach the point where you do more harm that good by rolling into a turn.
So, for a turn to be effective, you have to grasp the bull by the horns and get on with it early during the encounter.
7. What about pitching to avoid an intruder visually?
For completeness, you should be aware that humans are almost useless at judging relative altitude. Read this for some background:
http://www.pprune.org/tech-log/58559...-altitude.html
This is one reason why its a bad idea to disregard an RA in favour of visual spotting. If you don't have an RA (10 day TCAS MEL perhaps) then your best bet is a turn, not a pitch.
In summary:
TA - Comply with your ATC clearance as normal. If you have a big vertical speed, strongly consider reducing it but still comply with your clearance. Turn as normal for navigation or if given a heading. If its an 'avoiding action' turn make it immediately.
RA - Whatever else you do
Don't manouevre opposite the RA. Your RA will almost certainly be achievable and if so, you are required to follow it. If your RA is not achievable, get as close to it as you can whilst protecting the stall envelope. This applies even if it means you have to break your ATC clearance.
Turn if you think it will help, based either on ATC avoiding action or visual spotting. Do not use your interpretation of the traffic display to inform your decision to turn.
pb