TCAS RA in a turn ?

[lion-g]

Hi Guys,

To be very frank, been searching for an answer for the past week on the internet, be it PPRUNE, AIRLINERS.NET and smart cockpit but to no avail.

Anyone has any writeup on TCAS RA actions in a turn?

There are 2 camps after what I have read.

1) THE ACAS II system is based on wings level calcuation !!

2) All TCAS Manoeuvers must continue on the lateral flight path BUT and the TCAS vertical path to be followed.

I am kind of confused here ... initially, I would continue the turn IF i encountered RA in a turn but after reading about the ACAS II, I kind of quite buy the idea of wings level.

Next, most if not all procedures call for FDs OFF. How would you ensure that the aircraft contiunes it "lateral flight path" ? If it's a TCAS CLIMB, would it be better if we can climb on wings level ?

I have spoken to a couple of guys and all have been giving me different answers ....

I am really confused here .....

Cheers,
lion-g

[rudderrudderrat]

Hi lion-g,

I've seen nothing written about this either.

In the old days before TCAS, we would visually avoid one another by turning to Starboard. The same collision avoidance rules still hold.

TCAS RAs only avoid vertically and must be followed. So if I was turning left, then I'd level the wings. If I was turning right then I'd probably continue the turn and only level the wings if I was struggling for performance in order to satisfy the RA order.

[de facto]

Hello Lion G,

Pprune is a great source of information however always take what you read very carefully and check official data to confirm what you read.

This is a good example.

Tcas does not include a turning maneuver neither should you initiate or continue a turn but follow at all times Tcas guidance.(even if you receive a climb Ra at your optimum level).
Most registered tcas ra are around 3000 feet.

The sites I most often use are The UK Civil Aviation Authority (http://www.caa.co.uk/homepage.aspx) and EUROCONTROL (http://www.eurocontrol.int)

So for this one i will get your the answer on a silver platter::E

5.5.7 Horizontal manoeuvre made using the traffic display
5.5.7.1 Aircraft 1 and Aircraft 2 were both level at FL 290 under radar vectoring for separation. The
estimated horizontal distance at the crossing was 12 NM. However, Aircraft 1’s pilot saw Aircraft 2 on his
TCAS display. He thought that Aircraft 2 was “coming right at him”. He decided to turn even though he did
not have any ACAS advisory nor visual contact. Because of the turn, the horizontal distance was quickly
reduced to 2 NM.
5.5.7.2 This event suggests that the TCAS display was used inappropriately. TCAS bearing
information is only accurate enough to help identify intruder aircraft, but not good enough to suggest
horizontal manoeuvres. Furthermore, relative displays of this type are easy to misinterpret — as this
example has shown.

Collision avoidance:
i) TAs can be issued against any transponder-equipped aircraft even if the
aircraft does not have altitude-reporting capability;
ii) RAs can be issued only in the vertical plane and only against aircraft that
are reporting altitude;
iii) RAs issued against an ACAS-equipped intruder are coordinated to ensure
complementary RAs are issued; and
iv) Failure to respond to an RA deprives own aircraft of the collision protection
provided by its ACAS.

You can read more about tcas on this euro control link:http://www.eurocontrol.int/msa/gallery/content/public/documents/Doc9863_ACAS_Pilot_Training_chp5_1.pdf

I dont see any reason why you would switch off the FDs during a tcas ra..

In any case ALWAYS check for answers in official sites and NEVER take for granted what other pilots may tell you how senior they may be...

Safe flights!:ok:

[rudderrudderrat]

Hi de facto,

Some good references, but I can't find any advise about continuing (or not) the turn I happened to be making when the RA was triggered.

I dont see any reason why you would switch off the FDs during a tcas ra.. - it's SOP on Airbus to ensure autothrust in "Speed" mode.

[hawk37]

De Facto wrote

"Most registered tcas ra are around 3000 feet."

Can you please explain what this means?

thanks, Hawk.

[de facto]

Dear rudderat,

Of course if your airbus tells you to switch off the fds,by all means do it, it is not the case for my aircraft type, the 737.
Concerning the 3000feet, i found it in a report where it was mentioned that most RAs occur below fl 290 and 90 percent of them(from graph view) occurred around 3000feet.
They mentioned reduced rate of climb to 1500 feet to avoid unnecessary RAs trigger.
May have been in the UK CAA website.
Concerning continuing the turn or not I would level the wings,better performance to follow the RA /if your turn happen to get you closer to the conflict traffic the RA will have to continuously recalculate the rate of descent/climb..and for reasons mentionned in the euro control link i posted above.

[Microburst2002]

The lateral precision of the TCAS system is very poor, specially during turns. In addition, radius of turn of modern jets are great, specially at high altitude.

Even at low altitude (about 3,000) and low speed (about 210 kt), the radius of turn is aproximately one mile and it takes too many seconds to change the path.

In many cases, not following an RA would just mean that you are going to have an air miss. In others it would mean a collision. In order to have a collision we need the airplanes to be at the same 3D point at the same time. Changin one of the 3D is enough to avoid the collision.

TCAS makes us to avoid collisions exclusively by separating the airplanes vertically. You can change your vertical speed very promptly. The other airplane change it, too, in a coordinated way, and that's it. We may be in the same lat, long, to the millionth of a minute, but at a different altitude.

I would just focus on my VSI, and keep turning. It is a matter of seconds.

[Nightrider]

Horizontal manoeuvres are not incompatible with TCAS RAs and may increase the chance of successful collision avoidance. However, if the pilot is unable to fly the RA in turn (e.g. in case of heavy aircraft you may need to have wings level in order to reach the prescribed climb rate) the priority shall always be given to following the RA. If in doubt - follow the RA.

This was the answer I received from Eurocontrol after I asked a similar question as no one in the company was able to reply with any references.

[muduckace]

I do not see in any place where this situation is addressed. The only reason I can see even thinking about the turn is if you could not comply with the vertical command as a result of it.

Real simple TCAS does not care about your turn, just vertical seperation.

[lion-g]

Hi guys,

Thanks for all the reply. Well, 1 situation that I can think of is, if we encountered an RA in a turn is when you are departing on an SID which your track is the reciprocal of the R/W in use and you need to do a "180 turn" on the SID.

Below your turn is VFR traffic going inbound or outbound from a nearby local airport. In the midst of the turn, you encountered an RA with a VFR traffic.

SO do you wings level or continue the turn while following the vertical TCAS command ?

Thanks for all your various input.

Cheers,
lion-g

[de facto]

Dear lion-G,

What I am about to write you is how 'I' would react to such an event.
I would definitively query your Flight ops manager on how he would want you to deal with such a situation.
It is a very interesting subject as it never happened to me in flight so far and never practiced a turning RA in the sim,I will make sure to do so in my next OPC.

The worse situation would be as you mentioned during a SID at low level(ie flaps at take off setting) in a busy environment with step climbs,for example in London Stansted.

First of all as a pilot you should have a basic knowledge of your aircraft performance.
On the 737,with high weights your initial pitch will be around 15 degrees and you may be limited by the max pitch (yellow moustache:-) ) at around 20 degrees.
So there you dont have too much to play with,only a few degrees up extra.
Climb TCAS RA requires 737 to disconnect the A/T and advance thrust to max.You need all thrust available to increase your climbing rate.

So lets say you are in a turn and heavy,the PF will comply with the initial RA command,the PM will call ATC telling them about the RA maneuver and will then have to help the PF in the decision to continue the turn or not based on performance and traffic.I mention traffic as you may not have a traffic at your twelve o clock therefore levelling the wings a better option for traffic and perf.
PM should tell ATC about the RA and other actions such as stopping the turn and if unable to comply with the step climb(possible level bust).

In any case 1) fly the aircraft at the best of its performance(be aware of low QNH and wrong and late qnh settings)
2)Navigate
3)Communicate clearly your crew decisions(to ATC)
4)Help the flying pilot with all inputs he may need to perform the best possible maneuver.
5)Dont forget to look outside too as a PM for extra info on traffic.

CRM with your flight deck partner and ATC is of the outmost importance.

Remember the best way to avoid unnecessary RA at low level in busy airspace is to avoid climb rates higher than 1500ft/min.

[de facto]

Hi again Lion-g,

Ok here we go I finally got off my lazy butt and checked my BOEING manuals:

1)Flight Crew Training Manual:7.26(RA):"...when complying with an RA,flight director commands MAY be followed ONLY if they result in a VERTICAL speed that satisfies the RA command"

2)QRH:PF: "If maneuvering is required,disengage the autopilot and autothrottle.SMOOTHLY adjust pitch and thrust to satisfy the RA command.Follow the PLANNED LATERAL flight path unless visual contact with the conflicting traffic requires other action"

I am pretty sure you have the same guidance for your aircraft type,no?:E:8

[lion-g]

HI,

Thanks for all your reply. Really appreciate it. :)

In my Airbus FCOM, nothing mentioned about the lateral flight path. But what I found in the AP/FD TCAS RA,

"When the AP/FD ​TCAS mode engages, any engaged(armed) AP/FD lateral mode remains engaged(armed).
If both APs and both FDs are disengaged, when the AP/FD ​TCAS mode engages, ​HDG automatically engages. The heading target is the aircraft heading at ​HDG engagement."

Which is the new system available to our a/c in the very near future, where A/P will "fly" the TCAS RA and obey the LATERAL MODE.

In the above case, can we conclude that we should be doing the same thing as what was programmed by the AP/FD TCAS when we manually fly the TCAS RA?

On the other hand, since we put our FDs OFF during our RA, should we be "maintaining current HDG == wings level" as per what the AP is doing ?

A new can of worms ? :confused:

lion-g ;)

[Graybeard]

Whether you continue the turn, or level off, the TCAS will guide you for vertical separation. You need miss the target in only one dimension. If you are in a climbing turn as on departure, you are unlikely to get a climb or increase climb command, so not so likely to have to go wings level to increase climb rate.

GB

[Capt Pit Bull]

You should fly as close to your ATC clearance as an RA permits. So you should follow the planned / cleared horizontal profile if possible. However, if performance during a climbing turn is marginal then you must sacrifice the turn rather than the climb.

Generally, rolling the wings level during a SID turn is not a good idea, especially if your departure airfield is in close proximity to other airfields or terrain.

pb

[Checkboard]

The TCAS extrapolates your flight path, and checks for collision risks against the targets. It determines the target lateral position quite roughly using radio direction finding. If it determines a collision risk it gives you vertical separation instructions as the vertical information it has is much much more accurate than the lateral information.

SO - if you are in a turn - stopping the turn in order to follow the vertical RA (Resolution Advisory) may or may not bring you closer to the target - as the lateral position of the target is uncertain. Stopping the turn is thus not necessary for correct separation - and I wouldn't personally stop the turn as it then gives me a second (lateral navigation) problem to sort out after the RA has passed. The RA is such a small performance requirement that stopping a turn to achieve the performance should never be necessary - in those performance limited situations the RA is usually inhibited (Vref, low altitude landing config. or single engine where the QRH directs TA only be selected etc.)

[syedo]

simple.... if u are in turn to a certain track/heading, and suddenly u get, "CLIMB,CLIMB NOW", "DESCEND, DESCEND NOW", "MONITOR VERTICAL SPEED", and so on from Tcas RA, just initiate your recall items, in this case for airbus, "AP/FD.......OFF" and fly the green zone. if u reach your intended track/heading, just level off while maintaining the green zone till out of RA. dont worry bout what other aircraft is doing, coz his TCAS will definitely command him to do the opposite. ask for a practice in the sim, and u'll find it interesting....ehehehe... UNLESS, u have a multiple traffics RA commands... eg. a/c A is above and descending towards u, a/c B is below and climbing towards u at the same time.... it will be interesting to know what command will TCAS use....:E