TCAS in holding

[jacksmith]

Could someone please explain what happens when TCAS goes off in a hold? Presumably the pilots must follow the TCAS instructions, but does this affect the rest of the aircraft like a domino effect? Thanks for you help!

[smellysnelly2004]

You are correct

[Scuzi]

There was an incident in one of the Heathrow holding stacks a few years ago where TCAS had a domino effect in the hold. It certainly was interesting to watch! One of the aircraft at the top of the hold descended to the same level as another, the lower aircraft got a TCAS descent which resulted in the aircraft below it getting the same.

In such a scenario, the best a controller can hope to do is to pass traffic information and give vectors to separate the aircraft horizontally so that TCAS can step out.

[HEATHROW DIRECTOR]

I also saw it with a rapidly climbing outbound that levelled off under a stack. TCAS on the lower holding aircraft obviously didn't know the climb was going to stop and a certain amount of excitement ensued!!

[PPRuNe Radar]

In such a scenario, the best a controller can hope to do is to pass traffic information and give vectors to separate the aircraft horizontally so that TCAS can step out

In the UK, a controller should not be doing the second of those actions, and the first only in specified circumstances.

MATS Part 1 refers:

GEN Chapter 9 Para 5.2

When a pilot reports an ACAS RA, controllers shall not attempt to modify the aircraft flight path until the pilot reports 'Clear of Conflict'.

Para 5.3

Controllers should not routinely pass traffic information to aircraft conducting RA manouevres, or other aircraft affected by such manouevres. Nevertheless, there may be circumstances where the passing of traffic information is justified; consequently, controllers may provide traffic information under the following circumstances;

a) to aircraft conducting an RA manouevre if it is considered essential to flight safety (e.g. information on aircraft which are known to be in close proximity that are not transponding Mode C information).

b) to other aircraft affected by an RA manouevre if judged necessary by the controller (e.g in airspace where the carraige and operation of TCAS and/or SSR transponders is not mabdatory).

[anotherthing]

Very true PPRuNe Radar, but there is also the paragraph in MATS part 1 which says that nothing within the publication shall prevent an ATCO from doing what he or she deems necessary for flight safety (obviously paraphrased).

I'd defy any ATCO not to try to give vectors or suggest safe headings in the situation above - particularly when it happens in a hold for an LTMA airport, such as Scuzi mentioned in his post - the incident was shown on TRUCE a while back and it was horrendous.

[Scuzi]

MATS Pt. 1 also says:

Section 1, Chapter 1, Paragraph 1.2.

The Manual of Air Traffic Services contains instructions and guidance for controllers providing Air Traffic Services to cater for both routine and many emergency situations.
However, nothing in this manual prevents controllers from using their own discretion and initiative in response to unusual circumstances, which may not be covered by the procedures herein.


If I was in control and there was a TCAS domino effect, I very much doubt I'd sit and watch the show unfold. Although it does depend on the specific scenario.

[BEXIL160]

Earlier versions of TCAS were particularly prone to this sort of domino effect .... there were several incidents at Dallas-Fort Worth (DFW) in the early days of ACAS, leading to it being dubbed the Dallas (or Texas) "bump up"

rgds BEX

[phantomlurker]

Surely a solution is to clear a level in the stack above (maybe 2 levels or so) to allow the effect to come to a natural stop?
my 2p

[simfly]

don't forget that the chances of all aircraft in the hold being in a very similar spot when viewed from above may be slimmer, some may be at opposite ends not even pointing at each other etc so you may get a break in the chain there.... but no doubt there may be instances to the contrary!

[HEATHROW DIRECTOR]

<<Surely a solution is to clear a level in the stack above (maybe 2 levels or so) to allow the effect to come to a natural stop? >>

Doubt if there would be time to do it..... in any event, turning traffic out of a hold may introduce conflictions with other traffic.

[Jerricho]

Originally Posted by anotherthing

I'd defy any ATCO not to try to give vectors or suggest safe headings in the situation above - particularly when it happens in a hold for an LTMA airport, such as Scuzi mentioned in his post - the incident was shown on TRUCE a while back and it was horrendous.

Which is exactly what the controller did (if it's the incident I'm thinking of).......the STCA gave a few seconds notice and he stared trying to get some lateral (as luck would have it, 2 of them were just starting their turns back inbound to BNN) when TCAS took over.

[anotherthing]

Jerricho - your memory serves you well, I think we are talking of the same incident. A level bust at any of Heathrows stacks could be potentially serious if the controller does not try to use some vectors - especially as the SIDs of Heathrow and surrounding airports all interact and often route under the holding areas.

Obviously you would have to take any instance on its own merit, but at the onset, moving aircraft that are above/below the hold as a precaution would seem to be a sensible way of limiting multiple TCAS events.

[jacksmith]

Thanks everyone, much appreciated

[The Jolly Roger]

TCAS in the hold???....Your F***ED!!!!!!!

[OurSoul]

Some good points above.
Personally I would try to pre-empt the RA and turn an a/c that was maybe a level or two above..Obviously this would require some very quick thinking which,lets face it,is easier said than done.If you have cleared 2 to the same level you will not necessarily believe what you are hearing/seeing.
Also,what will a pilot be hearing from his TCAS with traffic below and above? Would he be in a position to also listen to ATC instruction on turns and listen to traffic information.I doubt it.

The reference in MATS 1 is there for a reason based on past lessons.It would be very hard to argue against it.