TCAS on Emer Descent

[MASsenger]

Which is a better selection on TCAS for emergency descent, TA or TRA?

[Arrowhead]

I am not aware of any guideance from Airbus, but some airlines SOPs do call for TA only, and some dont call for any change from TA/RA.

But common sense says you probably should switch to TA only (you dont want to have to make a climb in an emerg des). Switching to TA only will make the other aircraft manoeuvre out of your way, and not vice versa.

[ICT_SLB]

Don't forget that where there is an Autopilot Emergency Descent Mode, the first thing that is done is a 90 degree turn off flight plan heading and then the emergency descent with a level off at a preset altitude. This would probably make choice of TCAS mode moot.

[Gary Lager]

I think that would depend very much on where you are...presumably you are referring to MNPS airspace contingency procedures? The same behaviour in busy continental airspace may not always be the most sensible option.

I would suggest TA as well, and certainly A7700 as a priority - gets ATC on your side v. quickly and may preclude needing TCAS at all

[mad_jock]

This is just question from a newly qualified TCAS pilot.

I thought that the whole TCAS event takes less than 60seconds from intial traffic traffic to the event finishing.

If your high up and already got the rubber jungle out in the back and the crew are on O2 surely the addition of 60secs isn't going to effect the safety of the decent. In fact if your going down like a bat into hell. I would suspect that you will be given a decent rate TCAS which will still have you going down but just not as fast.

Seems strange that you would turn a quite important peice of safety equipment off, in fact your only one for avoiding collision. When you have another emergency when your attention will be diverted away from maintaining your SA.

As I said this is just my thoughts and please educate me if i have got my head round the TCAS thing the wrong way.

Summary is that my risk assement is that the additional risk of fatal outcome is higher with the TCAS mode downgraded than the additional risk of the emergency decent profile being not the optimal one.

MJ

[Gary Lager]

The TCAS 'TA' mode isn't the same thing as turning it off: while it means that the 'TA' crew would no longer get instructions on traffic avoidance, they still get advisory information. More importantly, all the other TCAS units in the sky know that that particular aircraft won't be manouevring and so can adjust their own response (RA) to compensate

[mad_jock]

By off I ment the RA was turned off.

[Piltdown Man]

If you have the time, might I suggest that you select "TA only" unless your company SOP's say otherwise. My reasoning is that if whilst doing this descent, you don't want to be told to start climbing again. Otherwise, what's the point of the descent? In TA only mode, you are still letting other Mode S people "see you" and their aircraft knows that you will not be taking action to avoid.

[FlexibleResponse]

I was taught that the worst crime in aviation was flying a perfectly serviceable aircraft into the ground.

I was also taught that the second worst crime was to fly a perfectly serviceable aircraft into another aircraft.

Two wonderful pieces of equipment were designed and subsequently incorporated into airliners in an effort to stop pilots from committing these crimes; firstly, the GPWS (and now EPGWS) and secondly, the TCAS.

Now I don’t think any one in their right mind would turn off the GPWS in an emergency descent as they would defeat their last line of protection from flying into the ground.

So I have to agree with mad_jock and ask why anyone in their right mind would turn off the TCAS RA function during an emergency descent thereby disabling their last line of protection against flying into another aircraft?

Strangely enough, if during an emergency descent, the GPWS tells me to climb to avoid flying into the ground, that is exactly what I am going to do. Likewise if during an emergency descent the TCAS tells me to climb to avoid flying into another aircraft, that is exactly what I am going to do.

Passenger Supplementary Oxygen Systems typically last 22 minutes for chemical type oxygen generators and substantially longer for gaseous bottled type systems.

Therefore, what would be the better choice between the following?

1. Flying a textbook emergency descent and having a midair collision during the manouevre and killing all onboard; or

2. Taking about one or two minutes longer to complete the emergency descent due to action taken to follow a TCAS Resolution Advisory on the way.

[mad_jock]

And not everything which you might conflict with is fitted with TCAS.

Your just coming in over the north west of scotland inbound to MAN. Just over Lewis you get a cabin pressure failure.

You start your emergency decent all goes well until you have a RA at FL 150 against a Tomcat which is mid exercise on the west coast of Scotland who has just done a pull up from 500ft (opps thats way to low for the yanks say 1500ft) to head off to the fuel bowser. IF your lucky scottish might just see the conflict before the RA but proberly not.

MJ

[Gary Lager]

You need to recognise what kind of airpsace you are in, naturally, and tailor your response accordingly.

You might be just as likely to descend into a airliner flown by crews who are not able to use TCAS correctly. What can you do then?

At least a functioning TCAS system on the other aircraft will recognise that you are TA only - it's reponse will be altered acordingly to (possibly) a more aggressive response, but still a safe one for both aircraft.

[Empty Cruise]

Let's try a different approach, then...

Let's say that you have left the switch in TA/RA. During your emergency descent, you get an RA.

Now, gents, please raise your hand if you would not follow said RA....

...anyone...???

Good, then - that should wrap it up :}

I know that TA only will modify the response of the other aircraft - but what would that response be, and would it be any different by you selecting TA only? Bear in mind, you are descending @ 4-5000 fpm on a steep profile (that alone reducing the likelihood of conflict, but lets leave that aside for now). If the other aircraft was climbing, cruising or descending on a normal profile - the easiest way to solve the puzzle would (geometrically speaking) be to issue the other aircraft with a climb command and you with a "Monitor vertical speed".

I'm fully aware that a lot of factors could cause the scenario to play out differently (i.e. you getting a climb command) - but it is a game of statistics in the first place. What if - as stated above - the other crew don't know how to deal with their end of the stick? What if you accidentially in the rush of an emerency descent select ALT on your transponder? What if the other a/c is for some reason or another also in TA ONLY mode? Or what if your SOP says nothing about selecting TA ONLY during an emergency descent?

As long as ICAO have not drafted any policies on the matter @ hand, my airline has not implemented the procedure i our SOP and nobody can find a QRH reference for such a drill - I'd be very tempted to get on with the emergency descend & leave the TCAS alone. You are not gonna die from a decompression - both the F/D & C/C are all on oxygen. Nor is a fire gonna kill you in the 1 minute it takes to resolve an RA (if you reacted quickly enough in the first place & initiated the descent & diversion ASAP i.s.o. waiting to find out that it - God forbid - is uncontrollable).

But hitting the other aircraft IS gonna kill you - no matter what your previous actions & subsequent plans!

Any thoughts? Best reagrds - and a merry Chistmas/Holiday to all
Empty

[Giles Wembley-Hogg]

I disagree with Empty Cruise. I think that during an emergency descent the chance of triggering an RA is MUCH higher. Your (necessarily) high rate of descent will bring many more targets below you into the "threat zone". TCAS does not know that you are going to level off at a particular level (typically FL100ish), so an awful lot of aeroplanes that would be ignored during your normal 2000fpm descent rate become important at your 5000fpm+ rate of descent.

We set TA only if an engine fails "to prevent climb commands which can exceed single engine performance". So there is a precedent for changing TCAS mode during abnormal operations.

The good thing about setting TA only is that conflicting aircraft will manoeuvre around YOU. You can just carry on unhindered with the emergency procedure you are already carrying out rather than have to start another one.

Who can say that a crew receiving an RA whilst they are carrying out an emergency descent will follow it? At the very least I would suggest that they would pause longer than normal whilst their poor maxed out brains compute the information they are being presented with loud airflow noise/uncancelled alarms/bleeding eardrums* delete as applicable. The last thing we want is crews manoeuvring against an RA, Uberlingen anybody?

Having said all the above, selecting TA only is not part of our emergency descent checklist.

G W-H

[Piltdown Man]

I'm missing something here. You are doing an "Emergency Descent" because you can't stay for a second longer than where you are. Presumably because something is very, very wrong. And now you set the TCAS to a position whereby you can possibly go back up again! I'm missing something very big here!

[petitfromage]

You have to deal wth the most imminent threat 1st......and thats the depress 1st.

I had this recently in the sim and although our company provides no specific guidance (its impossible to legislate for every possible senario) it was quite obvious that we were unable to comply with Climb RAs.
*If we reacted to the "climb" RA we would actually increase our chance of collision as our pull-out from the dive is limited to 2.5g......ie: 3-4,000ft

1. The extra high RoD meant that they were many more 'threats' and consequently RAs. It was disconcerting!!

2. We tended to get an RA "Climb" and then the very next second it changed to an RA "Descend".

I dont know (and cant find) of any software within the TCAS that takes account of aircraft with significantly high RoDs but we certainly noted that the TCAS very quickly changed its mind due to our vertical profile.

I hope someone with specific TCAS knowledge can join this thread.

On you next sim, ask to see this in action. Its well worth it!

[LEM]

but it is a game of statistics in the first place.
Exactly.

If we were sure everybody else has got a TCAS, it would be better to set TA only.

If we were sure nobody else has got a TCAS, it would be better to leave TA/RA.

The important thing to remember is:
always follow the RA!

This has been covered in other threads, and one can download the Eurocontrol bulletins on this matter.

And in the extremely unlikely case in which you get a CLIMB RA during the descent, follow it!

LEM

[Gary Lager]

Other way round , surely - if no-one else has got TCAS then there's no point setting TA/RA at all, since you don't get RAs from conflicts with non-equipped traffic.

[Empty Cruise]

Petitfromage,

V. good points, especially about the change from "Climb" to "Descend, descend now".

This link (http://www.arinc.com/downloads/tcas/tcas.pdf), specifically p. 35, rh coloumn, second-last paragrpah, indicates that TCAS manouvres are designed around a 0,33 delta-G manoeuvre. Therefore, nothing suggests that a 2,5/-1,0 (or shouldn't it rather be +2,0/-0,0, since flaps may be extended?) G manoeuvre would ever be commanded.

The above link also (in several places) points to TCAS being designed for vertical speeds of up to 10.000 fpm - and therefore that any manoeuvre - even during an emergency descent - will be subject to the same manouvre laws as your "everyday" TCAS.

The word "emergency descent" seems to jumble a few priorities (see above) - how long does your crew oxygen last? Cabin crew? OK, so nobody requred for the safe operation of the aircraft will be incapacitated by interrupting your emergency descent. Even the passengers will not be incapacitated - the emergency descent would be resumed long before their oxygen systems run out (worst case I've heard of is 12 min.)

Trying a bit of math here: -1 min. from the masks drop to start the descent, -3 min. to resolve an RA - that's 8 minutes left to descend from FL410 to FL130 - this requires an average rate of 3500 fpm. Or 3875 fpm to descend all the way to FL 100. Does this sound undoable?

None of the above has altered my argument - a collision will kill everybody - here and now. The lack of oxygen might kill some passengers in a future 10 minutes away. Now - balancing those two scenarios against eachother, I have no doubt in my mind what I'll do. I would do the same if I knew that I would - with absolute certainity - kill 10% of the passengers doing so. Even if I knew it would ultimately kill the flightdeck & cabin crew - but save some of the passengers for now (not talking about how they'd get the thing landed - in that instant, there is a long way to the tollbooth across that bridge) - I would do it.

Statistics - don't you just love them :(

Empty

[LEM]

Wrong, Gary Lager, you indeed get TA/RAs from traffic without TCAS, provided it has a transponder with altitude reporting.

Instead of getting coordinated RAs, you will be the only one maneuvering to avoid a collision.

Cheers, LEM

[5milesbaby]

1: Initiate emergency descent
2: Squalk 7700
3: Shout Mayday at ATC
I understand that there are lots of other things going on in the cockpit but these are the important things as far as I am concerned. I would say that 3 isn't crucial if 2 is done as I would guess that an emergency descent is in progress with the descent rate, but it would be nice to know whats happenning and that you aren't uncontrollably falling. If there is any traffic below expect avoiding action to be given to all the conflicts (BIG vectors), it would be very rare to ask the emergency a/c to make any turns but there are some senarios that may need this to happen. Hopefully there will be no TCAS RA's if we can get our job done quickly!! :ouch:

[Gary Lager]

LEM - you are right , of course, misunderstanding on my part.

Again, shouldn't it go back to the decision on the day being based not only on the type of airspace in which you are in (Mid-Atlantic vs. overhead London vs. open FIR) but also the likely occupants of that airspace?

In Class A airspace throughout most of the UK, I would not expect to find much non-TCAS-equipped traffic above FL100, so a decision to select TA may be more useful to others around.

Descending to FL100 directly overhead the CPT VOR with RA selected will probably mean in takes a LONG time to get down, after completing numerous TCAS RAs - that said, prudent decision in that case would be to turn and take the shortest route away from other traffic.

I would never suggest not following an RA - just placing the burden of manouvre on the other aircraft...with emergency descent required we have enough to worry about, for at least the first few thousand feet.

If we are on an unchanging descnet path, no matter what rate, then even if other traffic were on a direct collision course then they only limited manouevring would be required to remove them from that previous unsafe flightpath, hence I'll select TA and let TCAS do the co-ordinating and the other chap do the RA.

[Giles Wembley-Hogg]

I'm with Gary Lager on this one. For most of the airspace in which I operate, TA only offers the best solution.

I disagree with LEM that the chance of getting a CLIMB RA during the descent is "extremely unlikely". In fact I would contend that the chances of getting one (or more) are actually fairly high. If you don't believe me, take a trip to your local ATC Centre and have them switch off the filters so that you can see all the transponding traffic.

5milesbaby has a good idea suggesting that we select 7700. Unfortunately, our rule book (supposedly following consultation with ATC) states that we must keep our assigned squawk unless told to squawk 7700. As a result of this, from his list: item 1 will occur fairly briskly, item 2 will probably not occur and item 3 will occur once the situation is under control (ie all the memory items have been completed and the flight profile is stable).

Just my thoughts.

G W-H

[Capt Pit Bull]

The obvious question to ask here is "how often might this sort of scenario occur".

When we instituted our TCAS training program this scenario was one that seemed in need of consideration.

My thoughts were that TA only was the way to go. After all, if you do that you are (at worst) no worse off than in the pre TCAS era, versus a scenario compelling you to manouevre opposite. General strategy here is that when a scenario occurs that prevents full manouverability, TCAS should go to TA only. The aircraft we flew at that time had "TA only" in several abnormal checklists, e.g. flying control malfunctions.

TA only mode is effectively a 'priority' button (as long as, as others have pointed out, your general operating environment is one of TCAS use by other traffic). I could talk more about this, but it would be mostly repeating others observations so I'll get back to the case in point.

I really felt that we should be pre empting this sort of scenario. In '99, as part of an attempt to quantify the problem I got in touch with the CAA and asked them to do a MOR database search and send me a list, for the previous year, of MORs including the terms "emergency descent" and "RA" or "TA".

We were hoping we might get hit or two and see how things had panned out in real life. It was therefore somewhat sobering to receive back a list of several dozen!

(cautionary note - this was u6.04 days, before u7 's improved horizontal miss distance filtering)

So folks, the bottom line is these scenarios are happening! For real!

- TCAS has been mandated in the UK for what, 6 years now?
- These scenarios are entirely likely and predictable.
- Lack of a clear policy is, frankly, negligent.

If your company doesn't have a policy on this, it should do. Urge your training departments / standards folks / flight safety team to make it happen!


BTW I am available to consult for a very reasonable fee ;)




The former pilot known as Pitbull

[LEM]

Hi Giles Wembley-Hogg,
I insist it is extremely unlikely to get a CLIMB RA while descending like a rocket.

Don't get me wrong, I'm not saying it is extremely unlikely to get an RA while doing so, on the contrary, I believe the chances for it increase.

But you definitely won't get a CLIMB RA.

One of the basic principles of TCAS is that it is designed to suggest the smallest correction necessary to the actual flight path.

Can you believe an airplane descending like a rocket getting a CLIMB RA?
Nope!

The rate is so high, that you will get a MONITOR VERTICAL SPEED RA, to reduce it, or maybe even maintain it, as the TCAS will take advantage of this high rate and make the other traffic climb.

But certainly you won't get a CLIMB RA.


A bit of speculation, I know, but if you know the system and some previous incidents, you will come to this conclusion.

Having said that, I'd like to add that in the beginning it was encouraged to selecy TA ONLY in single engine and similar circumstances.

Not anymore.
Now the trend is to ALWAYS leave TA RA selected, as TCAS is perfectly capable of calculating the right maneuvres to be accomplished, even if one of the two airplanes (the one in the emergency descent) does not follow it.
The other one will get a revised RA, but as long as you don't make the worst possible mistake of maneuvering CONTRARY to the RA, everything will be fine.

Remember, if in doubt about the RA, at worst don't do anything..
Still OK if the other one has got a TCAS.

[Giles Wembley-Hogg]

LEM

Alright, you've won me over to thinking that a CLIMB RA may not be as likely as I first thought. BUT "ADJUST VERTICAL SPEED" is a distinct possibility. It was also the RA that I received last time I tried a very high rate of descent into a piece of airspace that turned out to be busier than it looked! It resulted in me levelling out. Not something I would want to do if I was in the middle of an emergency descent. Would you want to?

The trouble with an RA like this is that you do not have the option to "don't do anything". If you carry on with your emergency descent you are automatically manoeuvring opposite to the RA (ie remaining in the red sector).

It is for this reason that I think your comment that if you are not sure about the RA, then do nothing is possitively dangerous in this situation. Furthermore, all the problems that lead you to make this suggestion could be resolved by selecting TA only.

Surely we want to reduce the workload on the aircraft suffering the emergency?

Incidentally. We still select TA only for single engine operations.

Just my thoughts

G W-H

[Mr Angry from Purley]

I thought TCAS meant Training Captain Avoidance System :\

[LEM]

Hi Giles Wembley-Hogg, as I said in the beginning it would be better to select TA only if we were sure everybody else has got a functioning TCAS.

Unfortunately, for Murphy's law, the day you are going to have a close encounter during an emergency descent, the other guy will have a transponder with NO TCAS :}

Thus my choice is for always leaving this device fully ON.

If a momentary leveloff will prevent a collision during an emergency descent, so be it.
That's the price to pay, and not a big one I think.

But at least you'll be sure not to hit anybody.

Regarding the "not doing anything", I repeat AT WORST don't do anything.
Don't climb if you have a Descent RA, and don't descent if you have a Climb RA.

Keeping the same path is not considered a maneuver OPPOSITE to the RA, I think.
It's bad, but not catastrophic.

This possibility is accounted for by the TCAS, which will instruct the other guy to increase his maneuver.
At Uberlingen, the system failed because the Russian maneuvered opposite to the RA.
Had he maintained level flight (yes, in the red arc), there would have been no collision.


I would definitely follow the RA even in an emergency descent.

Cheers, LEM

[MrBernoulli]

At a quick glance, I haven't seen it mentioned here that TCAS will not display traffic descending at more than 10000 ft/min. I know that is an extreme descent rate but my previous old T-tail 4-jet could be persuaded to go there.

Whether that means that non-displayed traffic can still be reacted to by the TCAS kit, with TA or RA, I'm not sure.

[Gary Lager]

LEM - I disagree that we always have to consider 'Murphy's Law' when planning for these sorts of scenarios. As with everything else in Aviation we have to always also consider probabilities, otherwise everything ends up being so over-engineered and doubly- triply-redundant that we can't fly in the first case.

There are some aircraft around without TCAS, though nowadays very few in UK/Central European airspace above FL100 (I have no experience of Oceanic flying so my comments cannot be extended to that arena). Additionally, I would hope that one is able to talk to ATC within the first 30 seconds of beginning an Emergency Descent (when one will have likely descended only 3-4000 from cruising level) who should then be able to provide assistance as well.

Or are we expected to always consider a depressurisation, comms failure AND non-TCAS traffic above FL100/MSA? This is where it seems our respective opinions diverge.

As I have said before, the 'answer' really depends on the personal judgement of the crew on the day, taking into account airspace, traffic levels, ATC service, terrain, and aircraft equipment, amongst other things. I am not brave enough to say ALWAYS do this, or NEVER do that in such circumstances; in certain situations I would probably take your advice (and may not have considered that scenario until you mentioned it here, so thank you), but my advice will remain as it has been, for those of us in busy controller airspace: select TA and let those other black boxes out there know you won't be doing anything except descend!

For info, an ADJUST VERTICAL SPEED can be more than just a level off - this summer over East Germany I was descended to FL270 from FL350 and turned into traffic climbing into me - my 2000fpm descent was eventually transformed by an ADJUST VERTICAL SPEED into a 1800fpm climb (though the scenario was resolved very shortly afterwards).

MrB - you ask an interesting question, I'd like to know the answer to that as well!

[issi noho]

Dont know if this helps anybody but a while ago I had a airprox with 2 harriers pulling up from low level'ish. I was in crz about 160 they were climbing 7000 fpm plus ( about the same as I would be in ED) TCAS was norm TA/RA but couldn't or didnt resolve the conflict. Just got prox traffic with no brg info, and that only happened after the first one had passed thru my level.

[vikena]

Having the wherewithall to squawk 7700 in a Emer Desc probably will not happen.

Interesting thread , but only interesting . This is a double RED failure if you like .

The Mayday call should clear airspace beneath.

And I'm not going to worry about this combination .

vikena

[FlexibleResponse]

It is comforting to know that professional pilots operate their aircraft in accordance with their SOPs.

It is also comforting to reflect that non-professional pilots who think they personally have better ideas on operations than the aircraft manufacturers, avionics equipment manufacturers, civil aviation regulatory authorities, and airline company SOPs, don’t actually fly airliners.

[BANANASBANANAS]

I raised this issue quite a few years ago whilst on my 767 command course. No real joy from the trainers so I went to have a chat with the engineers and after several faxes and emails sent bacwards and forwards around the world the "technical" answer was to select TA as other TARA equipped aircraft will issue their own (and if multiple - coordinated) RA(s) based on the assumption that you will maintain your existing vertical profile.

I still haven't seen any formal guidance from any manufacturer though.

Personally, by the time I think of it (in the sim) I am usually at least half way towards 10,000 feet/MSA and I think it would be an extremely sharp mind that would remember to select TA in "anger."

[MrBernoulli]

As unlikely as it might seem, I just throw this in as food for thought:

Anyone selecting TA only is always told that other aircraft in the vicinity on TA/RA will do the avoiding. Sure. But what if other aircraft in the vicinity are also on TA only (single-engine; gear-down ferry)? Don't assume that everyone else will, or is 'programmed' to, get out of your way. Again, as unlikely as it is, I throw it in as something to keep at the back of your mind.