TCAS in an Emergency Descent

[TR4A]

With VHF radio's it's very rarely required.LJ-ABXQuote:
Originally Posted by Mr. Irrelevant
I always understood "traffic in sight" or "negative contact". Just a pet peeve.



I was starting to think that I was the only one who used the AIM phraseology.

It's in the AIM. Just pointing it out.

 

[Av8rPHX]

How ironic, was reading this thread while listening to PHX ATC, heard a guy in a small aircraft come on and say "uhh we're lookin', we got him on TCAS tho"...

 

[ISaidRightTurns]

Soooo - From what I gather from some of the actual replies,

TCAS in a normal mode only scans +/- 2k, so it wouldn't see the conflict until about 10 seconds before the a/c passes through your altitude.

Unless you have it in Up/Down mode, then it would give you about 30 seconds warning.

Is that about the jist of it?

 

[LarryinTN]

TCAS in a normal mode only scans +/- 2k, so it wouldn't see the conflict until about 10 seconds before the a/c passes through your altitude.

Unless you have it in Up/Down mode, then it would give you about 30 seconds warning.

NO! TCAS scans it's full range all of the time. The option that controls the display effects only what's displayed on the screen.

 

[Kingairrick]

See, there you go. You read through all the "mine's bigger than yours" BS, and you can learn something here...I just did. Thanks LJ!

 

[ISaidRightTurns]

So in a situation like I had, if the emergency aircraft was SB and a conflict a/c was NB, what would the resolution be? I would imagine that the emergency a/c's TCAS would be issuing an RA to reduce the rate of descent, or climb, which wouldn't really be an option. After a few updates and the descending a/c doesn't seem to be responding, will it come up with a new resolution that works in the emergency a/cs descent?


Sorry for the technical questions, just trying to get a hold of how this works.

 

[LJ45]

looking got him on TCAS

"looking for traffic got him on TCAS "I say this to save ATC the constant updates of the traffic while we look visually.

Is this not a good idea to save the constant updates and help ATC?

 

[ISaidRightTurns]

If I need it, I'll keep calling. Even pull out the ol 7110.65 "Do you have it in sight?". If one or the other says they are in the soup, I leave it alone and go to plan B. Which with me is normally plan M or N.

 

[LarryinTN]

So in a situation like I had, if the emergency aircraft was SB and a conflict a/c was NB, what would the resolution be?

That would depend on the unique geometry of the encounter. The TCAS software is biased against an altitude crossing RA but if the ROC/ROD is sufficient it will give a crossing RA.

I would imagine that the emergency a/c's TCAS would be issuing an RA to reduce the rate of descent, or climb, which wouldn't really be an option.

Why wouldn't it be an option? An extra few seconds at altitude is a lot better than a collision. If I was in a descent after losing pressurization I'd certainly follow any RA that I received. Both pilots are on oxygen.

 

[Snoopy58]

The two TCAS's will coordinate a plan of action with each other that keeps the planes apart... but that theory depends on BOTH AIRCRAFT ADHERING TO THE RA. Obvious problem: guys with an emergency descent may or may not do everything that the RA is telling them to do. What we're told as pilots is that no, the software does not have time to "realize" that one plane isn't doing what it's supposed to & then formulate an alternate plan. To avoid false alarms (like the guy zooming up to 10, while you're level at 11), the software waits until it's pretty sure that there really is a conflict, which means that you no longer have time to "see" if the other aircraft is doing its thing.

There was a crash over Europe (I think) a while back, where TCAS told the jets to do one thing (X goes up, Y goes down) but ATC had the opposite plan in mind & was giving it over the radio about the same time. Sure enough, one jet did what ATC said, the other did what TCAS said, both did the same thing, and they collided. Outcome, we're now told that TCAS takes precedence over the controllers, since you're guaranteed that both TCAS's are one the same sheet of music (they coordinate with each other over Mode S), but both aircraft may or may not be hearing the same controller, etc.

The solution to your scenario is straightforward enough, but probably not in the checklist: the emergency descent aircraft should switch his TCAS from TA/RA to TA only. This means that they don't get an RA and during the "coordination" the other jet's TCAS does what it needs to do in order to avoid the collision WITHOUT any action by the emergency aircraft. Of course, a rapid decompression isn't the ideal time for guys to be considering stuff beyond what's in their checklist, and I don't know if it's common to include "Transponder - TA only" as a step in the rapid descent checklist. It IS in our engine shutdown in flight checklist (and I understand that this is pretty common) -- if we're down to single engine performance, you don't want somebody's TCAS calculating a mutual solution that we don't have the power to give perform!

If you're screaming down with the cabin altitude horn blowing & your ears popping, wearing an O2 mask with the adrenaline flowing, and then you get a the "monitor vertical speed" RA, that's another oh %$#& moment in a day that's already going pretty crummy.