View Full Version : Emergency descent
SFI145
11th Dec 2002, 08:59
What is the current philosophy regarding a turn off the airway during an emergency descent?
Also has TCAS been evaluated during the emergency descent procedure?
18-Wheeler
11th Dec 2002, 10:37
Up to the person/company.
I usually turn 45° left/right when starting the drop, and go about 10 miles off track.
Slasher
11th Dec 2002, 18:19
Im the same as 18-wheeler, except Im more a 30 deg man and Im after 25nm off track.
Only exception is when in RVSM airspace which dictates 90 deg L or R if NOT in contact with an ATC unit. Only thing about RVSM emergency descent procedure is this minimise initial descent rate until 10nm off track which I disagree with. Given an explosive decompression at FL370 Ill get down as fast as I can, giving only prime consideration to the aircraft structural integrity and what the TCAS is showing (if fitted).
I havent read anywhere TCAS has been tested in an emerg descent. It may well have though. But remember TCAS operating laws take into account only 1 dimension of the 3-dimensional sky we fly in. Dont forget that the golden rule of "to avoid a colision each aircraft shall turn to the right" is just as valid now as it was 87 years ago. Might even save your life one day because TCAS only knows up and down. We pilots know theres a left and right too!
The question of turning off the airway keeps coming up ,in view of certain 'congested'corridors etc. The 'turn 'itself was advocated by boeing to allow the nose to 'fall' through and get on'speed' quicker..Tcas is now 'selected' down(if you can)to 'see' the airspace.:p
Young Paul
14th Dec 2002, 13:03
Yes, though TCAS itself "sees" the airspace anyway - it is to allow you to see it.
TCAS will not issue RA's if ROD exceeds 10000fpm, if your bird can manage that.
IMHO there is a strong case to be made for selecting "TA only" in an ED, in order to prevent a co-ordinated RA being issued between your own and another mode S txpdr. This ensures that all the avoidance will be taken by the other a/c, given that you will not be changing trajectory until below 14000'.
Comments welcome.
Slasher
15th Dec 2002, 00:01
That suggestion is certainley worth mulling over WOK, esp in IMC. The question I have is, if 2 TCAS RA aircraft are coordinating, and one aircraft is ignoring RA commands, does the other (RA-obeying) aircraft's TCAS realise that the other one ISNT obeying?
If the obeying TCAS doesnt realise it, then you have a very strong case.
*Lancer*
15th Dec 2002, 07:07
Slasher, TCAS II version 7 can command an RA reversal if the other aircraft doesn't follow it's own coordinated response - but only once. So if the aircraft DOES subsequently decide to follow it's RA (after you've been commanded to change direction), you will only get increase climb/descent commands rather than another actual reversal. That's how I read it anyway! :)
TCAS III is just around the corner and will allow lateral guidance as well as vertical...
Lancer
Slasher
16th Dec 2002, 03:55
Thanks Lancer. If TCAS 111 R.A.s provide avoidance in 2D Ill be far more confident in it than the present 1 dimensional avoidance envelope.
Capt Pit Bull
16th Dec 2002, 08:45
Further to Woks reply. Agree with the strong case for TA only mode.
The other thing to bear in mind is if you are doing an emergency descent, almost by definition you are fairly high, so there is a good chance that at least one of the aircraft is subject to climb inhibitions.
Got to go to work, so not a detailed response, but if you think about it, if the other guy can't climb, you are almost certain to get told to level off - ("Adjust Vertical Speed"), i.e. told to stop your emergency descent.
CPB
That's exactly the scenario I envisaged. Think I may raise this with the tech bods at my company, tho' I suspect the answer will be ".....no point in changing anything since ACAS 111 is imminent..." or suchlike.
*Lancer*
18th Dec 2002, 10:14
WOK and CPB,
Understand your points, but wonder if you would actually get a level off, or rather a reduction in the rate of descent... the computation would take into account your trajectory, so it wouldn't necessarily be anything as drastic as an 8000 fpm ROD change would it?
Having said that, will an intermediate reduction, or even level off for the 10 seconds an RA was commanded really affect the emergency descent significantly enough to consider switching RA off while the rest of the rapid decompression bedlam is going on?
Just trying to promote a bit of discussion here - it's an interesting topic! :D Let us know what the techies said!
Lancer