A question about TCAS

[Topslide6]

Hey everyone.

First forray into this forum but i've been pondering this one over the last week or two as I suddenly realised I had a gap in my knowledge about it. I can't find a definate answer to it in my tech manuals either so any input would be appreciated.

Does TCAS take into account aircraft performance/limitations when issuing RA's?

For example, i'm sat at 20,000ft flying level and someone is climbing up underneath me. The resolution in this instance (I assume) would be for our aircraft to climb and the aircraft underneath to either reduce rate of climb or descend. Now take this scenario upto 37,000ft which happens to be my type's ceiling. Would TCAS still issue the climb command?

Of course I appreciate that at that altitude (unless it was military traffic or something similar) the rate of climb of the lower aircraft would likely be low enough to allow the TA time to wake up both crews. My question is more hypothetical I guess but I must admit i've no idea what the answer is!!

I've had a spurious RA in the past which commanded a very high rate of climb over a long period (roughly FL100 upto FL190 ish) and it didn't appear to be taking into account that the speed was slowly bleeding off with the engines at full climb power. Again, I don't know if this was down to the 'faulty' nature of the RA.

Cheers,

TS6

[Mad (Flt) Scientist]

Originally Posted by Topslide6

Does TCAS take into account aircraft performance/limitations when issuing RA's?

The certification of a TCAS II on a particular aircraft model requires an analysis to evaluate the need to inhibit TCAS II Climb and/or Increase Climb RA because of inadequate aircraft performance.

Afraid it's not a direct quote from an AC - can't find it right now - but that's the gist of what WE do. From which I conclude that modern TCASs, at least, are smart enough not to command excessive climbs.

[gearpins]

I assume we are discussing TCASII version7:
Because of a/c climb performance limitation at high alt or in some flap / gear configs an a/c installation may be configured to inhibit climb or increase climb RA under some conditions.These inhibit conditions can be provided via pin programs in the tcas connector or in real time via an input through FMS.Also the lead time to the Closest Point ofApproach (CPA) called tau( i think thats a greek alfbet) is more at higher alt its 15sec at 1000' ----35sec at fl420

[gearpins]

"Introduction to TCAS 7.0" is available here (free with registration):
http://www.arinc.com/tcas/

And Bulletins 1-8 here:
http://www.eurocontrol.int/msa/publi...html#Bulletins

just forwarding these links from an earlier thread posted by Zeffy

[Capt Pit Bull]

Does TCAS take into account aircraft performance/limitations when issuing RA's?

In theory, yes.

In practice, not totally.

i.e. inputs can be provided to tell TCAS about the config / altitude / conditions of flight. On the basis of these inputs, certain RAs can be inhibited. (and these inhibitions are data linked to the threat aircraft as well, for the purposes of coordinating the RAs generated.

However, these inputs may well be fairly gross, and not be comprehensive enough to prevent an unachievable RA. E.G. airframe icing, engine out. Sometimes they appear just plain optimistic. But bear in mind that RAs should not be long duration. In a coordinated encounter you should not even need to maintain the adjusted rate until CPA. Unless the other aircraft is doing something dumb, all TCAS is doing is shifting your flight path up or down a few hundred feet, we are not in the business of long duration climb demands.

It would be interesting to know the cause of your spurious RA. Ultimately, TCAS does not directly protect the flight envelope. Every TCAS manual I've ever seen has warned that the flight crew must protect the flight envelope, although admitably its been a few years since I was directly involved.

pb

[captain_jeeves]

Originally Posted by Topslide6

Hey everyone.

Does TCAS take into account aircraft performance/limitations when issuing RA's?

Cheers,

TS6

I can't find a catagorical answer for you. But, here's what Airbus says about it.


If a RA is generated:
. The flight crew must always follow the TCAS RA orders in the correct
direction, even:
-- If they contradict the ATC instructions
-- At the maximum ceiling altitude with CLIMB, CLIMB or INCREASE CLIMB,


The PF will avoid excessive maneuvers, and keep the Vertical Speed outside
the red area of the VSI and within the green area. If necessary, the PF must
use the full speed range between Valpha max and Vmax.

[Piltdown Man]

The limit of FL370 (or whatever it is for your type) is for the pressurisation system to maintain 8,000'. If TCAS says climb, you do so! The cabin altitude will then exceed its design altitude. So what! The majority of TCAS events will command a vertical deviation of no more than -/+ 3-500' commenced at 1.25g. So we are not really talking about a big bust either. Hit the plane coming the other way though...

[Topslide6]

Cheers for the thoughts on this.

Pilotdown man, I understand what you're saying but the design ceiling is not just for pressurisation. Believe me, in a heavy 737 at 37,000' there can sometimes be very little manouvre margin to play with. I do agree, however, that if TCAS said climb...I would climb!! No question.

Capt Pit Bull, the cause of that spurious RA was an aircraft that had come back from maintenence with what we believe was an incorrectly wired TCAS system. It had various spurious 'events' over the course of a week. Not particularly pleasant when it doesn't resolve after a 5000'+ climb

Thanks once again,

TS6.

[bloggs99]

OK here's one. If an aircraft is descending to your level and you are only 2000 feet clear of terrain and generates a descend RA you may get a EGPWS/GPWS pull up before being clear of the conflict.

I am particularly interested in the A320 here. I cannot see that the EGPWC will prevent the descend RA. Therefore you will get a Descend RA swiftly followed by a pull up - which of course must be followed!

Anyone elaborate on this interesting discussion point?

[Olof]

GPWS takes precedence over RA. If you get a GPWS pullup you will follow that instead of the TCAS (not that smart to follow a TCAS down into the terrain.) The other aircraft will probably get an increased climb instruction to aviod the traffic. The same goes with windshear which also takes precedence!

[Bumz_Rush]

Does the new verson ie 7

take into consideration single engine situations ????...I know the earlier versions suggested that you turned off R/A if in single engine flight....Bumz

[gearpins]

tcasII mops7 is designed to inhibit increase RAs below 1450'agl;Descend RAs below 1100'agl;and all RAs below 1000'agl