View Full Version : tcas question

4th Apr 2011, 10:09
hello, i'm trying to determine what is the general consensus of the tcas RA function. if a jetliner has an engine out, are company sop's having crews secure the RA function due to a lack of performance in the engine out condition and potentially not being able to comply with an RA should a crew receive one?
my understanding is that the mode s portion is datalinked between aircraft and should you secure the RA function, the mode s would then send contradictory information to the other aircraft. therefore, you should keep the RA portion on inorder to allow the mode s to continue to provide complementary information to other mode s transponders. can anyone clarify?

thank you in advance.


4th Apr 2011, 11:03
On the 737 the advice in the relevant non normal checklists is to switch TCAS to TA only, which means we will still receive traffic advisories but no resolution advisories anymore, and TCAS of course will not coordinate RAs anymore as well therefore the intruder TCAS will have to calculate its avoidance manouver according to that, same as for any other non-TCAS aircraft.

Piltdown Man
4th Apr 2011, 11:07
When engine out, you select TA. This is not SOP as such but an action required by the checklist, written by the manufacturer. This means that if there is a potential conflict with another TCAS equipped aircraft, the one operating with the normal TA/RA selection will be issed with a the RA. Not doing so might mean that an "avoidance contract" might not be capable of execution for performance reasons - with unpleasant consequences.


4th Apr 2011, 11:56
Question: Will the TCAS command a climb, when to do so, initiate a stall, and perhaps then fall ON top of the other aircraft.

In other words, will the TCAS consider the possible rate of climb that the aircraft can maintain, in order to avoid the TRAFFIC....

Answer: if the aircraft can not climb at 1500 fpm or more a climb instruction will not be issued.

However my question is still of interest, and what if BOTH are limited by this constraint.


4th Apr 2011, 13:38
Answer: if the aircraft can not climb at 1500 fpm or more a climb instruction will not be issued.

However my question is still of interest, and what if BOTH are limited by this constraint.

How would you design the TCAS to handle that situation? You would advise one aircraft to not descend, while advising the other to descend at 1500 fpm. If that doesn't provide enough margin, then advise increased descent of 2500 fpm.

The plane's max altitude at which 1500 fpm climb can be achieved is determined by the aircraft maker, and programmed into the TCAS processor. That altitude is pretty low in a 747-100 in partial flaps configuration.


galaxy flyer
4th Apr 2011, 15:49
USAF procedure and Bombardier GLEX and Challenger QRH procedure to go to TA ONLY. I believe that is the recommended procedure universally.


5th Apr 2011, 00:00
Reminds of when I updated the avionics in the company jet.

'So is it TCAS I or II?'
'What's the difference?'
'They do the same thing, but for another $100k the box will tell you to climb or descend and you have to comply."
'Yeah? Last I checked I'm the guy flying the plane, so let's go with TCAS I and save the boss a $100k."

5th Apr 2011, 06:38
"engine out" you should be flying your aircraft. Seems a no brainer to me, conflicting aircraft should comply to RA.. You find a RWY despite traffic. This is very much an "non issue" in an emergency, ATC should be working with you in this event as well.

5th Apr 2011, 16:22
When two conflicting traffics reach de RA threshold but one of them is in TA only, What does the TA/RA one do?

I guess it executes an RA, right?

and this RA has to take into account the lack of RA on the other traffic. Probably it assumes that the conflicting traffic will maintain path. If it changes, then RA will change, too.

I'm just guessin, but I think this is the key. Airplanes wihich performance is compromised should not execute RAs. The conflicting traffics will.

In Airbus fbw, the procedure is TA only when engine out.