TCAS RA in VMC. Follow the RA or Not?
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Narada - agreed. My position is what the current regulations actually say. i.e. those that say 'follow the RA in all circumstances without exception' are actually the "ones who think they know better".
Rubik. Your scenario shouldn't happen. TCAS knows your approximate envelope, and although as I've said in some circumstances it can be a bit optimistic, the max certified flight level will be in there as a hard limit. You will not get a climb RA at your max FL.
Regarding Right Way Ups comment, not withstanding the fact that his basic position is wrong (see above) there is a germ of truth in what he is saying. it goes like this:
If you have a scenario where both aircraft are inhibited in the same sense - two aircraft at max FL are a good example, then the normal TCAS rules about generating complimentary RAs can not be followed.
i.e. usually one aircraft is commanded to pitch up and the other to pitch down. But if both aircraft are inhibited in the same sense, the best alternative is to direct one aircraft in the allowed sense, and tell the other aircraft NOT to manoeuvre in the 'allowed' sense.
Take our 2 aircraft both inhibited (due performance) from climbing. There are 2 possible solutions, selected essentially at random by the TCAS coordination process:
1. Aircraft 1 told to descend, aircraft 2 told not to descend (best alternative to climb).
2. Aircraft 2 told to descend, aircraft 1 told not to descend (best alternative to climb).
Let us say that the crew of aircraft 2, for some reason, are rabid TCAS haters and have sworn a pact never to follow an RA. If TCAS chooses solution 1, all is well. However, if TCAS chooses solution 2 there is a big problem.
Whereas: if aircraft 2 selects TA only, then the rules change because this is no longer a TCAS to TCAS coordinated encounter. The TCAS in aircraft one will *always* choose a manouvre to avoid. Therefore a possible evasion option is not wasted.
This is esentially (one of) the thought process's behind selecting TA only for engine failures or certain flying control failures. Basically selecting TA only is equivalent to a "get out of my way because I'm doing my own thing" button on your TCAS.
CPB
Rubik. Your scenario shouldn't happen. TCAS knows your approximate envelope, and although as I've said in some circumstances it can be a bit optimistic, the max certified flight level will be in there as a hard limit. You will not get a climb RA at your max FL.
Regarding Right Way Ups comment, not withstanding the fact that his basic position is wrong (see above) there is a germ of truth in what he is saying. it goes like this:
If you have a scenario where both aircraft are inhibited in the same sense - two aircraft at max FL are a good example, then the normal TCAS rules about generating complimentary RAs can not be followed.
i.e. usually one aircraft is commanded to pitch up and the other to pitch down. But if both aircraft are inhibited in the same sense, the best alternative is to direct one aircraft in the allowed sense, and tell the other aircraft NOT to manoeuvre in the 'allowed' sense.
Take our 2 aircraft both inhibited (due performance) from climbing. There are 2 possible solutions, selected essentially at random by the TCAS coordination process:
1. Aircraft 1 told to descend, aircraft 2 told not to descend (best alternative to climb).
2. Aircraft 2 told to descend, aircraft 1 told not to descend (best alternative to climb).
Let us say that the crew of aircraft 2, for some reason, are rabid TCAS haters and have sworn a pact never to follow an RA. If TCAS chooses solution 1, all is well. However, if TCAS chooses solution 2 there is a big problem.
Whereas: if aircraft 2 selects TA only, then the rules change because this is no longer a TCAS to TCAS coordinated encounter. The TCAS in aircraft one will *always* choose a manouvre to avoid. Therefore a possible evasion option is not wasted.
This is esentially (one of) the thought process's behind selecting TA only for engine failures or certain flying control failures. Basically selecting TA only is equivalent to a "get out of my way because I'm doing my own thing" button on your TCAS.
CPB
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CPB: "If you have a scenario where both aircraft are inhibited in the same sense - two aircraft at max FL are a good example, then the normal TCAS rules about generating complimentary RAs can not be followed."
The rules, I would submit, have already accounted for that.
If both a/c are at max FL all that happens if they get TCAS RA is that the one told to descend does so, and the one told to climb but cannot simply stays where it is, i.e. “not manoeuvring contrary to the RA”.
The rules, I would submit, have already accounted for that.
If both a/c are at max FL all that happens if they get TCAS RA is that the one told to descend does so, and the one told to climb but cannot simply stays where it is, i.e. “not manoeuvring contrary to the RA”.
Rubik & Capt Pit Bull,
Sorry for the late response but have just been enjoying a weeks R & R. My post was not specifically pointed at the max crz level situation, but to people who believe that RAs are only there for guidance. My hint to switch to TA was so that my TCAS will compute max performance to avoid their aircraft and not assume that they will follow their RA commands.
Sorry for the late response but have just been enjoying a weeks R & R. My post was not specifically pointed at the max crz level situation, but to people who believe that RAs are only there for guidance. My hint to switch to TA was so that my TCAS will compute max performance to avoid their aircraft and not assume that they will follow their RA commands.
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my TCAS will compute max performance
What will happen, in the event of coordination which is followed by the other aircraft, is an earlier weakening of the initial RA.
If the other aircraft is non compliant with its RA, you will get a strengthened or reversed RA. If the other aircraft is not TCAS equipped (or in TA only mode), and just coincidentally happens to manoeuvre in the wrong sense, again you will get a strengthened or reversed RA. The size and sense of these RAs is not influenced by whether TCAS is coordinating or not ((since v6.04 IIRC))
((note: earlier tcas versions did have reversals inhibited in coordinated encounters. No longer true.))
CPB
CPB,
Honest question - if what you say is true that having the other aircraft TA only will not make a difference, why with an engine failure do we switch to TA only?
Also why in that case did the Uberlingen collision occur?
Honest question - if what you say is true that having the other aircraft TA only will not make a difference, why with an engine failure do we switch to TA only?
Also why in that case did the Uberlingen collision occur?
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You select TA with an engine failure in order not to receive a RA indication.The other trafic,however, will get a RA to take it off the colision vector.
Regarding the uberlingen collision,you may check this http://www.cs.york.ac.uk/hise/safety...2004/0518.html
Regarding the uberlingen collision,you may check this http://www.cs.york.ac.uk/hise/safety...2004/0518.html
Excerpt from the Honeywell Acas 2 manual.
"The crew should promptly but smoothly follow the advisory. Since the maneuvers are coordinated, the crew should never maneuver in the opposite direction of the advisory."
It also says that with a normal RA the crew reaction time is 5 secs, but with a reversal or increase RA (other a/c not doing what it is told), the reaction time is 2.5 secs. Whichever way it is an erosion of safety, and I think backs up my opinion that if a crew thinks following an RA is negotiable then they should switch to TA only!
"The crew should promptly but smoothly follow the advisory. Since the maneuvers are coordinated, the crew should never maneuver in the opposite direction of the advisory."
It also says that with a normal RA the crew reaction time is 5 secs, but with a reversal or increase RA (other a/c not doing what it is told), the reaction time is 2.5 secs. Whichever way it is an erosion of safety, and I think backs up my opinion that if a crew thinks following an RA is negotiable then they should switch to TA only!
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Right Way Up,
Regarding TA only for engine out:
Bah. I wrote an explanation of why TA ONLY use is recommended, but it got far too wordy. I've shunted it down to the bottom of my post because I realised that it might not get to the heart of what you were asking.
Essentially, I didn't say TA mode made no difference, I just said that it made no difference to the size of the RA. I inferred that you thought you would get bigger RAs in uncoordinated encounters.
It does make a difference when you have aircraft in an encounter what are inhibited in the same sense. My earlier post looked at a high altitude encounter, down the bottom of this post is a discussion of a low altitude encounter.
Regarding Uberlingen:
That isn't anything to do with TA only mode, but everything to do with manoeuvering in the opposite sense to an RA. Its been discussed in several threads on this site. The bottom line is that if the intruder keeps pitching in the same sense as you, you're going to hit one another.
Uberlingen type encounters have happened before, and since, that collision. The only difference being the presence of a fortunate ammount of horizontal seperation such that collisions did not occur. The horrendous danger associated with manoeuvering in the opposite sense to an RA was well appreciated by certain segments of the aviation community, but somehow the message failed to get through to all pilots. Frankly, I get depressed every time I think about it. I see so many articles and posters banging the 'follow the ra' drum. What was needed, and perhaps still is, was an overriding 'don't manoeuvre opposite' campaign.
Safe flying guys.
CPB
Whats going on in a low level engine out encounter
The aircraft performance information programmed into a TCAS is based on all engines operating. Therefore, with an engine out, you can be given an RA that is not achievable.
If this happens in a situation where the other aircraft is uninhibited, then the failure of the engine out aircraft to comply would not actually be a problem - there is enough time for either aircraft to generate a miss - thats what happens against a non TCAS intruder.
To put some numbers on it (approximate as I am nowhere near a manual), at high altitude the RA time threshold is something like 35 seconds, at low altitude maybe 20. With 5 seconds allowed to respond to the RA this gives 30 and 15 seconds respectively, for 1500 fpm to generate a miss. The positive RA thresholds (i.e. size of the vertical miss TCAs is trying to arrange) are something like 700 feet at high altitude and 300 feet at low altitude, so you can see that a single RA is capable of providing the designed ammount of vertical 'miss'.
((Hence, in coordinated encounter, the usual progression is that both aircraft comply, so more than adequate seperation will be achieved. This permits both TCASs to weaken their RAs relatively early, minimise deviations from original flight paths, and decrease the likelihood of any 'knock on' effects against any other traffic.))
However, if our engine out aircraft meets another aircraft that is inhibited, then there is the possibility of losing an opportunity for a safe outcome. Basically, its the inverse of the scenario I described in an earlier post.
The easiest to imagine is a low altitude encounter. Perhaps you are making a single engine approach and for whatever reason somebody gets in your way. Since it is a low level encounter, both aircraft could be inhibited from descending. If the engine out aircraft is still in "TA/RA mode", then TCAS coordination will take place. Since neither aircraft can be given a descend RA, one will get a climb, and the other a preventative to not climb. Which way around is essentially random. If the engine out aircraft gets the climb, it likely won't manage it and the two aircraft will not be safely apart at the closest point of approach. However, if the other aircraft gets the climb, all is well.
So what we are seeing is a 50% chance of missing the opportunity to be safe.
Whereas: If the engine out aircraft is at "TA ONLY" then there will be no coordination. The TCAS in the other aircraft will certainly choose to climb. Problem solved. Note the size of the RA will be the same, its just that the TA/RA aircraft certainly gets the climb.
Regarding TA only for engine out:
Bah. I wrote an explanation of why TA ONLY use is recommended, but it got far too wordy. I've shunted it down to the bottom of my post because I realised that it might not get to the heart of what you were asking.
Essentially, I didn't say TA mode made no difference, I just said that it made no difference to the size of the RA. I inferred that you thought you would get bigger RAs in uncoordinated encounters.
It does make a difference when you have aircraft in an encounter what are inhibited in the same sense. My earlier post looked at a high altitude encounter, down the bottom of this post is a discussion of a low altitude encounter.
Regarding Uberlingen:
That isn't anything to do with TA only mode, but everything to do with manoeuvering in the opposite sense to an RA. Its been discussed in several threads on this site. The bottom line is that if the intruder keeps pitching in the same sense as you, you're going to hit one another.
Uberlingen type encounters have happened before, and since, that collision. The only difference being the presence of a fortunate ammount of horizontal seperation such that collisions did not occur. The horrendous danger associated with manoeuvering in the opposite sense to an RA was well appreciated by certain segments of the aviation community, but somehow the message failed to get through to all pilots. Frankly, I get depressed every time I think about it. I see so many articles and posters banging the 'follow the ra' drum. What was needed, and perhaps still is, was an overriding 'don't manoeuvre opposite' campaign.
Safe flying guys.
CPB
Whats going on in a low level engine out encounter
The aircraft performance information programmed into a TCAS is based on all engines operating. Therefore, with an engine out, you can be given an RA that is not achievable.
If this happens in a situation where the other aircraft is uninhibited, then the failure of the engine out aircraft to comply would not actually be a problem - there is enough time for either aircraft to generate a miss - thats what happens against a non TCAS intruder.
To put some numbers on it (approximate as I am nowhere near a manual), at high altitude the RA time threshold is something like 35 seconds, at low altitude maybe 20. With 5 seconds allowed to respond to the RA this gives 30 and 15 seconds respectively, for 1500 fpm to generate a miss. The positive RA thresholds (i.e. size of the vertical miss TCAs is trying to arrange) are something like 700 feet at high altitude and 300 feet at low altitude, so you can see that a single RA is capable of providing the designed ammount of vertical 'miss'.
((Hence, in coordinated encounter, the usual progression is that both aircraft comply, so more than adequate seperation will be achieved. This permits both TCASs to weaken their RAs relatively early, minimise deviations from original flight paths, and decrease the likelihood of any 'knock on' effects against any other traffic.))
However, if our engine out aircraft meets another aircraft that is inhibited, then there is the possibility of losing an opportunity for a safe outcome. Basically, its the inverse of the scenario I described in an earlier post.
The easiest to imagine is a low altitude encounter. Perhaps you are making a single engine approach and for whatever reason somebody gets in your way. Since it is a low level encounter, both aircraft could be inhibited from descending. If the engine out aircraft is still in "TA/RA mode", then TCAS coordination will take place. Since neither aircraft can be given a descend RA, one will get a climb, and the other a preventative to not climb. Which way around is essentially random. If the engine out aircraft gets the climb, it likely won't manage it and the two aircraft will not be safely apart at the closest point of approach. However, if the other aircraft gets the climb, all is well.
So what we are seeing is a 50% chance of missing the opportunity to be safe.
Whereas: If the engine out aircraft is at "TA ONLY" then there will be no coordination. The TCAS in the other aircraft will certainly choose to climb. Problem solved. Note the size of the RA will be the same, its just that the TA/RA aircraft certainly gets the climb.
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Since ueberlingen has been raised again, thought I would remark on what is often forgotten, that no reversal of the original RA could issued to the DHL (the "master" in this encounter) because both aircraft were at the same FL. Perhaps, CPB, you can enlighten us as to why TCAS should be inhibited from issuing reversal commands when both a/c are at same level.
I was told Eurocontrol raised this as a problem well before Ueberlingen but that it was thought not worth fixing by the FAA-ARINC bods. I think they are now in the process of fixing it.
I was told Eurocontrol raised this as a problem well before Ueberlingen but that it was thought not worth fixing by the FAA-ARINC bods. I think they are now in the process of fixing it.
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Frangible,
I'm not totally sure what you are asking. Is this a reference to non reversal of RAs during coordinated encounters? If so, I don't think its an issue - I think (but couldn't swear to it) that both a/c were v7 and therefore its not an issue.
If its a not a reference to that, I'm sorry, I'm not sure. I tried to look up the reports etc but I am on holiday at a mates place and for some reason his PC will not read the .pdfs that the relevant reports are on.
As I recall, (and its very hazy - its several years back now) the DHLs RA responses where just leading the Tupolevs - i.e. its flight path projection was passing fractionally underneath the other aircraft, hence why TCAS selected a strengthened RA rather than a reversal. I must emphasise however that is based on dim and distant memory. Sorry.
Back to beer and steak for me
Safe flying guys.
CPB
I'm not totally sure what you are asking. Is this a reference to non reversal of RAs during coordinated encounters? If so, I don't think its an issue - I think (but couldn't swear to it) that both a/c were v7 and therefore its not an issue.
If its a not a reference to that, I'm sorry, I'm not sure. I tried to look up the reports etc but I am on holiday at a mates place and for some reason his PC will not read the .pdfs that the relevant reports are on.
As I recall, (and its very hazy - its several years back now) the DHLs RA responses where just leading the Tupolevs - i.e. its flight path projection was passing fractionally underneath the other aircraft, hence why TCAS selected a strengthened RA rather than a reversal. I must emphasise however that is based on dim and distant memory. Sorry.
Back to beer and steak for me
Safe flying guys.
CPB
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Pardon my imprecision.
The accident report says, albeit in a roundabout way, that normally, under v7, which both a/c were equipped with, an RA can be reversed to the "master" if the "slave" a/c is not following his RA UNLESS both in level flight, according to analysis of the accident by Eurocontrol. Eurocontrol did a special report for the German investigators on this issue.
Page 37 of the BFU report says:
“Moreover, Eurocontrol conducted a further analysis how TCAS II would have reacted in this case with the modification CP 112 which had already been developed prior to the accident. According to the results provided, TCAS would have generated a Reversal RA after the initial RA which would have led to a sufficient vertical separation of both aircraft if the Boeing B757-200 crew would have reacted accordingly to the reversal RA.”
Which, being interpreted, means that although this was v7 in this particular case it was not capable of issuing a reversal because CP112 had not been implemented (still hasn’t I think).
CP (change proposal) 112 was a proposal from Eurocontrol that the V7 logic be changed so that reversals could be issued when both aircraft were in level flight, a suggestion made prior to Ueberlingen. CP112 has now been folded into a proposal for wider improvement of v7 called SA01. Where that is exactly in he regulatory process I don't know.
My question was, why would anyone want to inhibit TCAS from issuing reversals when both aircraft are in level flight? Seems to me very strange to assume that one or other of the a/c would always be climbing or descending.
The accident report says, albeit in a roundabout way, that normally, under v7, which both a/c were equipped with, an RA can be reversed to the "master" if the "slave" a/c is not following his RA UNLESS both in level flight, according to analysis of the accident by Eurocontrol. Eurocontrol did a special report for the German investigators on this issue.
Page 37 of the BFU report says:
“Moreover, Eurocontrol conducted a further analysis how TCAS II would have reacted in this case with the modification CP 112 which had already been developed prior to the accident. According to the results provided, TCAS would have generated a Reversal RA after the initial RA which would have led to a sufficient vertical separation of both aircraft if the Boeing B757-200 crew would have reacted accordingly to the reversal RA.”
Which, being interpreted, means that although this was v7 in this particular case it was not capable of issuing a reversal because CP112 had not been implemented (still hasn’t I think).
CP (change proposal) 112 was a proposal from Eurocontrol that the V7 logic be changed so that reversals could be issued when both aircraft were in level flight, a suggestion made prior to Ueberlingen. CP112 has now been folded into a proposal for wider improvement of v7 called SA01. Where that is exactly in he regulatory process I don't know.
My question was, why would anyone want to inhibit TCAS from issuing reversals when both aircraft are in level flight? Seems to me very strange to assume that one or other of the a/c would always be climbing or descending.
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Hmm. I wouldn't like to say.
I was under the impression that v7 could reverse in coordinated encounters.
Taking a wild guess:
If both aircraft are co-altitude and maintaining level flight, then that actually means neither has complied with its RA. In which case ordering a reversal is pretty pointless, better off to hope that at least one aircraft reacts at some point in the original sense?
But does this apply to Ueberlingen anyway, since both aircraft were in descent by the time the strengthened RA was given?
Straightforward, no BS answer: - I don't know.
CPB
I was under the impression that v7 could reverse in coordinated encounters.
Taking a wild guess:
If both aircraft are co-altitude and maintaining level flight, then that actually means neither has complied with its RA. In which case ordering a reversal is pretty pointless, better off to hope that at least one aircraft reacts at some point in the original sense?
But does this apply to Ueberlingen anyway, since both aircraft were in descent by the time the strengthened RA was given?
Straightforward, no BS answer: - I don't know.
CPB
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The sense of what the Germans were saying about the collision is, as I understand it, that TCAS had been inhibited from issuing reversals if the encounter originated with both a/c in level flight. These two were in level flight on a collision course long before they were in TCAS range, and that, as I understand it, is where the software inhibition to reversals arises. The software’s “assumption” is that when they come into each other’s TCAS range one or other would be changing level. For some reason the issuing of reversals was prohibited if the original encounter started with level flight for both parties.
Had either plane been climbing or descending at the time the original RA was issued, the “master” could have been issued with a reversal RA, but since the encounter began in level flight, it could not.
Had either plane been climbing or descending at the time the original RA was issued, the “master” could have been issued with a reversal RA, but since the encounter began in level flight, it could not.
