TCAS philosophies
Join Date: Aug 2003
Location: Not a million miles from EGTF
Age: 68
Posts: 1,579
Likes: 0
Received 0 Likes
on
0 Posts
Sorry
This bit of the thread is baffling me.
I assume that most of those posting are talking about the situation in Class A and B airspace.
In my limited experience, mainly outside of this type of airspace, taking TCAS over ATC is quite dangerous. In Class G, for example, there are many a/c either not squawking or using Mode A only. For them to ignore an ATC contact and only work off their TCAS seems to be perverse.
This bit of the thread is baffling me.
I assume that most of those posting are talking about the situation in Class A and B airspace.
In my limited experience, mainly outside of this type of airspace, taking TCAS over ATC is quite dangerous. In Class G, for example, there are many a/c either not squawking or using Mode A only. For them to ignore an ATC contact and only work off their TCAS seems to be perverse.
Pegase Driver
Thread Starter
Join Date: May 1997
Location: Europe
Age: 74
Posts: 3,681
Likes: 0
Received 0 Likes
on
0 Posts
I do not want to interfere too much in this fascinating debate but there are a couple of misconceptions and assumptions ( most coming from Fullwings ), that I’d like to correct :
Fullwings :
Not quite. In real life, late ATC intervention is rather common for various reasons, not all due to ATC errors, and 99,9999 % of the cases it solves the problem. There is no consequence. If a controller had to be replaced every time he use the word “expedite” there will be little staff left in my center !
ATC works on establishing separation ( typically 1000 ft and 5 NM )
TCAS was not designed to re-establish separation ..It was designed to prevent metal from touching each other.
(In most cases TCAS missed distance is unlikely to be more that 300 feet.)
Mixing the 2 leads to misconceptions on how both work in real life.
The misconception that once you get below “separation “ a controller should not issue any instruction is totally wrong and dangerous. ATC is responsible to provide separation , and anti collision avoidance at all times , unless the PIC in command has informed him that the crew is following an ACAS RA. From this moment on , pilot takes over responsibility of anti collision from ATC., but before that moment, the controller is responsible, above or below 5NM .
The problem we encountered in Ueberlingen ( and in many other less dramatic incidents ) is that ACAS intervenes BEFORE separation is lost , and pilots do not inform ATC that they are following an RA, enforcing the controller’s mind that everything is still under HIS control, and might even continue to issue instructions well after TCAS has interfered.
Note : As a controller you do not know if an aircraft has a functioning TCAS or not (e.g. : one can fly 10 days with a U/S unit) and the same service is given regardless of having TCAS or not.
Fullwings :
From my ATC experience and recorded data, until very recently, most RAs were not due to ATC but caused by excessive vertical rates by crew. Therefore the ICAO note to reduce vertical rate before leveling off.
There are many non-ATC other cases , such as ghosts targets, or level busts for instance , that are causing RAs.
The notion that an RA only occur when ATC has failed is wrong.
Lastly :
Fullwings again :
Not a UFO, but a military a/c ( quite a few where I come from ) or an out of tolerance SSR that will not be accepted by TCAS., but visible on radar.
In my ATC system, an SSR failure will be compensated and tracked on primary , that one too will be invisible on TCAS . Remember that I have legally to separate aircraft from what I know.
If ATC instructions differ from what TCAS is telling you , there you have a dilemna. Do I have , as a controller, the right picture, with every player, moving as my radar tells me, or does TCAS has the correct solution ? . You will never know until you try , but either way, it is not a 100% guarantee that what you chose is the correct action.
Finaly :
Bsieker :
This is indeed one of the real issues .Seeing the data I see , all post 2002, leads me to believe that we have not solved the problem yet and that another accident is probable.
Fullwings :
Yes but those instructions came too late to stop the aircraft coming within 7NM of each other at the same level, on a collision course. That, by definition, is a "failure of separation". "Late ATC intervention" is a "failure of separation", involving much form filling, suspension of the individuals involved, official reports, etc. I'm coming to think your definition of "separation" is "not actually hitting each other", which is not an industry standard!
ATC works on establishing separation ( typically 1000 ft and 5 NM )
TCAS was not designed to re-establish separation ..It was designed to prevent metal from touching each other.
(In most cases TCAS missed distance is unlikely to be more that 300 feet.)
Mixing the 2 leads to misconceptions on how both work in real life.
The misconception that once you get below “separation “ a controller should not issue any instruction is totally wrong and dangerous. ATC is responsible to provide separation , and anti collision avoidance at all times , unless the PIC in command has informed him that the crew is following an ACAS RA. From this moment on , pilot takes over responsibility of anti collision from ATC., but before that moment, the controller is responsible, above or below 5NM .
The problem we encountered in Ueberlingen ( and in many other less dramatic incidents ) is that ACAS intervenes BEFORE separation is lost , and pilots do not inform ATC that they are following an RA, enforcing the controller’s mind that everything is still under HIS control, and might even continue to issue instructions well after TCAS has interfered.
Note : As a controller you do not know if an aircraft has a functioning TCAS or not (e.g. : one can fly 10 days with a U/S unit) and the same service is given regardless of having TCAS or not.
Fullwings :
From a pilot's POV I have to assume that there has been a failure of ATC when in receipt of an RA and act appropriately.
There are many non-ATC other cases , such as ghosts targets, or level busts for instance , that are causing RAs.
The notion that an RA only occur when ATC has failed is wrong.
Lastly :
Fullwings again :
Or it's a cruise missile, a weather balloon, a flying saucer, a flock of birds, etc. Very easy to find edge cases where the system may not work but how realistic are they in a positive radar environment? If you stray into the path of a UFO during an RA it's just not your day is it?
In my ATC system, an SSR failure will be compensated and tracked on primary , that one too will be invisible on TCAS . Remember that I have legally to separate aircraft from what I know.
If ATC instructions differ from what TCAS is telling you , there you have a dilemna. Do I have , as a controller, the right picture, with every player, moving as my radar tells me, or does TCAS has the correct solution ? . You will never know until you try , but either way, it is not a 100% guarantee that what you chose is the correct action.
Finaly :
Bsieker :
The problem is when you get conflicting advice from ATC and TCAS, particularly if in the order:
- ATC advice
- starting following ATC
- TCAS RA
- ATC advice
- starting following ATC
- TCAS RA
Join Date: Jul 2006
Location: USA
Posts: 451
Likes: 0
Received 0 Likes
on
0 Posts
My comapny's SOP
At my company, the SOP is to ALWAYS FOLLOW THE TCAS RA, except in cases of windshear, terrain warnings, and stall warnings. We are to follow the RA, and as soon as practical, inform ATC of the RA maneuver.
THIS INCLUDES FLIGHT IN VMC WHERE THE OFFENDING TRAFFIC IS IN SIGHT. (Is what you're looking at really the conflicting traffic, or is there another aircraft out there that you don't see, that's about to hit you!!!!)
The logic is this: The RA is a coordination between two aircraft with TCAS. The two TCAS computers have identified a conflict and have devised a resolution. Failure of one aircraft to follow the plan...will screw things up...since nobody tells the TCAS computer of aircraft #2 that the pilot(s) of aircraft #1 has decided to do his own thing.
At my company, several years ago, there was a major $%%^&. One of our flights was departing a Florida airport...climbing out...received a TCAS RA. The flight was in IMC. They followed the RA...as per SOP...but, immediately afterward, got a second RA. They followed it, as per SOP, as well.
At first thought, one would assume that the second RA was due to the following of the first RA. After the BIG investigation, it was determined that the controller, that morning, suffered from a major brain fXrt...and the two conflicts were real...and the second one was not due to the crew following the RA of the first conflict...it was the real thing, too.
Now, the crew figured they were having a bad day, too. What's up with getting two RAs...one right after the other...in IMC!!!!!
Thank God, these guys followed the SOP! Otherwise, we'd be doing a 'coulda, shoulda, woulda' analysis on PPrune!
PantLoad
THIS INCLUDES FLIGHT IN VMC WHERE THE OFFENDING TRAFFIC IS IN SIGHT. (Is what you're looking at really the conflicting traffic, or is there another aircraft out there that you don't see, that's about to hit you!!!!)
The logic is this: The RA is a coordination between two aircraft with TCAS. The two TCAS computers have identified a conflict and have devised a resolution. Failure of one aircraft to follow the plan...will screw things up...since nobody tells the TCAS computer of aircraft #2 that the pilot(s) of aircraft #1 has decided to do his own thing.
At my company, several years ago, there was a major $%%^&. One of our flights was departing a Florida airport...climbing out...received a TCAS RA. The flight was in IMC. They followed the RA...as per SOP...but, immediately afterward, got a second RA. They followed it, as per SOP, as well.
At first thought, one would assume that the second RA was due to the following of the first RA. After the BIG investigation, it was determined that the controller, that morning, suffered from a major brain fXrt...and the two conflicts were real...and the second one was not due to the crew following the RA of the first conflict...it was the real thing, too.
Now, the crew figured they were having a bad day, too. What's up with getting two RAs...one right after the other...in IMC!!!!!
Thank God, these guys followed the SOP! Otherwise, we'd be doing a 'coulda, shoulda, woulda' analysis on PPrune!
PantLoad
FullWings, pursues the more practical and simpler route (#60 onwards), which often the regulatory segments of the industry choose to follow, albeit hidden in the paperwork. This practical approach has been most valuable in persuading the industry of the relevance of new equipment in the threat environment, and the need for standardised (world wide) procedures.
ACAS, like EGPWS and windshear warning systems, has enabled the industry to visualise a particular threat to safe operation, which previously many people did not believe existed (numerically), or did not whish to believe posed a significant risk. We now have growing evidence of the magnitude of the threat and a means (amongst others) of counteracting it.
I suspect that ACAS will continue to evolve (refinable) as did GPWS. I further suspect that any significant ‘failures’ in the overall ACAS safety system will originate from human error at the point of operation and not at the point of design, again compare this with EGPWS and the many incidents of incorrect crew action (again a suspicion, not provable).
The greater problem for the industry is how to get pilots to follow the ‘instructions on the tin’; something which applies to many systems and is fundamental to human behaviour.
ACAS may not assist our quest for correct behaviour in its use of terminology, e.g. using ‘advisory’ and ‘resolution’ (legal/engineering bias) when describing situations which warrant much higher or urgent levels of alerting and warning. These terms may mislead non English speaking pilots or the regulators (often with government/ legal structures) who may apply ICAO recommendations from their perspective and not from that of the crew. But such is the outcome of the worldwide adoption of a system based on a single national culture, perhaps without the necessary depth of understanding and debate on its design philosophy and operation.
ATC Watcher identifies a significant issue with the ATC/ACAS system, perhaps the issue; communications. The ATC based anti collision avoidance depends on communications at all times. Communications are fallible; even the alleviating action of the aircraft informing ATC of an ACAS manoeuvre depends on communication. Until this problem is resolved then there is a role for ACAS.
From the descriptions above, I see ATC anti collision avoidance as the strategic activity, generally proactive, but reactive when necessary and if the conditions (time) allow. ACAS is the tactical, reactive system; the last line of defence and thus at the time of warning it must have precedence.
There are similarities here with CFIT. ATC can provide early defences with safe altitudes and alerts, but still suffer problems of communication in the reactive sense, e.g. delays in transmitting MSAW warnings. EGPWS again provides the tactical, last line of defence.
For both ACAS and EGPWS, the success of these systems depends entirely on the crew following the instructions.
Peter Re PhD; x,y,z, = the 3 aircraft, but lets use your coordinate system. Am I correct in assuming that the 3D solutions to the quartic equation are inside the ACAS alerting area, and thus are realistic solutions for the risk of collision and not just imaginary space-time zones?
ACAS, like EGPWS and windshear warning systems, has enabled the industry to visualise a particular threat to safe operation, which previously many people did not believe existed (numerically), or did not whish to believe posed a significant risk. We now have growing evidence of the magnitude of the threat and a means (amongst others) of counteracting it.
I suspect that ACAS will continue to evolve (refinable) as did GPWS. I further suspect that any significant ‘failures’ in the overall ACAS safety system will originate from human error at the point of operation and not at the point of design, again compare this with EGPWS and the many incidents of incorrect crew action (again a suspicion, not provable).
The greater problem for the industry is how to get pilots to follow the ‘instructions on the tin’; something which applies to many systems and is fundamental to human behaviour.
ACAS may not assist our quest for correct behaviour in its use of terminology, e.g. using ‘advisory’ and ‘resolution’ (legal/engineering bias) when describing situations which warrant much higher or urgent levels of alerting and warning. These terms may mislead non English speaking pilots or the regulators (often with government/ legal structures) who may apply ICAO recommendations from their perspective and not from that of the crew. But such is the outcome of the worldwide adoption of a system based on a single national culture, perhaps without the necessary depth of understanding and debate on its design philosophy and operation.
ATC Watcher identifies a significant issue with the ATC/ACAS system, perhaps the issue; communications. The ATC based anti collision avoidance depends on communications at all times. Communications are fallible; even the alleviating action of the aircraft informing ATC of an ACAS manoeuvre depends on communication. Until this problem is resolved then there is a role for ACAS.
From the descriptions above, I see ATC anti collision avoidance as the strategic activity, generally proactive, but reactive when necessary and if the conditions (time) allow. ACAS is the tactical, reactive system; the last line of defence and thus at the time of warning it must have precedence.
There are similarities here with CFIT. ATC can provide early defences with safe altitudes and alerts, but still suffer problems of communication in the reactive sense, e.g. delays in transmitting MSAW warnings. EGPWS again provides the tactical, last line of defence.
For both ACAS and EGPWS, the success of these systems depends entirely on the crew following the instructions.
Peter Re PhD; x,y,z, = the 3 aircraft, but lets use your coordinate system. Am I correct in assuming that the 3D solutions to the quartic equation are inside the ACAS alerting area, and thus are realistic solutions for the risk of collision and not just imaginary space-time zones?
Join Date: Aug 2007
Location: Oerlinghausen, DE
Age: 49
Posts: 35
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by FullWings
If I'd set two aircraft on a collision course in error I'd want something to manipulate the outcome of my instructions!
Originally Posted by FullWings
Never heard/seen any complaints myself, especially from people who understand how the systems work.
If the TCAS system recognises this collision threat and if the second instruction was made not timely enough, TCAS issues an RA, which can be inverted to the controllers instructions.
Now, the controller cannot judge the sitiuation and actions of the aircraft under his control. He can only pray, that they solve the conflict on their own. Hardly a mental state I want to be in.
Join Date: Sep 2000
Location: Bielefeld, Germany
Posts: 955
Likes: 0
Received 0 Likes
on
0 Posts
robin,
The UK is unusual in offering some level of IFR separation through ATC in its Class G airspace. Most countries, such as the US, only offer separation in Class G airspace through flight planning (that is, ATM not ATC).
Most TCAS-equipped kit will be flying most of the time in Class A or B airspace, and (in most countries) ATC is not going to be providing separation under IFR for Class G airspace, so TCAS-ATC interactions in Class G airspace would not be an issue in most countries. However, I have no experience with IFR in Class G airspace in the UK, so I can't usefully comment on that.
PBL
The UK is unusual in offering some level of IFR separation through ATC in its Class G airspace. Most countries, such as the US, only offer separation in Class G airspace through flight planning (that is, ATM not ATC).
Most TCAS-equipped kit will be flying most of the time in Class A or B airspace, and (in most countries) ATC is not going to be providing separation under IFR for Class G airspace, so TCAS-ATC interactions in Class G airspace would not be an issue in most countries. However, I have no experience with IFR in Class G airspace in the UK, so I can't usefully comment on that.
PBL
Last edited by PBL; 25th Sep 2007 at 05:01. Reason: Disambiguation
Join Date: Sep 2000
Location: Bielefeld, Germany
Posts: 955
Likes: 0
Received 0 Likes
on
0 Posts
alf,
I don't necessarily agree that *the* issue with ACAS/ATC interaction is communication. The communication issue would theoretically be solved by having secondary radar interpret and display the Mode S interactions between two TCAS-equipped aircraft undergoing an RA sequence.
Under a TCAS RA manoeuvre, the aircraft involved can well depart cleared airspace. That means that the controller has suddenly to replan, and that may neither be desirable nor, in some circumstances, possible, perfect communication or not.
And since TCAS may (and does) propose manoeuvres when aircraft are still legally separated, (and in the case of "zoom climbs" the crews likely will not bust their clearance) that means this replanning problem may be artificially introduced.
I must say that it does surprise me that, even at this late date, many line aircrew seem to be unaware of the problems of TCAS/ATC interaction. (For example, FullWings suggested he wasn't aware of problems; ATC Watcher implicitly pointed out that he was aware of lots.) I could have understood it, say, a decade ago, but I don't understand it five years after Überlingen.
I think some of it may be due to Eurocontrol's and others' attempts to impose uniform RA behavior on aircrew. If you are trying to get all pilots everywhere to follow an RA, it works against that message to publicise all the incidents in which TCAS might prompt inadvisable or unnecessary manoeuvres. So the incident narratives are suppressed (to which phenomenon I can attest, having tried to get data on TCAS incidents).
I originally said that I am not taking a stand on whether this is a "good" or a "bad" thing; that I was simply noting a phenomenon. That is not quite true; I do take a stand. I am for transparency of information on all safety-relevant systems in which there is a public interest. Just as the UK has an Airprox Board that reports publically on all designated near misses, I think data on TCAS manoeuvres should be publically collected and publically analysed and displayed.
Then maybe we would not see such travesties as Eurocontrol's declaration in their EUR-RVSM Safety Case that they had not identified any anomalous ACAS-RVSM interactions and that therefore they did not need to address such potential interactions in the Safety Case (even after I pointed such out to them, they maintained their "view").
PBL
I don't necessarily agree that *the* issue with ACAS/ATC interaction is communication. The communication issue would theoretically be solved by having secondary radar interpret and display the Mode S interactions between two TCAS-equipped aircraft undergoing an RA sequence.
Under a TCAS RA manoeuvre, the aircraft involved can well depart cleared airspace. That means that the controller has suddenly to replan, and that may neither be desirable nor, in some circumstances, possible, perfect communication or not.
And since TCAS may (and does) propose manoeuvres when aircraft are still legally separated, (and in the case of "zoom climbs" the crews likely will not bust their clearance) that means this replanning problem may be artificially introduced.
I must say that it does surprise me that, even at this late date, many line aircrew seem to be unaware of the problems of TCAS/ATC interaction. (For example, FullWings suggested he wasn't aware of problems; ATC Watcher implicitly pointed out that he was aware of lots.) I could have understood it, say, a decade ago, but I don't understand it five years after Überlingen.
I think some of it may be due to Eurocontrol's and others' attempts to impose uniform RA behavior on aircrew. If you are trying to get all pilots everywhere to follow an RA, it works against that message to publicise all the incidents in which TCAS might prompt inadvisable or unnecessary manoeuvres. So the incident narratives are suppressed (to which phenomenon I can attest, having tried to get data on TCAS incidents).
I originally said that I am not taking a stand on whether this is a "good" or a "bad" thing; that I was simply noting a phenomenon. That is not quite true; I do take a stand. I am for transparency of information on all safety-relevant systems in which there is a public interest. Just as the UK has an Airprox Board that reports publically on all designated near misses, I think data on TCAS manoeuvres should be publically collected and publically analysed and displayed.
Then maybe we would not see such travesties as Eurocontrol's declaration in their EUR-RVSM Safety Case that they had not identified any anomalous ACAS-RVSM interactions and that therefore they did not need to address such potential interactions in the Safety Case (even after I pointed such out to them, they maintained their "view").
PBL
Last edited by PBL; 25th Sep 2007 at 05:12. Reason: Taking an appropriate moral stance
Join Date: Sep 2000
Location: Bielefeld, Germany
Posts: 955
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by PantLoad
The logic is this: The RA is a coordination between two aircraft with TCAS
Answer: no one actually knows, but many people seem to assume it will just be OK.
PBL
Join Date: Jul 2006
Location: USA
Posts: 451
Likes: 0
Received 0 Likes
on
0 Posts
Yes, good question!!!
PBL,
Yes, good question!!!!
In the case of my company's incident that occurred several years ago, I honestly don't know the version of TCAS that was installed on our aircraft at that particular time. But, the TCAS system worked for the two conflicts. The second conflict occurred almost immediately after the first.
But, who knows what would happen if two or more conflicts occurred at exactly the same time...from two or more different directions...two or more different altitudes!!!!!
I guess, if that happens to you, you can definitely classify that as a 'bad day'!!!!
PantLoad
Yes, good question!!!!
In the case of my company's incident that occurred several years ago, I honestly don't know the version of TCAS that was installed on our aircraft at that particular time. But, the TCAS system worked for the two conflicts. The second conflict occurred almost immediately after the first.
But, who knows what would happen if two or more conflicts occurred at exactly the same time...from two or more different directions...two or more different altitudes!!!!!
I guess, if that happens to you, you can definitely classify that as a 'bad day'!!!!
PantLoad
Wow, seems to be a lot of interest in this subject... Shame I can't post from the jet (yet!).
ATC Watcher,
Absolutely. I have stated that quite positively.
Not what I said. What I am saying is that the penalty for assuming that TCAS has got it wrong (and being wrong in that assumption) is much higher than for assuming ATC has got it wrong (and being wrong in that assumption). Apply a bit of logic to that and you can see why I'd follow an RA. It's a sort of Pascal's Wager for aeroplanes...
At the end of the day it's down to probabilities, isn't it? In order to hit an aircraft during an RA (assuming you're following the guidance correctly) there would have to be another aircraft in addition to the one(s) that gave you the RA that a) Doesn't have a working TCAS or transponder and b) just happens to be in exactly the right place at the right time. I would suggest (although I haven't published a paper on it yet) that you are much more at risk from the *known* target(s) you are avoiding because of a *definite* risk of collision than the *unknown* ones that *might* be there (or not)?
alf5071h,
The voice of reason, as always.
joernstu,
You assume that the instructions inevitably would have lead to a collision.
If you (as a controller) have sent two aircraft directly at each other and there is no further intervention (from you, pilots, TCAS, etc.) then they're going to collide. ATC in Africa have plenty of practice at setting this up!
We don't know. ATC voice instructions have a very high potential latency compared with TCAS interactions. If there are other aircraft on the frequency and one transmits for, say, 20 seconds then there is no chance to issue any avoiding instructions until too late. The situation becomes worse because if a potentially serious conflict is noticed by pilots, one of the first things that often happens is they check with ATC about the situation, thereby blocking information flow from ATC to the pilot(s); Standard VHF R/T is only half-duplex.
ATC Watcher,
ATC is responsible to provide separation , and anti collision avoidance at all times
The notion that an RA only occur when ATC has failed is wrong.
If ATC instructions differ from what TCAS is telling you , there you have a dilemna. Do I have , as a controller, the right picture, with every player, moving as my radar tells me, or does TCAS has the correct solution ? . You will never know until you try , but either way, it is not a 100% guarantee that what you chose is the correct action.
alf5071h,
The voice of reason, as always.
joernstu,
Originally Posted by FullWings
If I'd set two aircraft on a collision course in error I'd want something to manipulate the outcome of my instructions!
If I'd set two aircraft on a collision course in error I'd want something to manipulate the outcome of my instructions!
Controlling the airspace by radio communications takes time. A controller telling one aircraft to descend, the other to climb or maintain altitude can make these instructions only sequentially. In this scenario the controller's instructions would have prevented a collision.
PBL,
Taking some comments out-of-order...
I can't disagree with that, although "public" should really mean interested academic groups, IMHO, unless you want it all appearing in the Daily Mail under "Killer Deathjets in Head-on Plunge!!"
Without wishing to be rude, I think you've got it slightly backwards. What we really don't need is people going around responding to RAs with Manoeuvre($Random). I'm sure you can see that?
I'll take your word for it. I suppose you might have to cross proprietary and DPA boundaries to get this information as an individual, although I'm sure that agencies such as the AAIB and CAA would have as much access as they desired?
Maybe what needs to be done is to run a series of simulated encounters, no, let's make that a *lot* of encounters, using the actual software involved and analyse the results, possibly making changes to the software... Hmm I wonder if anyone has ever done that? (Sorry, been up for 36hrs after a delayed nightflight so am a bit sarky.
)
Taking some comments out-of-order...
...I do take a stand. I am for transparency of information on all safety-relevant systems in which there is a public interest. Just as the UK has an Airprox Board that reports publically on all designated near misses, I think data on TCAS manoeuvres should be publically collected and publically analysed and displayed.
If you are trying to get all pilots everywhere to follow an RA, it works against that message to publicise all the incidents in which TCAS might prompt inadvisable or unnecessary manoeuvres.
So the incident narratives are suppressed (to which phenomenon I can attest, having tried to get data on TCAS incidents).
Maybe what needs to be done is to run a series of simulated encounters, no, let's make that a *lot* of encounters, using the actual software involved and analyse the results, possibly making changes to the software... Hmm I wonder if anyone has ever done that? (Sorry, been up for 36hrs after a delayed nightflight so am a bit sarky.
)
Join Date: Aug 2007
Location: Oerlinghausen, DE
Age: 49
Posts: 35
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by FullWings
If you (as a controller) have sent two aircraft directly at each other and there is no further intervention (from you, pilots, TCAS, etc.) then they're going to collide.
I understand that you - and many company's SOPs - instruct the crew to follow the TCAS RA. This will be the solution for 2 aircraft situations, which are the most probable to occur. But this threat is not only about what TCAS can do, but also about what it cannot do - and the effects the TCAS design produce.
ATC in Africa have plenty of practice at setting this up!
But if the african ATC is so capable in producing dangerous situations, and the probablity of aircraft flying without operational TCAS is highest in poor regions like Africa, why don't we hear about more mid-air collisions over Africa in the news?
We don't know.
ATC voice instructions have a very high potential latency compared with TCAS interactions. If there are other aircraft on the frequency and one transmits for, say, 20 seconds then there is no chance to issue any avoiding instructions until too late. The situation becomes worse because if a potentially serious conflict is noticed by pilots, one of the first things that often happens is they check with ATC about the situation, thereby blocking information flow from ATC to the pilot(s); Standard VHF R/T is only half-duplex.
Perhaps the introduction of full-duplex digital communications and transmission of digitized instruction, with person-to-person communication being an exception for uncommon situations could help, but their impact on aviation safety is still unknown.
Join Date: Jul 2007
Location: Germany
Posts: 556
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by FullWings
If you (as a controller) have sent two aircraft directly at each other and there is no further intervention (from you, pilots, TCAS, etc.) then they're going to collide.
- they're going directly head-on
- both are flying very accurately.
The latter is usually true these days with high-precision navigation, although some oscillations around the exact altitude are normal for every feedback-control loop. (I don't know about other types, but Airbus FBW aircraft's autopilots have a "soft-altitude" mode for cruise that allows up to 50ft deviation to reduce thrust changes for comfort and economy.)
As to the first point, of the two recent Mid-Airs, only in one were they going in directly opposite directions (GOL/ExelAire), in the other (Bashkirian/DHL)they were at around 90 degrees, and for a collision in that setup you have to get it exactly right, otherwise they'll miss.
Not what I said. What I am saying is that the penalty for assuming that TCAS has got it wrong (and being wrong in that assumption) is much higher than for assuming ATC has got it wrong (and being wrong in that assumption). Apply a bit of logic to that and you can see why I'd follow an RA. It's a sort of Pascal's Wager for aeroplanes...
The penalty for both might be a Mid-Air. The DHL crew in the Ueberlingen accident followed the RA to the letter. And were rewarded with a collision.
Consider the Ueberlingen scenario with a slight modification: The Tuplolev has, as allowed for dispatch, an unserviceable TCAS system, but fully working transponder.
Thus, the DHL Boeing gets a resolution advisoary, but the Tupolev gets none. The Tupolev does get, however, a manoevering instruction from ATC ("descend FL350 expedite", iirc). The difference here being between not following an RA versus not getting it. Educate me about TCAS: would the DHL crew have known about the Baskirian's TCAS being inoperable?
Bernd
Last edited by bsieker; 26th Sep 2007 at 08:07. Reason: Another thought, formatting.
...how do you explain, that not every loss of separation situation inevitably lead to a mid-air?
why don't we hear about more mid-air collisions over Africa in the news?
Perhaps the introduction of full-duplex digital communications and transmission of digitized instruction, with person-to-person communication being an exception for uncommon situations could help, but their impact on aviation safety is still unknown.
Consider the Ueberlingen scenario with a slight modification: The Tuplolev has, as allowed for dispatch, an unserviceable TCAS system, but fully working transponder.
the DHL Boeing gets a resolution advisory, but the Tupolev gets none. The Tupolev does get, however, a manoevering instruction from ATC ("descend FL350 expedite", iirc). The difference here being between not following an RA versus not getting it. Educate me about TCAS: would the DHL crew have known about the Baskirian's TCAS being inoperable?
Would the DHL aircraft's TCAS know about this? Yes. It would treat the Tu-154 as a "dumb" target and expect to be doing all the manoeuvring whilst tracking it.
If two or more TCAS equipped aircraft come into conflict with each other there may be an element of coordination between the TCAS units to decide who does what; e.g. "You go up, I'll go down". This is what makes manoeuvring against an RA potentially serious as other aircraft may have begun to manoeuvre in expectation of a certain manoeuvre from you. It's a bit like indicating left on a busy road then suddenly turning right.
Join Date: Jul 2007
Location: Germany
Posts: 556
Likes: 0
Received 0 Likes
on
0 Posts
FullWings,
thanks for the reply,
I have no intimate knowledge of the TCAS algorithms, but my point was to see if this was a credible scenario:
- Two airliners, both with transponders, one with serviceable TCAS, the other unserviceable.
- On conflicting trajectories ("collision course")
- TCAS-aircraft gets RA ("Descend, Descend!"), complete with red area, possibly larger than in the case whin which the conflict is expected to climb, but I'm on shaky ground on this.
- Non-TCAS aircraft gets ATC-instruction to descend, but, significantly, does not get an RA. (With the instruction to descend to the next FL below, separation is supposedly restored, from an ATC point of view.)
This scenario is facilitated by:
- Lack of knowledge by ATC of RA
- Lack of knowledge by TCAS-aircraft flight crew of lack of TCAS in conflict aircraft
- Rules of dispatch for U/S TCAS unit (10 days, is it?).
If this isn't a clear hazard (read: danger) created by following an RA I don't know what is.
And for this scenario we don't even need to suppose one flight crew disregarding an RA, rules may well be for all operators to always follow the RA!
I'd be grateful for anyone to point out possible factual errors in this one, since I'm not familiar with TCAS internals and the exact conditions under which what kind of RA will be issued by TCAS on encountering a "dumb" target.
Bernd
thanks for the reply,
I have no intimate knowledge of the TCAS algorithms, but my point was to see if this was a credible scenario:
- Two airliners, both with transponders, one with serviceable TCAS, the other unserviceable.
- On conflicting trajectories ("collision course")
- TCAS-aircraft gets RA ("Descend, Descend!"), complete with red area, possibly larger than in the case whin which the conflict is expected to climb, but I'm on shaky ground on this.
- Non-TCAS aircraft gets ATC-instruction to descend, but, significantly, does not get an RA. (With the instruction to descend to the next FL below, separation is supposedly restored, from an ATC point of view.)
This scenario is facilitated by:
- Lack of knowledge by ATC of RA
- Lack of knowledge by TCAS-aircraft flight crew of lack of TCAS in conflict aircraft
- Rules of dispatch for U/S TCAS unit (10 days, is it?).
If this isn't a clear hazard (read: danger) created by following an RA I don't know what is.
And for this scenario we don't even need to suppose one flight crew disregarding an RA, rules may well be for all operators to always follow the RA!
I'd be grateful for anyone to point out possible factual errors in this one, since I'm not familiar with TCAS internals and the exact conditions under which what kind of RA will be issued by TCAS on encountering a "dumb" target.
Bernd
Bernd, when setting such a scenario it should be to qualified with the probability of it occurring.
Your example assumes one TCAS inoperative, that the strategic defence has already failed (ATC plan), and that the Controller’s command occurs after the TCAS warning. The latter involving a critical time span of approx 30sec.
Then there is the assumption that the two aircraft will manoeuvre precisely as required to result in a collision.
TCAS will command a minimum descent (1500ft/min ? (check spec values)), the manoeuvre assumes a standard crew response time and acceleration (2sec, 1.25g).
The aircraft responding to an urgent ATC command is not bounded by time or acceleration, but experience suggest that it will not be greater than the TCAS manoeuvre.
Thus there is a vast range of variables in both the start and action conditions that will not result in a collision.
However, given that the initial manoeuvres meet the collision criteria, then TCAS will continue to evaluate the situation and can increase the decent rate attempting to avoid the collision.
If you wish to debate the next level of what-if … ATC demands a higher rate of descent for the non TCAS aircraft, then the whole gambit of time, reaction, acceleration, and TCAS counter-counter response starts again. The practical number of what-ifs depends on the total time span, which provides a limit to the scenario.
No doubt in theory a collision scenario can be generated, but to a simplistic pilot this seems most unlikely and thus we are required to trust the overall controller / ACAS system; ergo training / discipline.
The safety certification should show that the probability of the collision scenario is sufficiently extreme to discount it; thence it meets an acceptable level of safety. The principle of aviation safety is that it is not absolute; it accepts that rare events can occur, but the probability of them resulting in death has to be extremely remote (10-9).
Your approach appears to take the limit case as ‘the’ hazard without qualifying (bounding) it with the probability of its occurrence. Yes it is a real hazard, but if by following TCAS it only occurs in one manoeuvre in 10 million, then the industry accepts that following TCAS is the safer option.
Your example assumes one TCAS inoperative, that the strategic defence has already failed (ATC plan), and that the Controller’s command occurs after the TCAS warning. The latter involving a critical time span of approx 30sec.
Then there is the assumption that the two aircraft will manoeuvre precisely as required to result in a collision.
TCAS will command a minimum descent (1500ft/min ? (check spec values)), the manoeuvre assumes a standard crew response time and acceleration (2sec, 1.25g).
The aircraft responding to an urgent ATC command is not bounded by time or acceleration, but experience suggest that it will not be greater than the TCAS manoeuvre.
Thus there is a vast range of variables in both the start and action conditions that will not result in a collision.
However, given that the initial manoeuvres meet the collision criteria, then TCAS will continue to evaluate the situation and can increase the decent rate attempting to avoid the collision.
If you wish to debate the next level of what-if … ATC demands a higher rate of descent for the non TCAS aircraft, then the whole gambit of time, reaction, acceleration, and TCAS counter-counter response starts again. The practical number of what-ifs depends on the total time span, which provides a limit to the scenario.
No doubt in theory a collision scenario can be generated, but to a simplistic pilot this seems most unlikely and thus we are required to trust the overall controller / ACAS system; ergo training / discipline.
The safety certification should show that the probability of the collision scenario is sufficiently extreme to discount it; thence it meets an acceptable level of safety. The principle of aviation safety is that it is not absolute; it accepts that rare events can occur, but the probability of them resulting in death has to be extremely remote (10-9).
Your approach appears to take the limit case as ‘the’ hazard without qualifying (bounding) it with the probability of its occurrence. Yes it is a real hazard, but if by following TCAS it only occurs in one manoeuvre in 10 million, then the industry accepts that following TCAS is the safer option.
Join Date: Jul 2007
Location: Germany
Posts: 556
Likes: 0
Received 0 Likes
on
0 Posts
alf5071h,
It is extremely hard, to quantify the probabilities of very rare events.
But here is the reason why I think that it is not too far-fetched, and is a realistic, if rare, scenario to be considered when advocating "Always follow TCAS!".
It is essentially the same scenario as happened at Ueberlingen, the only difference being that at Ueberlingen, the Tupolev did get an RA, but chose to follow ATC instead. And that scenario did result in a collision. In that case, too, the Boeing's TCAS would have monitored the Tuploev's course, and it issued an increase-descend RA.
It has been debated why the Boeing's TCAS did not issue a reversal RA ("CLIMB NOW! CLIMB!") in this instance, as would seem appropriate.
While it may be true that this particular accident could have been avoided, if both pilots had followed their respective TCAS RAs, the Bashkirian crew's action were rational in light of the perceived three-aircraft-conflict.
But let's look at the decisions of the individuals involved in the modified setup:
- TCAS-aircraft crew:
Get TCAS RA (possibly after a TA)
SOP: follow ATC, except when getting in RA, in which case follow RA
Rational choices available: follow RA
Rational choice taken: follow RA.
- non-TCAS-aircraft crew:
Get ATC instruction
SOP: follow ATC, except when getting in RA, in which case follow RA
Rational choices available: follow ATC
Rational choice taken: follow ATC (in the absence of an RA)
- ATC:
notice conflicting traffic at the same FL
SOP: separate traffic
Rational choices available: issue climb or descend instructions to either of the involved aircraft
Rational choice taken: issue descend order to non-TCAS aircraft
So, ATC could take any of four different choices, but without knowing the circumstances, neither is favoured over the others (perhaps descend instructions are favoured, because it is unclear if the aircraft can climb quickly enough, or if it is even at its operational ceiling). Other factours may make the descend instruction to the non-TCAS aircraft the preferred one.
Are there data availabel on the availability of TCAS? What, thus, are the chances of two aircraft with inop TCAS "meet" each other?
Bernd
It is extremely hard, to quantify the probabilities of very rare events.
But here is the reason why I think that it is not too far-fetched, and is a realistic, if rare, scenario to be considered when advocating "Always follow TCAS!".
It is essentially the same scenario as happened at Ueberlingen, the only difference being that at Ueberlingen, the Tupolev did get an RA, but chose to follow ATC instead. And that scenario did result in a collision. In that case, too, the Boeing's TCAS would have monitored the Tuploev's course, and it issued an increase-descend RA.
It has been debated why the Boeing's TCAS did not issue a reversal RA ("CLIMB NOW! CLIMB!") in this instance, as would seem appropriate.
While it may be true that this particular accident could have been avoided, if both pilots had followed their respective TCAS RAs, the Bashkirian crew's action were rational in light of the perceived three-aircraft-conflict.
But let's look at the decisions of the individuals involved in the modified setup:
- TCAS-aircraft crew:
Get TCAS RA (possibly after a TA)
SOP: follow ATC, except when getting in RA, in which case follow RA
Rational choices available: follow RA
Rational choice taken: follow RA.
- non-TCAS-aircraft crew:
Get ATC instruction
SOP: follow ATC, except when getting in RA, in which case follow RA
Rational choices available: follow ATC
Rational choice taken: follow ATC (in the absence of an RA)
- ATC:
notice conflicting traffic at the same FL
SOP: separate traffic
Rational choices available: issue climb or descend instructions to either of the involved aircraft
Rational choice taken: issue descend order to non-TCAS aircraft
So, ATC could take any of four different choices, but without knowing the circumstances, neither is favoured over the others (perhaps descend instructions are favoured, because it is unclear if the aircraft can climb quickly enough, or if it is even at its operational ceiling). Other factours may make the descend instruction to the non-TCAS aircraft the preferred one.
Are there data availabel on the availability of TCAS? What, thus, are the chances of two aircraft with inop TCAS "meet" each other?
Bernd