ACAS again
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ACAS again
hello,
in many operations manuals it is said that, when receiving a TA, to try to see the traffic.
I wonder if that procedure is not somewhere contradictory with the basic philosophy "follow the RA anytime", since in giving more information to the pilot it could lead him to misuse that information and finally not to follow the RA.
(this is not a post regarding the need to follow or not the RA)
in many operations manuals it is said that, when receiving a TA, to try to see the traffic.
I wonder if that procedure is not somewhere contradictory with the basic philosophy "follow the RA anytime", since in giving more information to the pilot it could lead him to misuse that information and finally not to follow the RA.
(this is not a post regarding the need to follow or not the RA)
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re from captain87
When operating the TCAS in TA mode, a pilot should look out of the window in order to be better aware of the situation and this not because the TA informations and RA instructions are not sure ! The responsibility is always of the captain, who must always well-understand the instruments and computers. If we are flying into a "crowded" controlled-airspace, we must use ATC instructions to be separated from the traffic and TCAS for situation awareness ... if the TCAS gives us a "Traffic Alert" even if ATC do not, we must always follow TCAS. If we are flying in uncontrolled airspaces we use TCAS as the primary source of traffic informations and Visual Inspection for situation awareness ... TCAS informations have anytime priority over visual. I think this is the traffic-philosophy We have to follow anyway.
Cheers
by captain87
Cheers
by captain87
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TCAS II evolved from TCAS I, which only warned the pilots about conflicting traffic (TA). When advancing an existing system you seldomly begin from scratch but rather keep large portions of the old system and add components and methods which enhance the system performance.
In addition to this historical reason, the TA introduces one possibility to solve the conflict situation, when the crew contacts ATC and ATC acts accordingly.
Because of timing issues with radio the last point can become counterproductive when ATC gives a late advice which comes into conflict with TCAS RA.
In addition to this historical reason, the TA introduces one possibility to solve the conflict situation, when the crew contacts ATC and ATC acts accordingly.
Because of timing issues with radio the last point can become counterproductive when ATC gives a late advice which comes into conflict with TCAS RA.
I agree with avionneta’s concerns.
Even with the relatively low maturity of ACAS, its reliability in terms of avoiding error is better than the human visual system which can be fooled. ACAS may suffer from unnecessary alerts due to system limitations, but as far as I recall it does not suffer from illusions or misjudgement.
A recent example from a busy hold south of LHR on a misty day between cloud layers.
During a turn (looking out) with an aircraft crossing 1000ft below, the other aircraft appeared to be above our altitude and descending. This was due to the lack of a visual horizon and the substitution of our wing axis as the horizon. Thus, the instinctive visual manoeuvre would be to descend, which might then trigger a correct, but potentially confusing RA to climb. The situation could be worse at night where an aircraft’s outline is not visible and any manoeuvre would be flown on lights only – another source of visual error.
For those who would wish to argue that a manoeuvre in misty cloud layer conditions or at night should be flown on instruments, then (a) it is not a visual manoeuvre, and (b) if you are head down then use the ACAS display anyway and always follow the warnings.
Thus the Ops Manual advice is superfluous and could lead to error.
Even with the relatively low maturity of ACAS, its reliability in terms of avoiding error is better than the human visual system which can be fooled. ACAS may suffer from unnecessary alerts due to system limitations, but as far as I recall it does not suffer from illusions or misjudgement.
A recent example from a busy hold south of LHR on a misty day between cloud layers.
During a turn (looking out) with an aircraft crossing 1000ft below, the other aircraft appeared to be above our altitude and descending. This was due to the lack of a visual horizon and the substitution of our wing axis as the horizon. Thus, the instinctive visual manoeuvre would be to descend, which might then trigger a correct, but potentially confusing RA to climb. The situation could be worse at night where an aircraft’s outline is not visible and any manoeuvre would be flown on lights only – another source of visual error.
For those who would wish to argue that a manoeuvre in misty cloud layer conditions or at night should be flown on instruments, then (a) it is not a visual manoeuvre, and (b) if you are head down then use the ACAS display anyway and always follow the warnings.
Thus the Ops Manual advice is superfluous and could lead to error.
ICAO Required Operational Procedures for the Use of ACAS II
Just a couple of years ago, ICAO reviewed the contents of PANS-OPS and subsequently published advice as shown in the excerpt from Part III Chapter 3that follows:
3.2 USE OF ACAS INDICATIONS
The indications generated by ACAS shall be used by pilots in conformity with the following safety considerations:
a) pilots shall not manoeuvre their aircraft in response to traffic advisories (TAs) only;
Note 1.- TAs are intended to alert pilots to the possibility of a resolution advisory (RA), to enhance situational awareness, and to assist in visual acquisition of conflicting traffic. However, visually acquired traffic may not be the same traffic causing a TA. Visual perception of an encounter may be misleading, particularly at night.
Note 2.- The above restriction in the use of TAs is due to the limited bearing accuracy and to the difficulty in interpreting altitude rate from displayed traffic information.
b) on receipt of a TA, pilots shall use all information to prepare for appropriate action if an RA occurs; and
c) in the event of an RA, pilots shall:
1) respond immediately by following the RA as indicated, unless doing so would jeopardise the safety of the aeroplane;
etc, etc (seven additional instructions)
It might help if I explained that when I was writing guidance for UK flight crews (published in CAP 579) before the first UK aeroplanes actually had TCAS II installed, we felt it was important to ensure that pilots responded to any TAs that were posted ONLY (a) by preparing for the RA that might follow (eg hands on control column/ sidestick and on throttles/power levers) so that no time would be lost initiating the advised manouevre if an RA was posted, and (b) by looking out for the potentially conflicting traffic.
Because we pilots of civil aircraft are taught to assess abnormal situations that come to our attention before we react, rather than by simply responding without first assimilating all information available, the CAP advice included looking outside to possibly locate the intruder. If there was going to be any inaccuracy in the sense posted by the RA (unlikely, but possible IN THOSE EARLY YEARS), it would help the pilot flying the aeroplane to satisfy him or herself that any RA that followed the TA was not incorrect.
When TCAS II versions 6.02. 04 and 04A were installed (ie before version 7, henceforward described as ACAS II), there was a fairly high proportion of what could later be described as 'nuisance' RAs. Later, with version 7/ACAS II installed, this proportion reduced significantly. Thus, it now became more important to put the emphasis on responding without delay to RAs, having been primed to initiate that response by the TA that would normally precede it. Hence the ICAO caution about the possibility of mis-identifying the traffic that was causing the TA/RA. It is is simply that the emphasis has shifted now towards 'trust ACAS II to get it right' rather than 'confirm in your mind that TCAS II hasn't got it wrong'!
Ultimately, however, the pilot may disregard the RA (subject to the very important seven additional instructions in subparagraph c)) only when in his/her opinion 'doing so would jeaopardise the safety of the aeroplane'.
I hope that this helps anyone who reads this to understand the relative importance of a TA as described by ICAO.
3.2 USE OF ACAS INDICATIONS
The indications generated by ACAS shall be used by pilots in conformity with the following safety considerations:
a) pilots shall not manoeuvre their aircraft in response to traffic advisories (TAs) only;
Note 1.- TAs are intended to alert pilots to the possibility of a resolution advisory (RA), to enhance situational awareness, and to assist in visual acquisition of conflicting traffic. However, visually acquired traffic may not be the same traffic causing a TA. Visual perception of an encounter may be misleading, particularly at night.
Note 2.- The above restriction in the use of TAs is due to the limited bearing accuracy and to the difficulty in interpreting altitude rate from displayed traffic information.
b) on receipt of a TA, pilots shall use all information to prepare for appropriate action if an RA occurs; and
c) in the event of an RA, pilots shall:
1) respond immediately by following the RA as indicated, unless doing so would jeopardise the safety of the aeroplane;
etc, etc (seven additional instructions)
It might help if I explained that when I was writing guidance for UK flight crews (published in CAP 579) before the first UK aeroplanes actually had TCAS II installed, we felt it was important to ensure that pilots responded to any TAs that were posted ONLY (a) by preparing for the RA that might follow (eg hands on control column/ sidestick and on throttles/power levers) so that no time would be lost initiating the advised manouevre if an RA was posted, and (b) by looking out for the potentially conflicting traffic.
Because we pilots of civil aircraft are taught to assess abnormal situations that come to our attention before we react, rather than by simply responding without first assimilating all information available, the CAP advice included looking outside to possibly locate the intruder. If there was going to be any inaccuracy in the sense posted by the RA (unlikely, but possible IN THOSE EARLY YEARS), it would help the pilot flying the aeroplane to satisfy him or herself that any RA that followed the TA was not incorrect.
When TCAS II versions 6.02. 04 and 04A were installed (ie before version 7, henceforward described as ACAS II), there was a fairly high proportion of what could later be described as 'nuisance' RAs. Later, with version 7/ACAS II installed, this proportion reduced significantly. Thus, it now became more important to put the emphasis on responding without delay to RAs, having been primed to initiate that response by the TA that would normally precede it. Hence the ICAO caution about the possibility of mis-identifying the traffic that was causing the TA/RA. It is is simply that the emphasis has shifted now towards 'trust ACAS II to get it right' rather than 'confirm in your mind that TCAS II hasn't got it wrong'!
Ultimately, however, the pilot may disregard the RA (subject to the very important seven additional instructions in subparagraph c)) only when in his/her opinion 'doing so would jeaopardise the safety of the aeroplane'.
I hope that this helps anyone who reads this to understand the relative importance of a TA as described by ICAO.
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I must conclude that due to history, that mention "acquire visual" is still here but shouldn't be and pilots not aware of the new philosophy (most of the time not thorougly mentionned in operation manuals") might still use their own perception to go against RA orders, right ?
nice isn't it ?
nice isn't it ?
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I think it would be a good idea to visually aquire the other aircraft. What guarantee do you have that there will be coordinated RA's. You may have to visually manouver(or attempt to do so) around the other aircraft. Perhaps he will climb while you climb as well. Why, a visual illusion maybe. He may be ignoring his RA. Or maybe he isn't getting an RA because he is not TCAS equipped or his TCAS is unserviceable due to a unit defect or a defect of equipment that inputs information to the TCAS unit or improper equipment usage.
Many times you will get a TA for non mode C equipped aircraft. Therefore all you will ever get is a TA for traffic that may be on a collision course. So take a look. You will anyway if weather conditions permit.
Many times you will get a TA for non mode C equipped aircraft. Therefore all you will ever get is a TA for traffic that may be on a collision course. So take a look. You will anyway if weather conditions permit.
Last edited by punkalouver; 3rd Jan 2008 at 12:05.
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is it not standard for most operators with TCAS I/II, to follow the RA's then look for the traffic? since RA's tend to change in regards to the traffic?
just wondering...
just wondering...
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FD (the un-real)
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I must conclude that due to history, that mention "acquire visual" is still here but shouldn't be and pilots not aware of the new philosophy (most of the time not thorougly mentionned in operation manuals") might still use their own perception to go against RA orders, right ?
nice isn't it ?
nice isn't it ?
The problem with all of this is trying to apply a 'one size fits all' philosophy to collison avoidance.
Yes, its true, manouvres should definitely not be made solely on the basis of TAs.
It is however entirely possible that getting a visual spot on the other traffic might save the day.
What's needed is a sound appreciation of the relative merits of how you can avoid hitting another aircraft. Sometimes a turn is a sensible option, but it needs to be based on a visual spot rather than the traffic display.
Before someone jumps in and says that this suggestion is insane, and then gives an example when it would be, I accept that. The point is that one size does not fit all, and the suitable options depend on the circumstances; including of course that if an RA is given it must (if possible) be followed and must certainly never be manouevred against.
The most obvious example of a situationwhere a turn should be flown is the non altitude reporting intruder. On my 3rd most recent flight, a light aircraft blundered into my approach path. It was not altitude reporting. It was observed on the TCAS TD, and on that basis spotted visually, and on that basis I was just about to manouvre when it beat me to it (rolling on 80 degrees AOB and pulling like a bastard!). If it hadn't manouevred, and I had not spotted it, but rather just concentrated on my PFD waiting for the RA that was never going to come.....
People should receive theoretical training in the judgement of visual manoeuvres, for 2 reasons:
1. For the scenario above.
2. To learn how useless we are at judging vertical manoeuvres; to provide additional understanding of the reason why an RA must not be second guessed, thus reinforcing the need to follow RAs.
In summary the reasons for attempting a visual spot are valid as part of an overall strategy for collision avoidance. The reason its suggested is just as valid as the reason why an RA must be followed.
pb
