AF 447 Thread No. 6
DW
Hi DW, being evasive?
I described a flight phase as opposite to another flight phase (like one being close to the ground) where autotrim could help in Alt2 even at the extremes of the flight envelope (i´m not saying it would). My above statement stays. Should have explained to you, looks like.
Is it of relevance to this discussion? You may accept, that I look at it from an operators point of view. If the system is able to prevent trimming into a stall in ALT1 and Direct Law, it should be able to design one, that can do the same in ALT2.
But you did not answer my question.
You don´t need to explain how it is done, the technical aspect does not matter to the operator. Graceful would be logical, but this degradation is not.
is exactly my point. Let Pete know, that autotrimming into a stall is no good idea, like Pete knows in Alt1 at Vprot and in direct law. Implement it, however you do it.
The original sense of trim is to aleviate loads on the system after a flightpath change / change of loadfactor has been achieved (long term).
FCPC will always try to hold a load factor demand, by using elevators in short term and autotrim in longterm. How to disable the elevators during manual trim? With SS, I know, but if properly used manual trim would not have been necessary in the first place.
But if the system would have reverted to manual trim like in ALT2 at Vprot or like in Direct LAW, THS would not have been trimmed Full NU by the FCPC via SS inputs.
You know very well that i´m fully aware about the functioning and the necessity of autotrim and do not question it. I even find it a clever and well thought out system. But the situation developing in ALT2 with AF447 was not being expected somehow and needs to be addressed and changed. It works in Alt1 at Vprot, why not do the same in Alt2? Nobody seems to be concerned to hinder autotrim in Direct Law, but you explain it would be difficult for the crew if autotrim would be hindered in Alt2 when predesigned values (aoa, speed, Trim value, take whatever would suit yourself) are reached?
You are going flying? Pull on the stick and start trimming while pulling (hope you have your chute with you. No need to use a Ouija board.
Overall design has to follow function, not vice versa.
Originally Posted by RetiredF4
there is no situation in cruise, where autotrim is improving a situation in ALT2 like that encountered by AF 447.
Reply by DW
The FCU has no concept of "cruise", as far as I am aware. It is a real-time processing system that does the job it was designed to do very well, but it is intentionally quite a simple beast in terms of design (because the simpler a system is, the less things there are to go wrong).
there is no situation in cruise, where autotrim is improving a situation in ALT2 like that encountered by AF 447.
Reply by DW
The FCU has no concept of "cruise", as far as I am aware. It is a real-time processing system that does the job it was designed to do very well, but it is intentionally quite a simple beast in terms of design (because the simpler a system is, the less things there are to go wrong).
Originally Posted by RetiredF4
If you can think of one, let me know. As we know, autotrim is inhibited in ALt1 at V-prot anyway and in direct law as well.
Reply by DW
You have to look at this from a systems perspective to understand. Autotrim as a system is never "inhibited" in any law other than Direct and Manual Trim Only - it is the *protections* that prohibit the aircraft from leaving the flight envelope by preventing any commands - either manual or automatic - from doing so, and if necessary providing corrective commands to keep the aircraft within the flight envelope. The protections are a completely discrete subsystem that is loosely-coupled to the others that make up the FCU system. .
If you can think of one, let me know. As we know, autotrim is inhibited in ALt1 at V-prot anyway and in direct law as well.
Reply by DW
You have to look at this from a systems perspective to understand. Autotrim as a system is never "inhibited" in any law other than Direct and Manual Trim Only - it is the *protections* that prohibit the aircraft from leaving the flight envelope by preventing any commands - either manual or automatic - from doing so, and if necessary providing corrective commands to keep the aircraft within the flight envelope. The protections are a completely discrete subsystem that is loosely-coupled to the others that make up the FCU system. .
But you did not answer my question.
Originally Posted by RetiredF4
Concerning your graceful degradation explain this degradation with autotrim of the THS:
In Normal LAW autotrim (a rally nice feature)
In ALT1 Law autotrim prohibit at VC-prot (looks sound to protect from entering unknown territory)
In ALT2 Law autotrim (that is your part to explain)
In Direct LAW autotrim off (looks sound to protect from entering unknown territory)
Reply by DW
Again, that is not correct if you look at the architecture as a whole. I'll repeat for clarity - Autotrim as a system is not "prohibited", nor "turned off" in any other mode or Law than Direct or Manual Trim mode. If the protections are active, then the autotrim commands will be treated as any other command that takes the aircraft out of the flight envelope and corrected accordingly. Think of it as two separate processes running alongside each other rather than as an integrated whole.
Put even more simply, imagine two people on either side of a wall that has a two-handled saw poking through it. The person on one side (let's call him Otto Trim) is told to push the saw forward and the person on the other side (who is physically stronger and called Pete Tection) is told to not let the saw through past, say, two-thirds of it's length. Pete will always stop the saw at the limit and will try to return it to the prescribed position if it goes past, but he is not explicitly aware that Otto's on the other side trying to push it because the wall is in the way, and as such does not interfere or communicate with Otto directly - all Pete knows is that he mustn't let it through past a certain point.).
Concerning your graceful degradation explain this degradation with autotrim of the THS:
In Normal LAW autotrim (a rally nice feature)
In ALT1 Law autotrim prohibit at VC-prot (looks sound to protect from entering unknown territory)
In ALT2 Law autotrim (that is your part to explain)
In Direct LAW autotrim off (looks sound to protect from entering unknown territory)
Reply by DW
Again, that is not correct if you look at the architecture as a whole. I'll repeat for clarity - Autotrim as a system is not "prohibited", nor "turned off" in any other mode or Law than Direct or Manual Trim mode. If the protections are active, then the autotrim commands will be treated as any other command that takes the aircraft out of the flight envelope and corrected accordingly. Think of it as two separate processes running alongside each other rather than as an integrated whole.
Put even more simply, imagine two people on either side of a wall that has a two-handled saw poking through it. The person on one side (let's call him Otto Trim) is told to push the saw forward and the person on the other side (who is physically stronger and called Pete Tection) is told to not let the saw through past, say, two-thirds of it's length. Pete will always stop the saw at the limit and will try to return it to the prescribed position if it goes past, but he is not explicitly aware that Otto's on the other side trying to push it because the wall is in the way, and as such does not interfere or communicate with Otto directly - all Pete knows is that he mustn't let it through past a certain point.).
- all Pete knows is that he mustn't let it through past a certain point. ).
Originally Posted by RetiredF4
That should straighten up the manual trim option once and for all. It´s even worse, because the functioning autotrim in ALT 2 prevents manual trim in its original sense of implementation.
Reply by DW
What do you mean by "original sense of implementation"?
In any case, using manual trim and then holding on to the wheel will prevent the autotrim from re-engaging.
That should straighten up the manual trim option once and for all. It´s even worse, because the functioning autotrim in ALT 2 prevents manual trim in its original sense of implementation.
Reply by DW
What do you mean by "original sense of implementation"?
In any case, using manual trim and then holding on to the wheel will prevent the autotrim from re-engaging.
FCPC will always try to hold a load factor demand, by using elevators in short term and autotrim in longterm. How to disable the elevators during manual trim? With SS, I know, but if properly used manual trim would not have been necessary in the first place.
But if the system would have reverted to manual trim like in ALT2 at Vprot or like in Direct LAW, THS would not have been trimmed Full NU by the FCPC via SS inputs.
Originally Posted by RetiredF4
There might be pilots insane enough to pull on the yoke or sidestick in conventional and FBW aircraft until the aircraft stalls, but no one would trim while pulling.
"No pilot would ever..." is an impossible statement to prove.
To understand that above statement let me explain normal trim behaviour. With the intention to climb the pilot uses the yoke to change the pitch, once that change is reached he uses the trim to get rid of the pressure on the yoke. The trim comes into play when the change is achieved and not in the timeframe, where the change takes place.
Reply by DW
Here there is no tactile feedback, so "trimming to the pressure" is impossible, and performing the same thing visually using the ADI as reference would be physically exhausting on a day-in, day-out basis. This was part of the reason autotrim was developed because the flight control design was a completely new paradigm.
There might be pilots insane enough to pull on the yoke or sidestick in conventional and FBW aircraft until the aircraft stalls, but no one would trim while pulling.
"No pilot would ever..." is an impossible statement to prove.
To understand that above statement let me explain normal trim behaviour. With the intention to climb the pilot uses the yoke to change the pitch, once that change is reached he uses the trim to get rid of the pressure on the yoke. The trim comes into play when the change is achieved and not in the timeframe, where the change takes place.
Reply by DW
Here there is no tactile feedback, so "trimming to the pressure" is impossible, and performing the same thing visually using the ADI as reference would be physically exhausting on a day-in, day-out basis. This was part of the reason autotrim was developed because the flight control design was a completely new paradigm.
Originally Posted by RetiredF4
In case of AF447 without autotrim the pilot would never ever have tried to achieve the desired flight path change with manual trim, because that is not the way to do it.
Reply by DW
The only way you could possibly know that is with a Ouija board.
If all goes to plan I'm going to be doing some exciting research this weekend and I'll be able to argue from a much surer footing. If it turns out I've been wrong about anything you guys will be the first to know.
In case of AF447 without autotrim the pilot would never ever have tried to achieve the desired flight path change with manual trim, because that is not the way to do it.
Reply by DW
The only way you could possibly know that is with a Ouija board.
If all goes to plan I'm going to be doing some exciting research this weekend and I'll be able to argue from a much surer footing. If it turns out I've been wrong about anything you guys will be the first to know.
Overall design has to follow function, not vice versa.
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Those who think yokes should be the cure-all for what the other guy is doing should look at this accident originally posted by Netstruggler in the current Rumors & News AF447 thread.
http://libraryonline.erau.edu/online...s/AAR97-05.pdf
http://libraryonline.erau.edu/online...s/AAR97-05.pdf
That report was also referred to way back in this thread: http://www.pprune.org/tech-log/17316...ver-stall.html
Post #18 (from Old Smokey) on that thread is particularly precient regarding the stall training issue. Since then, Colgan, Perpignan, Schipol, 447 etc. and finally in 2011 maybe something is getting done about stall training.
Some other bits from the report:
Following a December 20, 1995, fatal accident involving an American Airlines
(AAL) B-757 near Buga, Colombia, the Safety Board recommended that the FAA:
A-96-94
Require that all transport-category aircraft present pilots with angle of attack30
information in a visual format...
(AAL) B-757 near Buga, Colombia, the Safety Board recommended that the FAA:
A-96-94
Require that all transport-category aircraft present pilots with angle of attack30
information in a visual format...
the Safety Board concludes that this
accident might have been prevented if the flightcrew had been provided a clear, direct indication
of the airplane’s angle of attack. Therefore, the Safety Board reiterates Safety Recommendation
A-96-94.
accident might have been prevented if the flightcrew had been provided a clear, direct indication
of the airplane’s angle of attack. Therefore, the Safety Board reiterates Safety Recommendation
A-96-94.
The crew never formally identified the stall situation. Information on angle of attack is not
directly accessible to pilots. ... Only a direct readout of the angle of attack could enable crews to rapidly identify the aerodynamic situation of the airplane and take the actions that may be required.
Consequently, the BEA recommends:
that EASA and the FAA evaluate the relevance of requiring the presence of an
angle of attack indicator directly accessible to pilots on board airplanes.
directly accessible to pilots. ... Only a direct readout of the angle of attack could enable crews to rapidly identify the aerodynamic situation of the airplane and take the actions that may be required.
Consequently, the BEA recommends:
that EASA and the FAA evaluate the relevance of requiring the presence of an
angle of attack indicator directly accessible to pilots on board airplanes.
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You know very well that i´m fully aware about the functioning and the necessity of autotrim and do not question it. I even find it a clever and well thought out system. But the situation developing in ALT2 with AF447 was not being expected somehow and needs to be addressed and changed. It works in Alt1 at Vprot, why not do the same in Alt2? Nobody seems to be concerned to hinder autotrim in Direct Law, but you explain it would be difficult for the crew if autotrim would be hindered in Alt2 when predesigned values (aoa, speed, Trim value, take whatever would suit yourself) are reached?
The big problem with what you suggest is that the plane is in Alt22 because it doesn't have trustworthy values for speed, aoa and possibly other airdata. So what should it stop trimming based on ?
The whole reason the protections drop out is because it has been held to be more dangerous to "protect" based on invalid (or not trusted) data, than to hand full unprotected control to the pilot. I suspect this is a certification issue. Boeing FBW is exactly the same (not sure about the newer bizjets).
That whole assumption may need to be challenged now - possibly we're reaching the point where safety will be improved by systems overriding / protecting pilots even based on known-bad airdata. Be careful what you wish for...
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Stall warning shouldn't happen if everything is working, but in fact can and does happen in normal law. If it does, at least in some (possibly all) cases it is a signal to the systems that something is wrong (maybe airdata), and normal law will then drop out.
So, SW more or less inhibits normal law, not the other way round.
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When the wings started rocking-it was like they had both forgotten all they should know.
When the wings rock, you have to reduce AOA if you want to be in charge of a flying machine. Otherwise, you are just a passenger.
I thought all real pilots knew that instinctively.
Thrustworthy values
infrequentflyer789
The big problem with what you suggest is that the plane is in Alt22 because it doesn't have trustworthy values for speed, aoa and possibly other airdata. So what should it stop trimming based on ?
The big problem with what you suggest is that the plane is in Alt22 because it doesn't have trustworthy values for speed, aoa and possibly other airdata. So what should it stop trimming based on ?
With thrustworthy values the protections would stop the trim reaching special values , without those values it will continue to trim also it can cause harm.
Where is the logic?
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Infrequentflyer789
Yes, SW inhibits Normal Law, then. Do you have a thought as to the relevance of degrade at the Initial SW just after handoff? Are you convinced the ADRdoubled out and that was the immediate cause of ALT2? The SW had no effect? Any chance that the SW chirped and was silenced as in Protection? Which? If any?
Yes, SW inhibits Normal Law, then. Do you have a thought as to the relevance of degrade at the Initial SW just after handoff? Are you convinced the ADRdoubled out and that was the immediate cause of ALT2? The SW had no effect? Any chance that the SW chirped and was silenced as in Protection? Which? If any?
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It cannot logically limit trim to "safe" values if it no longer knows what those values are.
Autotrim didn't down this a/c - it was stalled by elevator alone and elevators were held nose-up all the way down. There was no diagnosis of stall let alone recovery attempt.
In contrast, the cessation of autotrim when things start to go wrong is a least a contributory factor in some crashes, if not a killer.
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@Franzl
Autotrim is a separate system from the protections and the autoflight, as such under manual control and without protections it does exactly what the pilot commands. It does exactly what it says on the tin and trims automatically in the same way that manual trim is applied on older aircraft - there have never been any hard limits on manual trim wheels, why should there be one on their modern automatic descendents?
In this case we have one incident where possibly as a result of panic or poor training the PF commanded the autotrim - *after* the aircraft had already stalled - into an inappropriate position. I can say with some confidence that had the PF realised what was happening and pushed forward for an equivalent time to the amount he had previously been pulling back, the trim would have righted and control would have been regained. One incident in however many tens or hundreds of thousands of flights that this type and it's sister ships have made safely and without incident every day.
As such, it's far more realistic to train pilots to understand what the autotrim does as part of their type conversion (over and above never making and holding large control inputs for more than a second or two) than it is to redesign a system that works as well, if not better that it's manual predecessors and end up sacrificing pilot authority in the process.
@infrequentflyer789 - While you are correct in asserting that one should not hear the Stall Warning in Normal Law, and as such hearing it is an indication that something has gone wrong - as I understand it, the warning and annunciator system is not directly connected to the flight control logic - so it is never switched off or suppressed by anything directly. The FCU is a network of loosely-coupled, real-time processes with a degree of redundancy built in - not a monolithic, closely-coupled design.
Autotrim is a separate system from the protections and the autoflight, as such under manual control and without protections it does exactly what the pilot commands. It does exactly what it says on the tin and trims automatically in the same way that manual trim is applied on older aircraft - there have never been any hard limits on manual trim wheels, why should there be one on their modern automatic descendents?
In this case we have one incident where possibly as a result of panic or poor training the PF commanded the autotrim - *after* the aircraft had already stalled - into an inappropriate position. I can say with some confidence that had the PF realised what was happening and pushed forward for an equivalent time to the amount he had previously been pulling back, the trim would have righted and control would have been regained. One incident in however many tens or hundreds of thousands of flights that this type and it's sister ships have made safely and without incident every day.
As such, it's far more realistic to train pilots to understand what the autotrim does as part of their type conversion (over and above never making and holding large control inputs for more than a second or two) than it is to redesign a system that works as well, if not better that it's manual predecessors and end up sacrificing pilot authority in the process.
@infrequentflyer789 - While you are correct in asserting that one should not hear the Stall Warning in Normal Law, and as such hearing it is an indication that something has gone wrong - as I understand it, the warning and annunciator system is not directly connected to the flight control logic - so it is never switched off or suppressed by anything directly. The FCU is a network of loosely-coupled, real-time processes with a degree of redundancy built in - not a monolithic, closely-coupled design.
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Hi DozyWannabe,
FCOM: DSC Aircraft Systems, 27, 20, Alternate Law:
"In addition, audio stall warnings (crickets + “STALL” synthetic voice message) is activated at an appropriate margin from the stall condition."
This seems to suggest that the "Stall" warning is only activated when Normal Law is no longer active.
the warning and annunciator system is not directly connected to the flight control logic - so it is never switched off or suppressed by anything directly.
"In addition, audio stall warnings (crickets + “STALL” synthetic voice message) is activated at an appropriate margin from the stall condition."
This seems to suggest that the "Stall" warning is only activated when Normal Law is no longer active.
DW
@Franzl
Autotrim is a separate system from the protections and the autoflight, as such under manual control and without protections it does exactly what the pilot commands. It does exactly what it says on the tin and trims automatically in the same way that manual trim is applied on older aircraft - there have never been any hard limits on manual trim wheels, why should there be one on their modern automatic descendents?
That is quite a difference to your saying and to older conventional aircraft. The FCPC will trim the THS all the way nose up with the stick untouched, if in 1 g flight (like in a climb) the speed decays. We discussed that before more then once.
And it is an operating difference as well. Trimming by turning a wheel is a deliberate act to trim, whereas automatic trim by the FCPC is a programmed syytem behaviour. Wether that is a seperate system or an incorporated one is of no relevance at all. Therefore there can be measures incorporated to prevent the trim reaching values, which are insane in FL350. There are enough data available after a pitot failure (altitude, attitude, AOA, GPS data, you name some more) to compute a normal trim zone (with trimming by the FCPC) and one where the pilot should decide wether he really wants the trim that far up by turning the wheel.
@ infrequentflyer
In contrast, the cessation of autotrim when things start to go wrong is a least a contributory factor in some crashes, if not a killer.
In contrast, the cessation of autotrim when things start to go wrong is a least a contributory factor in some crashes, if not a killer.
If ceasing of trimming action in ALT1 is not a bad thing, i cannot follow the argument, why it would be bad in ALT 2.
What trim values are reached in normal operation except takeoff and landing?
Is Full NU a player above FL100 in any kind of normal maneuvering? In which one?
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@infrequentflyer789 - While you are correct in asserting that one should not hear the Stall Warning in Normal Law, and as such hearing it is an indication that something has gone wrong - as I understand it, the warning and annunciator system is not directly connected to the flight control logic - so it is never switched off or suppressed by anything directly. The FCU is a network of loosely-coupled, real-time processes with a degree of redundancy built in - not a monolithic, closely-coupled design.
It's probably possible to take a broken bus and fly to SW in normal law and then recover without leaving normal law. Not sure I'd want to be in the back when someone tried it though. The guys at Perpignan were in direct law within 10secs of SW - see p112 (english version) of final report.
@rudderrudderrat:
See the traces on page 112 of the Perpignan report - SW clearly in normal law.
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@Franzl
You're comparing apples with oranges here, or to be more literal, comparing the conventional, or "way we used to do it" method with the method on the FBW Airbus series, which is intended to be used in a completely different way - unless something has gone so seriously wrong with the aircraft that a drop to Direct Law is necesssary.
The FCPCs may command the physical movements, but it is the pilot that commands the FCPCs in manual flight. As the speed decays under Normal Law, the elevator/trim effect is limited by the AoA protections. Under all load factor laws (i.e everything above Direct) - even without protections, as the speed decays the control mode gradually transfers from "G" command to pitch command, meaning that the autotrim will not trim to stall with stick neutral due to decaying speed alone.
In this case the autotrim was commanded to move as it did to support the demands on the elevators made by the PF, who was - possibly for reasons we will never know - holding the thing between 50-100% back for the majority of the accident sequence. It was for this reason as well that the speed decayed, so in effect both the pitch and airspeed factors of the extreme angle of attack leading to and during the stall sequence were because of the consistent back-pressure on the stick, and for no other reason that I can see.
You're comparing apples with oranges here, or to be more literal, comparing the conventional, or "way we used to do it" method with the method on the FBW Airbus series, which is intended to be used in a completely different way - unless something has gone so seriously wrong with the aircraft that a drop to Direct Law is necesssary.
The FCPCs may command the physical movements, but it is the pilot that commands the FCPCs in manual flight. As the speed decays under Normal Law, the elevator/trim effect is limited by the AoA protections. Under all load factor laws (i.e everything above Direct) - even without protections, as the speed decays the control mode gradually transfers from "G" command to pitch command, meaning that the autotrim will not trim to stall with stick neutral due to decaying speed alone.
In this case the autotrim was commanded to move as it did to support the demands on the elevators made by the PF, who was - possibly for reasons we will never know - holding the thing between 50-100% back for the majority of the accident sequence. It was for this reason as well that the speed decayed, so in effect both the pitch and airspeed factors of the extreme angle of attack leading to and during the stall sequence were because of the consistent back-pressure on the stick, and for no other reason that I can see.
Last edited by DozyWannabe; 1st Nov 2011 at 23:48.
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It would have been better if the broken thing wasn't broken, but it was. Thereafter the loss of autotrim, coupled with lack of crew re-trim, caused deaths. Having got to that situation, it would have been better if autotrim had stayed engaged so that the trim did not impede the recovery.
If ceasing of trimming action in ALT1 is not a bad thing, i cannot follow the argument, why it would be bad in ALT 2.
What trim values are reached in normal operation except takeoff and landing?
Is Full NU a player above FL100 in any kind of normal maneuvering? In which one?
What trim values are reached in normal operation except takeoff and landing?
Is Full NU a player above FL100 in any kind of normal maneuvering? In which one?
As to what trim values are normally used, I don't know - but I doubt the designers put the mechanical end-stops where they are without reason. If no more than, say, 9deg nose up was ever needed, then the stop would have been at 9deg not 13.
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Airbus went to great lengths to design their FBW system to fly like a normal airplane. But in reality, the FBW Bus's do NOT FLY LIKE NORMAL AIRCRAFT. Auto trim proves that point. The Bus trims for "G", and normal aircraft trim for speed.
Airbus, EASA, FAA, etc, could fix all our problems with flying these computer games with one simple directive; change the training syllabus to state the opposite of what it states at present. Change "it fly's like any other airplane", to " it fly's like NO other airplane"........and train accordingly. Every Airbus pilot should be exposed to degraded flight characteristics at every training event, in addition to all other required maneuvers.
Airbus, EASA, FAA, etc, could fix all our problems with flying these computer games with one simple directive; change the training syllabus to state the opposite of what it states at present. Change "it fly's like any other airplane", to " it fly's like NO other airplane"........and train accordingly. Every Airbus pilot should be exposed to degraded flight characteristics at every training event, in addition to all other required maneuvers.
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I think you'll find that as speed decays, more elevator / trim will be needed to maintain stable attitude, and autotrim will happily provide it - up to the stops.
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Infrequentflyer789
Yes, SW inhibits Normal Law, then. Do you have a thought as to the relevance of degrade at the Initial SW just after handoff? Are you convinced the ADRdoubled out and that was the immediate cause of ALT2? The SW had no effect? Any chance that the SW chirped and was silenced as in Protection? Which? If any?
Yes, SW inhibits Normal Law, then. Do you have a thought as to the relevance of degrade at the Initial SW just after handoff? Are you convinced the ADRdoubled out and that was the immediate cause of ALT2? The SW had no effect? Any chance that the SW chirped and was silenced as in Protection? Which? If any?
UAS -> A/P drop & Alt law -> stick-back -> elevator up -> acceleration up -> AOA up -> SW
Causality is not in doubt.
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@if789
Wasn't the pitot-static system effectively completely hosed in the Perpignan case though (the systems must have been very badly compromised)? I must confess I didn't have an opportunity to follow that one closely at the time.
Wasn't the pitot-static system effectively completely hosed in the Perpignan case though (the systems must have been very badly compromised)? I must confess I didn't have an opportunity to follow that one closely at the time.