AF 447 Thread No. 12
Chris, thanks for your reply #1157.
I had overlooked the first point, but the recovery technique is important and is directly associated with perception.
Much of this thread has been concerned with 'why didn't the crew know that they were stalled' - disbelief, biased by hindsight, and now because 'we' know the aircraft was stalled, all that is required is recovery.
RAT 5 - I judge not, why would VMC change the crew's perception, would this be sufficiently attention-getting to change the focus of attention - would the crew have looked out given the mental workload and puzzling flight deck warnings?
Uplinker's experience demonstrates the difficulties in understanding the situation.
Classic certification requires stall warning and then stall ident. 'Warning' (stick shake) requires interpretation for awareness, but 'Ident' (stick push)- after Vss awareness, is a direct indication of recovery action, don't interpret; act.
I suspect that Airbus argued that in protected aircraft these functions are not required, and/or the EFIS display supplemented awareness, particularly if the protections degraded. However, neither were available/reliable in this accident, thus the modifications primarily address awareness.
I don't know the details of the Airbus AoA - speed computation, but other systems (MD 80, Avro RJ) were sufficiently accurate for awareness, and that the computation accommodated different weights and 'g', but neither were the primary reference for certification.
My dated military AoA experience suggests that it was primarily a warning opposed to stall identification, not used to direct recovery; constant AoA for approach, or maximising performance, do not exceed the limit ... ... loss of control, losing the fight.
I had overlooked the first point, but the recovery technique is important and is directly associated with perception.
Much of this thread has been concerned with 'why didn't the crew know that they were stalled' - disbelief, biased by hindsight, and now because 'we' know the aircraft was stalled, all that is required is recovery.
RAT 5 - I judge not, why would VMC change the crew's perception, would this be sufficiently attention-getting to change the focus of attention - would the crew have looked out given the mental workload and puzzling flight deck warnings?
Uplinker's experience demonstrates the difficulties in understanding the situation.
Classic certification requires stall warning and then stall ident. 'Warning' (stick shake) requires interpretation for awareness, but 'Ident' (stick push)- after Vss awareness, is a direct indication of recovery action, don't interpret; act.
I suspect that Airbus argued that in protected aircraft these functions are not required, and/or the EFIS display supplemented awareness, particularly if the protections degraded. However, neither were available/reliable in this accident, thus the modifications primarily address awareness.
I don't know the details of the Airbus AoA - speed computation, but other systems (MD 80, Avro RJ) were sufficiently accurate for awareness, and that the computation accommodated different weights and 'g', but neither were the primary reference for certification.
My dated military AoA experience suggests that it was primarily a warning opposed to stall identification, not used to direct recovery; constant AoA for approach, or maximising performance, do not exceed the limit ... ... loss of control, losing the fight.
IcePack, what you report might not be negative training, instead it reflects the choice of scenario and training expectation.
The simulator may well replicate a 'normal' stall, it would be surprising if it did not; however if you wish to replicate AF447 then the abnormal aircraft state has to be considered, particularly the trim setting, systems operation, and control input.
As discussed this is an example of point specific training - a stall as per AF447, opposed to training for low speed awareness, stall avoidance, or identification that the aircraft has stalled because of a surprising upset or self inflicted manoeuvre..
Most refresher stall training, simulator or real aircraft, adds little training value for minimising the risk in surprising upsets. In most scenarios the situation is 'framed' - i.e. 'stall training'; there is no difficulty or ambiguity in establishing the situation because you have already have the expectation of the event. Hence the value of Uplinker's 'surprising' experience
The simulator may well replicate a 'normal' stall, it would be surprising if it did not; however if you wish to replicate AF447 then the abnormal aircraft state has to be considered, particularly the trim setting, systems operation, and control input.
As discussed this is an example of point specific training - a stall as per AF447, opposed to training for low speed awareness, stall avoidance, or identification that the aircraft has stalled because of a surprising upset or self inflicted manoeuvre..
Most refresher stall training, simulator or real aircraft, adds little training value for minimising the risk in surprising upsets. In most scenarios the situation is 'framed' - i.e. 'stall training'; there is no difficulty or ambiguity in establishing the situation because you have already have the expectation of the event. Hence the value of Uplinker's 'surprising' experience
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Regarding the VMC question: I doubt, in a low-level stall, it would have had much effect; time would have been very compressed and attention fixed inside. At FL350 there is plenty of time to allow you attention to search for all clues available. A V.2004 says the high nose attitude might have been easier to understand with an horizon in your peripheral vision. It would have been an extra sense when you could use all the help you could get in a moment of confusion.
The horizon provides a reference for attitude, it does not indicate a stall. Depending on aircraft flap, cg, thrust, the aircraft can stall at a range of attitudes, e.g. normal training stall against the AF447 stall, or a stall in a climb (nose high attitude) with a high thrust setting opposed to an approach stall (nose low attitude).
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Probably you will find that post unrealistic and excessive but I try it to suggest more imaginative solutions to blocked situation and discussion . Sorry.
We are worrying from no real inflight stalls by test pilots. But the aircraft seems able to stall without damage. Why couldn't the manufacturer build a mimick without pilot to achieve these stalls until we learn enough to fly the plane like former wholly tested planes ? Perhaps my suggestion is crazy
So I already apologize...
We are worrying from no real inflight stalls by test pilots. But the aircraft seems able to stall without damage. Why couldn't the manufacturer build a mimick without pilot to achieve these stalls until we learn enough to fly the plane like former wholly tested planes ? Perhaps my suggestion is crazy
So I already apologize...
Hello RAT 5 and vapilot2004,
IIRC, both VMC and horizons are fairly nebulous concepts at FL350. Ever tried flying straight and level at that altitude without an artificial horizon?
What I would say is that perhaps one is more susceptible to panic in darkness, particularly in the early hours. And yes, as Mr Optimistic implies, clouds can be more helpful than empty space.
IIRC, both VMC and horizons are fairly nebulous concepts at FL350. Ever tried flying straight and level at that altitude without an artificial horizon?
What I would say is that perhaps one is more susceptible to panic in darkness, particularly in the early hours. And yes, as Mr Optimistic implies, clouds can be more helpful than empty space.
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What I would say is that perhaps one is more susceptible to panic in darkness, particularly in the early hours.
That was exactly my point. I would expect some pilots to be more calm in daylight than others. Calmness might help solve the dilemmas in time.
That was exactly my point. I would expect some pilots to be more calm in daylight than others. Calmness might help solve the dilemmas in time.
Roul, you suggest that no stall tests were conducted for the accident configuration; you may be right.
This accident appears to have been beyond anything foreseen by both the manufacturer and regulator, yet with hindsight, imagination - stating the obvious, it is suddenly easy.
The lack of foresight probably involved similar mental processes as the crew's inability to comprehend the situation; the regulators had several years to think about this, the crew much less.
There is no guarantee that even the highest standard of aircraft certification or an exceptional crew will prevent all accidents. Aircraft certification and crew regulation depend on judgement of reasonably low risk; aircraft system failures use probability (e.g. 10-9), but human performance and training is much more subjective; these aspects include awareness and decision making.
This accident appears to have been beyond anything foreseen by both the manufacturer and regulator, yet with hindsight, imagination - stating the obvious, it is suddenly easy.
The lack of foresight probably involved similar mental processes as the crew's inability to comprehend the situation; the regulators had several years to think about this, the crew much less.
There is no guarantee that even the highest standard of aircraft certification or an exceptional crew will prevent all accidents. Aircraft certification and crew regulation depend on judgement of reasonably low risk; aircraft system failures use probability (e.g. 10-9), but human performance and training is much more subjective; these aspects include awareness and decision making.
From request by Chris:
So your friendly Viper pilot answers.......
- The stall AoA when "clean" does not vary with gee or attitude, as most here already know, hence
- Our AoA indicators were primarily for the approach configuration, although we could use AoA when clean if it was displayed. The early Vipers did not display AoA unless gear was down. The A-7 displayed it all the time in the HUD and using the indexer lights. The VooDoo I flew in the 60's had a large round indicator with two needles - one for the stall/pitchup and the other for existing AoA ( that sucker was notorious, but McAir put a stick pusher on it and also an AoA limiter you could use that took over 60 pounds of force to override, then the 28 pound pusher! Heh heh).
- Stall in the Viper was almost impossible, but it could be had and exactly the way 447 did - keep nose higher than necessary and slow down quickly before the limiters prevented a stall AoA.
- Stall recovery using AoA was very simple, as the stall AoA was independent of attitude or gee ( previously asserted).
PUSH FORWARD OR RELAX BACK STICK/YOKE!!!
The small jets I flew showed an immediate reaction. The heavies take time due to inertial and possibly less sensitive aero reactions to control inputs. We saw a few posts from folks that duplicated the accident in the sim and it took thousands of feet to get flying again.
- I do not feel a lotta training is required to learn about AoA versus attitude vs speed versus control inputs. The key is representing the AoA with respect to one of those, and additionally WRT actual flight path in the inertial frame of reference. Our AoA "bracket" in the HUD was a no-brainer. Push the bracket down to reduce AoA, pull back and it went up. Sheesh. The position of the bracket WRT the flight path marker showed high or low or "just right" AoA for your configuration. The 777 prang at SFO was a great example of not having and using an effective AoA indication and/or HUD.
- I really support those that wish a tactile indication of AoA. We could have used one in the Viper, but the designers said we could just pull and roll as hard as we wished and nothing bad would happen. You know, like the 'bus, "you can't stall this jet". We did not lose anyone due to a deep stall back in those early years, but we had ejection seats!! Training and iproved flight control laws reduced incidents to nearly zero, best I can determine.
The problem with using the existing minimum-speed marker-bugs on the ASI (such as Alpha Max or Vsw) is the enormous variation of Vs with normal G. Any bug allowing for that would go down and up like a yo-yo during the recovery. And with UAS, IAS is not a usable tool.
Perhaps one of our ex-military guys will comment on the practicalities of using an AoA gauge for stall recovery.
Perhaps one of our ex-military guys will comment on the practicalities of using an AoA gauge for stall recovery.
- The stall AoA when "clean" does not vary with gee or attitude, as most here already know, hence
- Our AoA indicators were primarily for the approach configuration, although we could use AoA when clean if it was displayed. The early Vipers did not display AoA unless gear was down. The A-7 displayed it all the time in the HUD and using the indexer lights. The VooDoo I flew in the 60's had a large round indicator with two needles - one for the stall/pitchup and the other for existing AoA ( that sucker was notorious, but McAir put a stick pusher on it and also an AoA limiter you could use that took over 60 pounds of force to override, then the 28 pound pusher! Heh heh).
- Stall in the Viper was almost impossible, but it could be had and exactly the way 447 did - keep nose higher than necessary and slow down quickly before the limiters prevented a stall AoA.
- Stall recovery using AoA was very simple, as the stall AoA was independent of attitude or gee ( previously asserted).
PUSH FORWARD OR RELAX BACK STICK/YOKE!!!
The small jets I flew showed an immediate reaction. The heavies take time due to inertial and possibly less sensitive aero reactions to control inputs. We saw a few posts from folks that duplicated the accident in the sim and it took thousands of feet to get flying again.
- I do not feel a lotta training is required to learn about AoA versus attitude vs speed versus control inputs. The key is representing the AoA with respect to one of those, and additionally WRT actual flight path in the inertial frame of reference. Our AoA "bracket" in the HUD was a no-brainer. Push the bracket down to reduce AoA, pull back and it went up. Sheesh. The position of the bracket WRT the flight path marker showed high or low or "just right" AoA for your configuration. The 777 prang at SFO was a great example of not having and using an effective AoA indication and/or HUD.
- I really support those that wish a tactile indication of AoA. We could have used one in the Viper, but the designers said we could just pull and roll as hard as we wished and nothing bad would happen. You know, like the 'bus, "you can't stall this jet". We did not lose anyone due to a deep stall back in those early years, but we had ejection seats!! Training and iproved flight control laws reduced incidents to nearly zero, best I can determine.
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@PEI_3721
" aircraft system failures use probability (e.g. 10-9), but human performance and training is much more subjective; these aspects include awareness and decision making"
With my math hat I have to say that use of numbers (e.g. 10-9) is often an illusion!
And human performance may be more objective that numbers if you use experience from professional guys like you. At least experience is the only way to validate numbers and system models ! (Descartes said that already)
" aircraft system failures use probability (e.g. 10-9), but human performance and training is much more subjective; these aspects include awareness and decision making"
With my math hat I have to say that use of numbers (e.g. 10-9) is often an illusion!
And human performance may be more objective that numbers if you use experience from professional guys like you. At least experience is the only way to validate numbers and system models ! (Descartes said that already)
Stall recovery using AoA
Quote:
"Stall recovery using AoA was very simple, as the stall AoA was independent of attitude or gee ( previously asserted)."
Thanks gums: knew I'd get a clear view of practical experience from you. So would it be fair to propose that, having stalled the a/c, one simply pushes to achieve an AoA of a few degrees below the stall, and then maintains that AoA as accurately as possible during the recovery from the dive? Once the a/c is climbing again, the pull-up can be eased to maintain a suitable pitch and the AoA will gradually reduce as the IAS or IMN rises to the desired climb speed. At that stage the stall recovery itself will have been completed.
I'm likely to get some stick from the likes of yourself, Owain and others, but at high altitude the stall (or low-speed buffet) AoA is much lower than low-down, due to compressibility effects, and therefore varies also with Mach. So I'm guessing that would have to be taken into account when choosing a recovery AoA?
Normally, the Airbus FACs (augmentation computers) or whatever would be able constantly to calculate and display the stall AoA, but in UAS that would not apply. So a ball-park, conservative figure of stall AoA would need to be displayed to enable the pilot to choose a safe target AoA for the recovery profile.
In either case, if terrain clearance was critical, the pilot might have to shoot for a higher AoA (i.e., a lower margin below the stall AoA displayed).
Does that make any sense?
"Stall recovery using AoA was very simple, as the stall AoA was independent of attitude or gee ( previously asserted)."
Thanks gums: knew I'd get a clear view of practical experience from you. So would it be fair to propose that, having stalled the a/c, one simply pushes to achieve an AoA of a few degrees below the stall, and then maintains that AoA as accurately as possible during the recovery from the dive? Once the a/c is climbing again, the pull-up can be eased to maintain a suitable pitch and the AoA will gradually reduce as the IAS or IMN rises to the desired climb speed. At that stage the stall recovery itself will have been completed.
I'm likely to get some stick from the likes of yourself, Owain and others, but at high altitude the stall (or low-speed buffet) AoA is much lower than low-down, due to compressibility effects, and therefore varies also with Mach. So I'm guessing that would have to be taken into account when choosing a recovery AoA?
Normally, the Airbus FACs (augmentation computers) or whatever would be able constantly to calculate and display the stall AoA, but in UAS that would not apply. So a ball-park, conservative figure of stall AoA would need to be displayed to enable the pilot to choose a safe target AoA for the recovery profile.
In either case, if terrain clearance was critical, the pilot might have to shoot for a higher AoA (i.e., a lower margin below the stall AoA displayed).
Does that make any sense?
TNX for nice words, Chris.
Due to the mach relationship with the stall AoA, the numbers may vary but the procedure does not. The 447 accidenty had unreliable air data, so allowing for the mach effects was not possible. Nevertheless, you can see the change in AoA when you pull or push most of the time.
In AF447 the inertia and THS trimmed position would not show a quick change in AoA but it WOULD SHOW a high, stalled AoA!! In the A-7 HUD you would see the flight path marker off the bottom and the AoA bracket way up at the top. In the Viper you would only have the FPM, as AoA was not displayed in the HUD with gear up ( "you can't stall this jet", heh heh).
As far as physical stall indications go, the 'bus seems to have a really good "approach to stall" feeling. So good that you can fly thru the protections and have decent roll authority and no gross buffet.
Hope that helps....
Due to the mach relationship with the stall AoA, the numbers may vary but the procedure does not. The 447 accidenty had unreliable air data, so allowing for the mach effects was not possible. Nevertheless, you can see the change in AoA when you pull or push most of the time.
In AF447 the inertia and THS trimmed position would not show a quick change in AoA but it WOULD SHOW a high, stalled AoA!! In the A-7 HUD you would see the flight path marker off the bottom and the AoA bracket way up at the top. In the Viper you would only have the FPM, as AoA was not displayed in the HUD with gear up ( "you can't stall this jet", heh heh).
As far as physical stall indications go, the 'bus seems to have a really good "approach to stall" feeling. So good that you can fly thru the protections and have decent roll authority and no gross buffet.
Hope that helps....
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https://fightersweep.com/6296/angle-...fm-engagement/
AOA vs Critical AOA
AOA systems have been on fighters for decades, and on heavy transport planes as well. But AOA has only recently gained more attention from the FAA and legislators for applications in General Aviation (GA) and business aircraft. One such highly publicized event that highlighted the significance of AOA was the crash of the Air France flight 447 Airbus in June 2009 that killed all 216 people on board.
Investigators concluded that the aircraft’s airspeed probes had iced over providing erroneous indications to the pilots, and that the AOA system and it’s “gauge” was not available to pilots in the cockpit. They also concluded that having an AOA gauge could have saved the stricken crew.
AOA vs Critical AOA
AOA systems have been on fighters for decades, and on heavy transport planes as well. But AOA has only recently gained more attention from the FAA and legislators for applications in General Aviation (GA) and business aircraft. One such highly publicized event that highlighted the significance of AOA was the crash of the Air France flight 447 Airbus in June 2009 that killed all 216 people on board.
Investigators concluded that the aircraft’s airspeed probes had iced over providing erroneous indications to the pilots, and that the AOA system and it’s “gauge” was not available to pilots in the cockpit. They also concluded that having an AOA gauge could have saved the stricken crew.
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All this discussion of the merits of an AOA presentation in the AF447 accident is interesting and doubtless instructive, but the fact is surely that once the aircraft got to 40+ deg AOA, there was precious little chance of any stall recovery. Theoretically a sustained pitch down... to -30 deg or so... would have been effective, but who on earth would ever have done that in a large civil transport at night, AOA presentation or no AOA presentation? The authorities would be far better employed thinking how to ensure that crews have the required skills to keep the aircraft straight and level in all circumstances where controls are effective, than running down the UPRT road. The crew spectacularly failed on that flight and any suggestion that a crew with that skill set could cope successfully with UPRT is ridiculous IMHO.
Yes there are additional measures that could conceivably be taken to further improve the stall warning system on the A330, but as an old boss of mine once said in a slightly different context 'this [wind tunnel] is fool-proof, but it's not bloody fool proof'. Most improvements come at the expense of further complexity, which brings its own problems.
Yes there are additional measures that could conceivably be taken to further improve the stall warning system on the A330, but as an old boss of mine once said in a slightly different context 'this [wind tunnel] is fool-proof, but it's not bloody fool proof'. Most improvements come at the expense of further complexity, which brings its own problems.
Hate to disagree, "gone", but you do not need to get the nose down well below the horizon to break the stall.
- problem is pitch authority. In our little jet we simply ran out of pitch authority and the sucker also had a point on the pitch moment curves due to our ceegee where we would settle into a nice stall and the FLCS kept commanding "nose down" even tho "nose up" stabilator command still had some authority. So they put in a pure manual override feature and we could "rock" the sucker outta the deep stall.
The 447 jet was in a "deeply stalled" condition ( not classic "deep stall") and the THS and elevators were still capable of inducing a nose down pitch moment or at least reducing what existed.
- By manually trimming nose down and holding nose down stick, 447 would have reduced AoA and maybe sooner than most folks think. No need to have the nose down 10 or 15 degrees or more. Hell, prolly could have broken the stall with 4 or 5 degrees nose down, It's the AoA!!! It is not the pitch attitude.
The folks here that duplicated the accident in sims knew what they had to do and still took over 10,000 or 15,000 feet to recover. I think a good test pilot could beat that, but you have to play the cards you are dealt with the crew.
- The trimmed THS played a large role, IMHO. We had the same problem in the Viper, but we could use our trim roller wheel and the coolie hat to help. Nevertheless, we did not have a clear indication that the FLCS had trimmed to the max position for the stabilators, and I am not sure the 'bus does either.
Oh well, .......
- problem is pitch authority. In our little jet we simply ran out of pitch authority and the sucker also had a point on the pitch moment curves due to our ceegee where we would settle into a nice stall and the FLCS kept commanding "nose down" even tho "nose up" stabilator command still had some authority. So they put in a pure manual override feature and we could "rock" the sucker outta the deep stall.
The 447 jet was in a "deeply stalled" condition ( not classic "deep stall") and the THS and elevators were still capable of inducing a nose down pitch moment or at least reducing what existed.
- By manually trimming nose down and holding nose down stick, 447 would have reduced AoA and maybe sooner than most folks think. No need to have the nose down 10 or 15 degrees or more. Hell, prolly could have broken the stall with 4 or 5 degrees nose down, It's the AoA!!! It is not the pitch attitude.
The folks here that duplicated the accident in sims knew what they had to do and still took over 10,000 or 15,000 feet to recover. I think a good test pilot could beat that, but you have to play the cards you are dealt with the crew.
- The trimmed THS played a large role, IMHO. We had the same problem in the Viper, but we could use our trim roller wheel and the coolie hat to help. Nevertheless, we did not have a clear indication that the FLCS had trimmed to the max position for the stabilators, and I am not sure the 'bus does either.
Oh well, .......
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Buss trim wheel will show it is full up but crew is not expected to look there nor there is a need. It is the pitch. Keep it sufficiently below the horizon and if it doesn't stay there use the trim to keep it there it will work fine. Full up THS in Perpignan caused the problem because of low altitude it became impossible to do anything.
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Gums,
Thanks for those thoughts - I've not seen a thorough analysis of the dynamics of 447 post stall, maybe there is a link somewhere in this thread and I missed it, but it's surely not just a question of adequate pitch authority. I know that the aircraft was, as you say, merely deeply stalled, not deep stalled, and so it was capable of being pitched down, had the crew ever tried to do so. But that doesn't alter the fact that its flight path angle was very very negative and even going to say 5 deg nose down would still have left it fully stalled with massive AOA and drag. I don't know how realistic simulators are in that area, not very I suspect, so assertions that the sim recovered are not totally convincing. I believe Airbus stopped at about 20 deg in their flight test... and I thought they deserved a medal for going that far.
Thanks for those thoughts - I've not seen a thorough analysis of the dynamics of 447 post stall, maybe there is a link somewhere in this thread and I missed it, but it's surely not just a question of adequate pitch authority. I know that the aircraft was, as you say, merely deeply stalled, not deep stalled, and so it was capable of being pitched down, had the crew ever tried to do so. But that doesn't alter the fact that its flight path angle was very very negative and even going to say 5 deg nose down would still have left it fully stalled with massive AOA and drag. I don't know how realistic simulators are in that area, not very I suspect, so assertions that the sim recovered are not totally convincing. I believe Airbus stopped at about 20 deg in their flight test... and I thought they deserved a medal for going that far.
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Would this A330 have been able to roll out of the stall to gain some pitch authority later on as the stall was broken?
If so, and considering SA parameters would be improved in daylight, you'll understand my earlier thinking about what effect night had on this scenario. The high nose attitude, i.e. seeing only sky, IMHO might have helped them understand where they were and what they were doing and thus hinted to stop doing it. Ultimately they might have sensed what to do and how to do it and appreciated they had lots of air underneath them and where the horizon was. In the dark, and a panic sets in and takes over, that loss if visual sense surely did not help any.
If so, and considering SA parameters would be improved in daylight, you'll understand my earlier thinking about what effect night had on this scenario. The high nose attitude, i.e. seeing only sky, IMHO might have helped them understand where they were and what they were doing and thus hinted to stop doing it. Ultimately they might have sensed what to do and how to do it and appreciated they had lots of air underneath them and where the horizon was. In the dark, and a panic sets in and takes over, that loss if visual sense surely did not help any.