American Airlines Flight 742 "flight control system" problems
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Originally Posted by NTSB
there was some additional angle of attack margin available to flare the airplane and reduce the descent rate at touchdown
there was some additional angle of attack margin available to flare the airplane and reduce the descent rate at touchdown
Originally Posted by CONFiture
Originally Posted by NTSB
there was some additional angle of attack margin available to flare the airplane and reduce the descent rate at touchdown
Originally Posted by NTSB report AAR-10/03 pages 97-98
The A320 alpha-protection mode incorporates features that can attenuate pilot sidestick pitch inputs. Because of these features, the airplane could not reach the maximum AOA attainable in pitch normal law for the airplane weight and configuration; however, the airplane did provide maximum performance for the weight and configuration at that time.
Originally Posted by Ashling
If you think there isn't a risk of a stall in that speed regime you are a dangerous fool.
Now if a real pilot would operate his aeroplane while believing it, that would be entirely different ballgame.
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Confiture
My question
When I said what would you do if you got a stall warning I was referring to a Boeing not an Airbus in normal law. If your in a Boeing (or a Bus in alternate/direct law) what are you going to do when the stall warning goes off at 150' because your too slow.
You replyed
The same thing that at 3000 ft, release some back pressure should be enough to stop the STALL warning or the stick shaker and keep the final pull for the last 50 ft.
YOU mentioned 3000 not me, it's you who don't appreciate or give enough credence to correct stall recovery procedures. That "Generic" stall recovery procedure happens to be the initial stages of the one used by both Boeing and Airbus, strange that.
Point being in your Boeing releasing the back pressure may do little as you may have trimmed back, If you haven't you may or may not succeed in getting rid of your warning but at the price of a higher rate of descent for no or very little speed gain. At 150ft they were going down at @ 1200 fpm 19 kts below VLS @ 25 kts slow on F speed. Your solution would increase that rate of descent and the only way you would have to recover would be to pull back into the warning with no idea what your margin is.
In the event the Bus managed to reduce their descent rate all the way to splashdown.
Maybe as Clandestino suggests, your not a dangerous fool, your just not a pilot
Confiture
My question
When I said what would you do if you got a stall warning I was referring to a Boeing not an Airbus in normal law. If your in a Boeing (or a Bus in alternate/direct law) what are you going to do when the stall warning goes off at 150' because your too slow.
You replyed
The same thing that at 3000 ft, release some back pressure should be enough to stop the STALL warning or the stick shaker and keep the final pull for the last 50 ft.
YOU mentioned 3000 not me, it's you who don't appreciate or give enough credence to correct stall recovery procedures. That "Generic" stall recovery procedure happens to be the initial stages of the one used by both Boeing and Airbus, strange that.
Point being in your Boeing releasing the back pressure may do little as you may have trimmed back, If you haven't you may or may not succeed in getting rid of your warning but at the price of a higher rate of descent for no or very little speed gain. At 150ft they were going down at @ 1200 fpm 19 kts below VLS @ 25 kts slow on F speed. Your solution would increase that rate of descent and the only way you would have to recover would be to pull back into the warning with no idea what your margin is.
In the event the Bus managed to reduce their descent rate all the way to splashdown.
Maybe as Clandestino suggests, your not a dangerous fool, your just not a pilot
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Originally Posted by Ashling
Point being in your Boeing releasing the back pressure may do little as you may have trimmed back
Maybe as Clandestino suggests, your not a dangerous fool, your just not a pilot
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Depends how far back you trim, essential point being you have to ensure you lower the nose to remove the warning
Thought this was interesting from the performance study relating to the accident
The elevator response plotted in Figure 15b indicates that during this time, the elevators move trailing-edge up starting at 15:30:37, reaching about 4° at 15:30:38, and then move abruptly down to about -1° at 15:30:39 before increasing again to about 4.5° at 15:30:41. In the last 2 seconds of flight, the elevator deflection increases about 1°, from 4.5° to 5.5°.
Figure 7 shows that between 15:30:36 and the touchdown at 15:30:43, the pitch angle increases from 9.5° to 11° and then settles back to 9.5°, even though in the last two seconds the left longitudinal side stick is at its aft limit, and α is below αmax.
… the aircraft was in angle-of attack (AoA) protection from about 150 ft RA. When in AoA protection law, stick command is AoA objective. Stick neutral commands alpha-prot and full back stick commands alpha-max.
However, AoA protection shall take care of the A/C trajectory and, thus, looks after phugoid damping as well as AoA control: there are feedbacks within the AoA protection law aiming at damping the phugoid mode (low frequency mode). The feedbacks are CAS and pitch attitude variations. Without these feedbacks, an aircraft upset from its stabilized flight point up to constant high AoA would enter a phugoid (which is, by definition, a constant AoA oscillation) without possibility to stabilize the trajectory. As a consequence, commanded AoA is modulated as a function of speed and attitude variations: for instance, if A/C speed is decreasing and/or pitch attitude is increasing, pilot's commanded AoA is lowered in order to avoid such a situation to degrade.
On the last 10 sec of the "Hudson" event, it is confirmed that pitch attitude is increasing and CAS decreasing. Then, the phugoid damping terms are non nul and are acting in the sense to decrease the finally commanded AoA vs. the stick command, in order to prevent the aircraft from increasing the phugoid features.
Thought this was interesting from the performance study relating to the accident
The elevator response plotted in Figure 15b indicates that during this time, the elevators move trailing-edge up starting at 15:30:37, reaching about 4° at 15:30:38, and then move abruptly down to about -1° at 15:30:39 before increasing again to about 4.5° at 15:30:41. In the last 2 seconds of flight, the elevator deflection increases about 1°, from 4.5° to 5.5°.
Figure 7 shows that between 15:30:36 and the touchdown at 15:30:43, the pitch angle increases from 9.5° to 11° and then settles back to 9.5°, even though in the last two seconds the left longitudinal side stick is at its aft limit, and α is below αmax.
… the aircraft was in angle-of attack (AoA) protection from about 150 ft RA. When in AoA protection law, stick command is AoA objective. Stick neutral commands alpha-prot and full back stick commands alpha-max.
However, AoA protection shall take care of the A/C trajectory and, thus, looks after phugoid damping as well as AoA control: there are feedbacks within the AoA protection law aiming at damping the phugoid mode (low frequency mode). The feedbacks are CAS and pitch attitude variations. Without these feedbacks, an aircraft upset from its stabilized flight point up to constant high AoA would enter a phugoid (which is, by definition, a constant AoA oscillation) without possibility to stabilize the trajectory. As a consequence, commanded AoA is modulated as a function of speed and attitude variations: for instance, if A/C speed is decreasing and/or pitch attitude is increasing, pilot's commanded AoA is lowered in order to avoid such a situation to degrade.
On the last 10 sec of the "Hudson" event, it is confirmed that pitch attitude is increasing and CAS decreasing. Then, the phugoid damping terms are non nul and are acting in the sense to decrease the finally commanded AoA vs. the stick command, in order to prevent the aircraft from increasing the phugoid features.
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Originally Posted by A de M
bubbers44 - It is always better to counter arguments by refuting what was said rather than what was not said. No one said 'it is ok for your pilots to lose airspeed and pull up into an 11 degree pitch up attitude at FL350 which we all know will result in a full stall'. I can see you are not an Airbus pilot and therefore have limited grasp of the situation, but the whole point of an Airbus is that normally you cannot stall it. 'Lack of pilot skills', as you describe it, is not simply stick and rudder proficiency, all very good as that is. Operating a large passenger jet like an Airbus requires a whole host of other skills, including a thorough grasp of the failure modes and being able to separate the wood from the trees in a high stress environment - a rare skill in my experience. Unfortunately the aircraft in this case had degraded into a reduced flight law status, which was not recognised by the crew. Furthermore you say that 'at FL350 level flight AOA would be nose attitude'. That statement does not entirely make sense to me, but from what I understand you are saying that if the pitch attitude on the artificial horizon (PFD in Airbus parlance) says 2 deg then you would see that as an accurate readout of AoA. That is absolutely not the case and the crew's misunderstanding of that was ultimately what killed everyone. It is entirely possible to have the pitch attitude around 2-3 degrees and still have an AoA of 20-25 degrees, which was the case here. They got into a stall because they pulled back on the sidestick in a reduced flight mode (Alternate Law in Airbus-speak). That induced a stall (an aural warning went off 75 times in the descent sating, 'Stall, Stall' but they never acknowledged it once). The actual attitude on the horizon was only a few degrees but their AoA was massive - something they never recognised. There were two compouding factors - the RHS First Officer for some reason (overwhelming anxiety?) kept his sidestick deflected fully back throughout the whole experience which was not spotted by the LHS First Officer and it therefore made his inputs largely redundant. The second factor was that they had experienced an erroneous speed indication earlier on one side, due to icing of a pitot probe, that had completely blown their mental understanding of the situation. Combine that with a host of strange warnings they could not process, night time, bad weather, no Captain present to take absolute control clearly and positively - you have a cocktail for catastrophe present. There is absolutely no doubt their training was inadequate, but that is another issue. To simply see them as two idiots who lacked basic flying skills is a gross and unhelpful simplification of the situation. Were they ultimately to blame? Without a doubt. Were there a number of other factors which contributed to a totally recoverable situation? Absolutely.
As an Airbus trainer and examiner (check airman in US-speak), my observation would be that if a genuinely unexpected loss of airspeed takes place (not one everyone is expecting because that is what they are doing in that particular recurrent training cycle) it has about a 50% chance of being recovered by the crew. Many would disagree with that view, but that is my view nonetheless. Crashes are rarely one cause as we know - a whole series of events come together in a particular moment of time which together lead to disaster. Many lessons have already been learnt by Airbus operators about this accident - including the need to have stall training on an aircraft that is not theoretically able to stall! When I did my type rating many years ago we never did stall training. That has now changed dramatically and I believe that the vast majority of Airbus pilots facing the same situation as the AF447 crew would be able to recover the aircraft to safe flight. Sadly, it often takes an accident for the right training to be in place.
As an Airbus trainer and examiner (check airman in US-speak), my observation would be that if a genuinely unexpected loss of airspeed takes place (not one everyone is expecting because that is what they are doing in that particular recurrent training cycle) it has about a 50% chance of being recovered by the crew. Many would disagree with that view, but that is my view nonetheless. Crashes are rarely one cause as we know - a whole series of events come together in a particular moment of time which together lead to disaster. Many lessons have already been learnt by Airbus operators about this accident - including the need to have stall training on an aircraft that is not theoretically able to stall! When I did my type rating many years ago we never did stall training. That has now changed dramatically and I believe that the vast majority of Airbus pilots facing the same situation as the AF447 crew would be able to recover the aircraft to safe flight. Sadly, it often takes an accident for the right training to be in place.
! Bubbers is correct; 10 pitch-up is not what you want at FL350. It was a basic lack of SA.... No excuses at a large Euro airline.No need for extra stall training (which as PILOTS we should all know from our C-152 days). Just need crew who can FLY an aeroplane
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Originally Posted by bubbers44
This shows the splashdown from surveilance video. It looks pretty normal to me.
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Confiture you misunderstand
Trimming does not mean you are in trim. How far the nose travels, and in which direction, when you release the stick will depend on were you or the aircraft have trimmed to. I made the point about trimming to illustrate one reason why releasing the back pressure is an inappropriate response to a stall warning. You have to lower the nose attitude, to reduce AoA, by looking out the front or, if IMC, at your AI or PFD.
No one disputes that there was an AoA margin spare, the issue is whether you can access it. In a 737 you would be into the shaker so you wouldn't do any better unless your into ignoring stall warnings which would be insane.
The following is from Airbus's submission to the NTSB
"During the remaining portion of the flight, the Aircraft remained in Normal Law, and on occasion was flown within the alpha protection range. Notably from approximately 150 ft down to the water impact the Aircraft was in slats/flaps configuration 2. During this time period the Aircraft was in the alpha protection mode which allowed the flight crew to remain focused on their priorities, conversely if the Aircraft had been a non fly-by-wire aircraft, the flight crew would have had to fly in and out of the stick shaker to maintain the desired descent profile."
Trimming does not mean you are in trim. How far the nose travels, and in which direction, when you release the stick will depend on were you or the aircraft have trimmed to. I made the point about trimming to illustrate one reason why releasing the back pressure is an inappropriate response to a stall warning. You have to lower the nose attitude, to reduce AoA, by looking out the front or, if IMC, at your AI or PFD.
No one disputes that there was an AoA margin spare, the issue is whether you can access it. In a 737 you would be into the shaker so you wouldn't do any better unless your into ignoring stall warnings which would be insane.
The following is from Airbus's submission to the NTSB
"During the remaining portion of the flight, the Aircraft remained in Normal Law, and on occasion was flown within the alpha protection range. Notably from approximately 150 ft down to the water impact the Aircraft was in slats/flaps configuration 2. During this time period the Aircraft was in the alpha protection mode which allowed the flight crew to remain focused on their priorities, conversely if the Aircraft had been a non fly-by-wire aircraft, the flight crew would have had to fly in and out of the stick shaker to maintain the desired descent profile."
Last edited by Ashling; 24th Apr 2013 at 12:44.
I made the point about trimming to illustrate one reason why releasing the back pressure is an inappropriate response to a stall warning.
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It's not nonsense, what is nonsense is a pilot suggesting that the correct way to recover from a stall warning in a commercial jet is to release the back pressure.
You reduce the AoA by moving the stick forward as per Boeing and Airbus QRH, anything else is unacceptable. Both QRH's also refer to the possibility that you may need to reduce power and trim the THS forward in order to gain enough elevator authority if moving the stick alone is ineffective.
Depending on the aircrafts trim you could be holding a high stick load, none or anything in between its even feasible, especially with electric/auto trim, that you could be trimmed fully nose up. If your holding lots of stick force then maybe releasing the pressure will remove the warning, no guarantee, but that response will reduce the closer to in trim the aircraft is and that is why saying "release the back pressure" is wrong and dangerous.
As for pulling to the shake sticker on a base turn! What aircraft and operation are you referring to that would advocate releasing any back pressure is an adequate response?
You reduce the AoA by moving the stick forward as per Boeing and Airbus QRH, anything else is unacceptable. Both QRH's also refer to the possibility that you may need to reduce power and trim the THS forward in order to gain enough elevator authority if moving the stick alone is ineffective.
Depending on the aircrafts trim you could be holding a high stick load, none or anything in between its even feasible, especially with electric/auto trim, that you could be trimmed fully nose up. If your holding lots of stick force then maybe releasing the pressure will remove the warning, no guarantee, but that response will reduce the closer to in trim the aircraft is and that is why saying "release the back pressure" is wrong and dangerous.
As for pulling to the shake sticker on a base turn! What aircraft and operation are you referring to that would advocate releasing any back pressure is an adequate response?
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I do believe this handbagging has gone on long enough - can you both accept that a reduction in alpha is required? Whether it be from relaxing the 'backpressure' when pulling or physically moving the c/c (or stick) forward when not, is it not the same?
Maybe then we can return to "American Airlines Flight 742 "flight control system" problems"
Maybe then we can return to "American Airlines Flight 742 "flight control system" problems"
Whether it be from relaxing the 'backpressure' when pulling or physically moving the c/c (or stick) forward when not, is it not the same?
Originally Posted by Ashling
It's not nonsense, what is nonsense is a pilot suggesting that the correct way to recover from a stall warning in a commercial jet is to release the back pressure.
Handbagging at 20 paces.
I just what to confirm that one of the handbaggers understands what is going on.
As for pulling to the shake sticker on a base turn! What aircraft and operation are you referring to that would advocate releasing any back pressure is an adequate response?
Last edited by Capn Bloggs; 24th Apr 2013 at 15:41. Reason: added the bottom bit...
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Sorry BOAC,
While I can happily agree that a reduction in AoA is required I cannot accept that releasing backpressure is the correct response to a stall warning, it may be effective but it may not. Stall recovery procedures do not say "release any backpressure then if ineffective move the control column centrally forward". There is a reason for that. There are too many variables, rate of speed reduction, trim, power setting, nose attitude, bank angle, g load, control authority etc.
In the right circumstances releasing backpressure may be sufficient to remove a warning but what we have to have is a procedure that works and for people to understand that there is no ambiguity in how it is applied, that is why Boeing and Airbus rewrote their procedures in the light of recent tragic events.
If we're talking max rate turns or air combat in the buffet then things are different but that's a specialised environment.
While I can happily agree that a reduction in AoA is required I cannot accept that releasing backpressure is the correct response to a stall warning, it may be effective but it may not. Stall recovery procedures do not say "release any backpressure then if ineffective move the control column centrally forward". There is a reason for that. There are too many variables, rate of speed reduction, trim, power setting, nose attitude, bank angle, g load, control authority etc.
In the right circumstances releasing backpressure may be sufficient to remove a warning but what we have to have is a procedure that works and for people to understand that there is no ambiguity in how it is applied, that is why Boeing and Airbus rewrote their procedures in the light of recent tragic events.
If we're talking max rate turns or air combat in the buffet then things are different but that's a specialised environment.
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I am basically with you but trying to draw this endless thread somewhere
. It is certainly NOT 'the correct response' as a rote, but it might be an effective one sometimes. Whatever, alpha reduced is the aim. How about that? After all, 'relaxing the back pressure' is in fact 'pushing' the stick forward.
. It is certainly NOT 'the correct response' as a rote, but it might be an effective one sometimes. Whatever, alpha reduced is the aim. How about that? After all, 'relaxing the back pressure' is in fact 'pushing' the stick forward.
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BAOC
I appreciate your frustration and am quite happy to accept that releasing any back pressure MAY be enough to remove a warning and that it MIGHT be effective sometimes, in fact I've said that already I think. It's just I can't accept it as the "correct technique", ex standards QFI, hence being a touch pedantic. There is a difference between releasing a pressure and a positive forward movement of the stick IMHO. The later emphasises the need to keep going until the warning/ stall is broken, the former does not.
After recent events it would seem to be something worth being pedantic about!
I appreciate your frustration and am quite happy to accept that releasing any back pressure MAY be enough to remove a warning and that it MIGHT be effective sometimes, in fact I've said that already I think. It's just I can't accept it as the "correct technique", ex standards QFI, hence being a touch pedantic. There is a difference between releasing a pressure and a positive forward movement of the stick IMHO. The later emphasises the need to keep going until the warning/ stall is broken, the former does not.
After recent events it would seem to be something worth being pedantic about!
Last edited by Ashling; 24th Apr 2013 at 18:32.
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Originally Posted by Ashling
Trimming does not mean you are in trim.
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releasing back pressure may work. pushing forward on the controls/stick DOES work.
maybe you will only have one chance...so PUSH THE THING FORWARD.
and yes, you will probably lose altitude. but if you got into a stall at all...well, shame on you (training aside)
maybe you will only have one chance...so PUSH THE THING FORWARD.
and yes, you will probably lose altitude. but if you got into a stall at all...well, shame on you (training aside)