AF447 Thread No. 3
Stall and aerodynamics
18 years ago it was the last time i was in an aerodynamic course, so forgive me if i get it totally wrong, and feel free to correct my thoughts.
In nearly all contributions on this thread and on the one in R+N stall and the associated AOA is viewed in relation to the whole airframe. Basically there are two major lifting devices, the wings (upward lift) and the THS (downward lift). The efficiency of those two devices is influenced by airspeed and aoa and some other points like CG, configuration, bank angle, and so on. In straight and level unaccellerated flight the AOA of the THS with itīs downforce balances the weight and the lift of the airframe with the wings. To get the nose up without changing any other parameter there is the need to increase the lift of the THS by increasing the THS AOA thus forcing the tail down and the nose up, and vice versa. This is done by changing the elevators in the short term and by changing the THS trim (autotrim) in the long term.
Letīs just concentrate on the wings and the THS with regard to AF447.
When AP +AThr disengaged with speed of 0.8 mach, and the initial roll correction and NU input of the PF the airframe reached an Pitch of 10° and increasing, and a climb rate up to 7.000 f/min. The PF made a ND input with the SS and reduced the AOA to 4° and the climbrate to 700 f/min.
BEA doesnīt say what kind of pitch it finally reached. The AOA reduction does not necessarily cause an immidiate gross reduction in pitch, the nose might still have been a lot higher above the horizon than desired. The reduction in climb rate might as well not have been the direct result of the decrease of AOA and pitch, but the result of rapidly decaying forward speed thus reducing the initial peek climb speed. In other words, the energy state of the aircraft could already have been that of a stalled one, only the unloading had prevented the stall until right now.
At 2:10:51 the stall warning came on, the AOA was increasing above 6°.
Lets see in what kind of AOA the lifting devices wing and THS had been at that point. For me as a non engineer its easy to get those figures simplified and wrong, so feel free to correct me. The whole frame had 6° AOA, so i put the wings to the same value. The THS has the AOA of the fuselage, the AOA of the THS (3° nose up trim as later stated) plus the full nose down input of the elevators (full deflection 15° down, i take the half of it as AOA change) My sum here -6+3-7= -10 AOA. If the trend of my thinking is not totally wrong, the THS at that low speed might have been close to a negativ tail stall.
At that point PF applies TO/GA power and replaces his ND SS input by a NU SS input. The pitchup moment of the engines plus the pitchup NU SS input (which moves the THS trim from 3° to 13°) settles the airframe in a solid state of stall with an AOA of 16° and an increasing sinkrate up to 10.000 f/min. Applying my above right or wrong formula the AOA of the wing would equal 16°, the AOA of the THS would be -16+13+ the elevator NU input (max 38°, producing an THS AOA change of estimated 15°). That somes up to an THS AOA of 0 to 12 AOA, due to low speed a full aft stick probably produced a possitive Tail stall.
What does full ND SS input produce at that point?
-16 + 13 -7 brings us to - 10 again.
My values might be totally off, but i cant get rid of the feeling that with the low speed there was only one THS position favorable for recovery, but that the THS was stallable in the positive and in the negative THS AOA region. The crew had no means available to find out what that exact point would have been and how long it would have taken to get the nose below the horizon. Again, an old style AOA gauge might have helped to find the point, where the input produced a positive effect in total AOA.
Thats why our problem solving in the f-4 stall recovery was getting the nose down:
With the stick forward we tried to reach the ideal position of the stabilator (full forward might have caused the stabilator stall on the opposite side).
If we missed that point the drag of the chute would kick our tail up and force the nose down.
Edited for latest version of BEA statement, sorry i missed it first. Thanks for the heads up to Conf and Bear
By the way, BEA has it still wrong in the german version.
In nearly all contributions on this thread and on the one in R+N stall and the associated AOA is viewed in relation to the whole airframe. Basically there are two major lifting devices, the wings (upward lift) and the THS (downward lift). The efficiency of those two devices is influenced by airspeed and aoa and some other points like CG, configuration, bank angle, and so on. In straight and level unaccellerated flight the AOA of the THS with itīs downforce balances the weight and the lift of the airframe with the wings. To get the nose up without changing any other parameter there is the need to increase the lift of the THS by increasing the THS AOA thus forcing the tail down and the nose up, and vice versa. This is done by changing the elevators in the short term and by changing the THS trim (autotrim) in the long term.
Letīs just concentrate on the wings and the THS with regard to AF447.
When AP +AThr disengaged with speed of 0.8 mach, and the initial roll correction and NU input of the PF the airframe reached an Pitch of 10° and increasing, and a climb rate up to 7.000 f/min. The PF made a ND input with the SS and reduced the AOA to 4° and the climbrate to 700 f/min.
BEA doesnīt say what kind of pitch it finally reached. The AOA reduction does not necessarily cause an immidiate gross reduction in pitch, the nose might still have been a lot higher above the horizon than desired. The reduction in climb rate might as well not have been the direct result of the decrease of AOA and pitch, but the result of rapidly decaying forward speed thus reducing the initial peek climb speed. In other words, the energy state of the aircraft could already have been that of a stalled one, only the unloading had prevented the stall until right now.
At 2:10:51 the stall warning came on, the AOA was increasing above 6°.
Lets see in what kind of AOA the lifting devices wing and THS had been at that point. For me as a non engineer its easy to get those figures simplified and wrong, so feel free to correct me. The whole frame had 6° AOA, so i put the wings to the same value. The THS has the AOA of the fuselage, the AOA of the THS (3° nose up trim as later stated) plus the full nose down input of the elevators (full deflection 15° down, i take the half of it as AOA change) My sum here -6+3-7= -10 AOA. If the trend of my thinking is not totally wrong, the THS at that low speed might have been close to a negativ tail stall.
At that point PF applies TO/GA power and replaces his ND SS input by a NU SS input. The pitchup moment of the engines plus the pitchup NU SS input (which moves the THS trim from 3° to 13°) settles the airframe in a solid state of stall with an AOA of 16° and an increasing sinkrate up to 10.000 f/min. Applying my above right or wrong formula the AOA of the wing would equal 16°, the AOA of the THS would be -16+13+ the elevator NU input (max 38°, producing an THS AOA change of estimated 15°). That somes up to an THS AOA of 0 to 12 AOA, due to low speed a full aft stick probably produced a possitive Tail stall.
What does full ND SS input produce at that point?
-16 + 13 -7 brings us to - 10 again.
My values might be totally off, but i cant get rid of the feeling that with the low speed there was only one THS position favorable for recovery, but that the THS was stallable in the positive and in the negative THS AOA region. The crew had no means available to find out what that exact point would have been and how long it would have taken to get the nose below the horizon. Again, an old style AOA gauge might have helped to find the point, where the input produced a positive effect in total AOA.
Thats why our problem solving in the f-4 stall recovery was getting the nose down:
F-4 hard memory item for "out of control":
Stick - forward (it was not full forward!!)
Ailerons and rudder - neutral
If aircraft not recovered - maintain full forward stick and deploy drag chute
Stick - forward (it was not full forward!!)
Ailerons and rudder - neutral
If aircraft not recovered - maintain full forward stick and deploy drag chute
If we missed that point the drag of the chute would kick our tail up and force the nose down.
Edited for latest version of BEA statement, sorry i missed it first. Thanks for the heads up to Conf and Bear
By the way, BEA has it still wrong in the german version.
Last edited by RetiredF4; 8th Jun 2011 at 22:41.
Retired F4
Thinking back to the long post in re the two year symposium on stalls and stall training, and given that the pilot was apparently able to make some roll inputs, at some point would a roll toward 90 deg force the nose to fall (I am guessing the computer at this point would not curtail roll at 67 degrees, which is the advertised limit) and do sort of what your drag chute did for the Phantom?
Granted, if you are in a stall, rolling (and likely an out of balance condition if the rudder control isn't just right) could lead to something like an erect spin, or a stall-spiral hybrid ... but once the nose starts pointing down, would that not unstall the THS in your illustration?
PS: appreciate your pointing out that there are two airfoils in question, either of which has a critical AoA, and either of which could stall.
But ... and I may be arse backwards here, if THS stalls, would that not lead to a pitching down moment?
(If I have that arse backwards, apologies)
Thinking back to the long post in re the two year symposium on stalls and stall training, and given that the pilot was apparently able to make some roll inputs, at some point would a roll toward 90 deg force the nose to fall (I am guessing the computer at this point would not curtail roll at 67 degrees, which is the advertised limit) and do sort of what your drag chute did for the Phantom?
Granted, if you are in a stall, rolling (and likely an out of balance condition if the rudder control isn't just right) could lead to something like an erect spin, or a stall-spiral hybrid ... but once the nose starts pointing down, would that not unstall the THS in your illustration?
PS: appreciate your pointing out that there are two airfoils in question, either of which has a critical AoA, and either of which could stall.
But ... and I may be arse backwards here, if THS stalls, would that not lead to a pitching down moment?
(If I have that arse backwards, apologies)
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Bearfoil remember that in Alt Law Vsw is based on an AoA value which is lower than V Aprot in Norm Law and if you drop into Alt Law at Hi LvL the warning can be triggered by turbulence or heavy handed inputs on the controls (less damping at altitude) especially if your at Turb speed. Its for this reason that Airbus suggest you cruise 4000 below your max alt for weight. So I would suggest that the 2 stall warnings early on were genuine.
The article in safety first explains all of this much better than I can.
Narval's advice about not changing anything is spot on. Once they'd lost the A/P if they'd simply disconnected the A/T, set crz pwr and held the attitude it would all have come back for them.
The article in safety first explains all of this much better than I can.
Narval's advice about not changing anything is spot on. Once they'd lost the A/P if they'd simply disconnected the A/T, set crz pwr and held the attitude it would all have come back for them.
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Originally Posted by RF4
When AP +AThr disengaged with speed of 0.8 mach, and the initial roll correction and NU input of the PF the airframe reached an AOA of 10°
The airplanes pitch attitude increased progressively beyond 10 degrees
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"The airplanes pitch attitude increased progressively beyond 10 degrees..."
"And then began to climb...." (BEA)
ashling, thanks for the reply. However, Normal Law may have been retained for eleven seconds after a/p loss. If the Stall warnings had been legit, how is that? At Cruise? On Autopilot? Why didn't she then Stall at the beginning of climb? Not until 2500 feet later, and loss of all her energy (well, most).
waddya think?
"And then began to climb...." (BEA)
ashling, thanks for the reply. However, Normal Law may have been retained for eleven seconds after a/p loss. If the Stall warnings had been legit, how is that? At Cruise? On Autopilot? Why didn't she then Stall at the beginning of climb? Not until 2500 feet later, and loss of all her energy (well, most).
waddya think?
When AP +AThr disengaged with speed of 0.8 mach, and the initial roll correction and NU input of the PF the airframe reached an AOA of 10°
It is possible you still have the first version of the BEA note, the second was different :
The airplanes pitch attitude increased progressively beyond 10 degrees
The airplanes pitch attitude increased progressively beyond 10 degrees
Thank you for the correction, i edit it in my post as it makes my thinking even easier.
FBW versus "conventional" one mo' time
I am going with Smilin', Retired and Wolf for several things.
Meanwhile, and for many here that have not flown A FBW system, regardless of the model.....
We must remember that except for the Viper manual pitch override feature, that 99.9% of all pilot commands to the jet are filtered and interpreted and adjusted and the beat goes on.
Between you and the control surfaces are filters and gains and such that use control surface deflection capabilities and limitations and rates. The term "direct" control law just ain't so. Maybe A33Z can chime in here, but I'll bet a bottle of scotch that even in "direct" law that the confusers are tailoring your control stick inpouts to allow for aero and mechanical properties of the control surfaces and some of the aircraft's aero and structural considerations.
Secondly, I have not seen an AoA-based FBW system. I HAVE flown a sim with a rate law ( AFTI F-16). It had an additional bias for attitude. In short, it sucked!! That being said, most folks relate trim to pitch. So if you have to hold back stick for an AoA, you trim a bit so you have a "neutral" stick. The Airbus doesn't work that way.
I prolly should ask one of my shuttle friends to tell us how its FBW system works, but I would bet another bottle of scotch that is resembles the 'bus and the Viper.
Lastly, I am against this term "autotrim". We early Viper instructors did our best to expunge the term. In the case of the 'bus, all it means is that a) if you increase or decrease speed, the THS will change to provide the one gee command, b) if you roll into a bank the confusers will add enuf gee command to stay in level flight , and c) at a pitch attitude above zero the confusers will subtract a bit of the gee command to climb at an attitude, but that attitude is not the major player. It's still gee! How many times do we have to say this? Unlike the 'bus, our litle jet assumed the "trimmed" gee if you let go of the stick. And unlike the 'bus, we could manually trim the jet for a gee. Let go and the sucker went to whatever gee you had trimmed for. Didn't care about speed or attitude or roll angle. It went for "normal" gee. The Airbus does not do this.
later, and I await A33's inputs.
Meanwhile, and for many here that have not flown A FBW system, regardless of the model.....
We must remember that except for the Viper manual pitch override feature, that 99.9% of all pilot commands to the jet are filtered and interpreted and adjusted and the beat goes on.
Between you and the control surfaces are filters and gains and such that use control surface deflection capabilities and limitations and rates. The term "direct" control law just ain't so. Maybe A33Z can chime in here, but I'll bet a bottle of scotch that even in "direct" law that the confusers are tailoring your control stick inpouts to allow for aero and mechanical properties of the control surfaces and some of the aircraft's aero and structural considerations.
Secondly, I have not seen an AoA-based FBW system. I HAVE flown a sim with a rate law ( AFTI F-16). It had an additional bias for attitude. In short, it sucked!! That being said, most folks relate trim to pitch. So if you have to hold back stick for an AoA, you trim a bit so you have a "neutral" stick. The Airbus doesn't work that way.
I prolly should ask one of my shuttle friends to tell us how its FBW system works, but I would bet another bottle of scotch that is resembles the 'bus and the Viper.
Lastly, I am against this term "autotrim". We early Viper instructors did our best to expunge the term. In the case of the 'bus, all it means is that a) if you increase or decrease speed, the THS will change to provide the one gee command, b) if you roll into a bank the confusers will add enuf gee command to stay in level flight , and c) at a pitch attitude above zero the confusers will subtract a bit of the gee command to climb at an attitude, but that attitude is not the major player. It's still gee! How many times do we have to say this? Unlike the 'bus, our litle jet assumed the "trimmed" gee if you let go of the stick. And unlike the 'bus, we could manually trim the jet for a gee. Let go and the sucker went to whatever gee you had trimmed for. Didn't care about speed or attitude or roll angle. It went for "normal" gee. The Airbus does not do this.
later, and I await A33's inputs.
Last edited by Jetdriver; 9th Jun 2011 at 01:21.
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There is a BA training guide here http://www.737ng.co.uk/a320training.pdf.
At p14:
"If an overspeed occurs, perhaps because of a sudden unexpected increase
in headwind, the autopilot will disconnect, auto-pitch trim is frozen and
overspeed protection will activate. The auto-pilot disconnect aural
warning will be masked by the ECAM overspeed warning. Spiral static
stability is reduced to zero bank and the maximum bank angle is reduced
to 45°. As the speed increases, the side-stick nose-down authority is
progressively reduced, and a permanent nose-up order is applied to aid
recovery. To recover from an overspeed, reduce thrust and select
(carefully) speedbrake."
This description relates to normal law. The CVR extracts released by the BEA suggest (without confirming) that alternate law became engaged during the incident. However it is not clear (to me anyway) whether reversion to normal law occurred during the incident, possibly without coming to the attention of the pilots.
Given the incongruous / invalid speed readings, is it conceivable that (i) normal law was resumed and (ii) a "permanent nose-up order" contributed to the difficulty of recovery? Or is it certain that the invalid data or other circumstances would have prevented a "permanent nose-up order"?
Sorry if this has been asked previously.
At p14:
"If an overspeed occurs, perhaps because of a sudden unexpected increase
in headwind, the autopilot will disconnect, auto-pitch trim is frozen and
overspeed protection will activate. The auto-pilot disconnect aural
warning will be masked by the ECAM overspeed warning. Spiral static
stability is reduced to zero bank and the maximum bank angle is reduced
to 45°. As the speed increases, the side-stick nose-down authority is
progressively reduced, and a permanent nose-up order is applied to aid
recovery. To recover from an overspeed, reduce thrust and select
(carefully) speedbrake."
This description relates to normal law. The CVR extracts released by the BEA suggest (without confirming) that alternate law became engaged during the incident. However it is not clear (to me anyway) whether reversion to normal law occurred during the incident, possibly without coming to the attention of the pilots.
Given the incongruous / invalid speed readings, is it conceivable that (i) normal law was resumed and (ii) a "permanent nose-up order" contributed to the difficulty of recovery? Or is it certain that the invalid data or other circumstances would have prevented a "permanent nose-up order"?
Sorry if this has been asked previously.
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A/P drops out at 1 degree above Alpha Prot in normal law, you can still get back to Alpha Max with full back stick and the A/T out (Alpha Floor) which is still short of Alpha Stall albeit not by much. In alt law Vsw or the Alpha that it triggers at is higher than Alpha Prot so although the warning is there you still have a little way to go to the actual stall. In severe turbulence close to max alt you will get transitory spikes in AoA (which is why you disconnect things if its bad enough and hold the attitude) so in Alt law at turb mach with an aft CoG you could well get a couple of transitory warnings as the AoA threshold is temporarily exceeded and not be particularly close to the stall. Certainly with TOGA and full aft stick given the lack of damping you could get a pretty decent rate of climb for a short while at least.
So I figure they were in Alt Law from the point the A/P and the A/T kicked out and it was the PF's roll and nose up demand that triggered the initial 2 blips of the stall warning which would have triggered at a lower AoA than in Normal Law.
So I figure they were in Alt Law from the point the A/P and the A/T kicked out and it was the PF's roll and nose up demand that triggered the initial 2 blips of the stall warning which would have triggered at a lower AoA than in Normal Law.
GIGO
gums:
And that's exactly why, when the confusers get so confused that they give up, they need to stop changing things like pitch trim. With garbage in, they will produce more garbage.
...but I'll bet a bottle of scotch that even in "direct" law that the confusers are tailoring your control stick inpouts to allow for aero and mechanical properties of the control surfaces and some of the aircraft's aero and structural considerations.
I would like to offer the following as a basis for understanding the C* flight control parameter (law). The paper was written in 1993 when research into "FBW" was already thirty years old. As the paper suggests, this is primarily a military endeavour with precious little devoted to civilian transport aircraft. The author states he wished to change that. The paper is not easy reading but may lend a deeper understanding of C* laws and may therefore be of interest in the present discussion. - PJ2
The Application of a C* Flight Control Law to Civil Transport Aircraft - Edmund Field, College of Aeronautics, Cranfield University, Cranfield, Bedford, England
The Application of a C* Flight Control Law to Civil Transport Aircraft - Edmund Field, College of Aeronautics, Cranfield University, Cranfield, Bedford, England
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Originally Posted by Ashling
... which would have triggered at a lower AoA than in Normal Law.
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]Aldrin could also tell exactly where Armstrong was aiming because the DSKY gave him that information. I believe it displayed an offset between the current flight path and the desired landing point, which could be matched up to surface features using a reticule on the window. Though for most of the approach he was probably too busy trying to debug the computer alarms to be able to look out of the window.
While this seems offtopic, it does actually have some similarities to what we seem to know of the AF447 crash as some people have suggested that Armstrong was so distracted by the computer errors that he wasn't monitoring the landing site properly during the approach and if they'd got unlucky that could have resulted in landing on bad terrain (e.g. on top of a rock or on the side of a small crater) or an abort. Computer failures overloading human pilots seems to have been a problem for quite a while.
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Some thoughts (and a first post) from a humble engineer
On the right turn:
In the BEA google earth graphic, the right turn commences after point "6." It is stated after this point in the text that the PF makes inputs to the nose up and roll left stops. Assuming (note: I have far less than the near-encyclopedic knowledge of the Airbus Flight Control system than many others here) that in this case, spoilers 2-6 and the IB/OB aileron on the left wing are raised to a significant degree, the effective camber of the left wing is reduced. We know that at least in the conventional stall and approach to stall, decreased camber results in decreased overall lift, but also increased stall AoA (see Figure 2. Effect of Trailing-Edge Flap and Speed Brake Position for the cool graphic).
These guys were so far to the right of a conventional Cl/alpha curve, I won't comment as to the lift, but it does make sense that for some AoA near or beyond the stall AoA that the deflected ailerons and spoilers delayed separation or at least moved the separation point rearward compared to the right wing. Seems to me it's plausible this could result in lower drag on the left wing and a net yaw moment to the right. It would take those more in the know that me to comment on which direction the roll moment would be in this case.
I speculate that at some AoA, the left wing, like the right wing, becomes a very finely machined barn door, and the aileron/spoiler position becomes irrelevant.
Further speculating beyond my cubicle, this presents a weird control situation. Stick left would produce a yaw to the right. Stick back increases AoA and cancels any lift or drag asymmetry (zero roll/yaw moment) between the two wings. And, it has the nasty effect of canceling the stall warning. Stick forward gives the stall warning again and possibly the yaw sensitivity to roll inputs. In this (speculative) scenario, I can easily the psychological appeal of stick back and TOGA power (more to come on this later).
I can positively say that I'd never solve a puzzle like this in 38,000 ft, but my pilot in command experience is measured in airsick bags instead of hours - I'd appreciate thoughts from those of you that actually did this for a living.
On the right turn:
In the BEA google earth graphic, the right turn commences after point "6." It is stated after this point in the text that the PF makes inputs to the nose up and roll left stops. Assuming (note: I have far less than the near-encyclopedic knowledge of the Airbus Flight Control system than many others here) that in this case, spoilers 2-6 and the IB/OB aileron on the left wing are raised to a significant degree, the effective camber of the left wing is reduced. We know that at least in the conventional stall and approach to stall, decreased camber results in decreased overall lift, but also increased stall AoA (see Figure 2. Effect of Trailing-Edge Flap and Speed Brake Position for the cool graphic).
These guys were so far to the right of a conventional Cl/alpha curve, I won't comment as to the lift, but it does make sense that for some AoA near or beyond the stall AoA that the deflected ailerons and spoilers delayed separation or at least moved the separation point rearward compared to the right wing. Seems to me it's plausible this could result in lower drag on the left wing and a net yaw moment to the right. It would take those more in the know that me to comment on which direction the roll moment would be in this case.
I speculate that at some AoA, the left wing, like the right wing, becomes a very finely machined barn door, and the aileron/spoiler position becomes irrelevant.
Further speculating beyond my cubicle, this presents a weird control situation. Stick left would produce a yaw to the right. Stick back increases AoA and cancels any lift or drag asymmetry (zero roll/yaw moment) between the two wings. And, it has the nasty effect of canceling the stall warning. Stick forward gives the stall warning again and possibly the yaw sensitivity to roll inputs. In this (speculative) scenario, I can easily the psychological appeal of stick back and TOGA power (more to come on this later).
I can positively say that I'd never solve a puzzle like this in 38,000 ft, but my pilot in command experience is measured in airsick bags instead of hours - I'd appreciate thoughts from those of you that actually did this for a living.
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From two posts from COMPUSERVE'S AVSIG forum in 1994:
____________________________________________________________ _
#: 637883 S4/SAFETY
13-Aug-94 13:54:56
Sb: #637565-#A330 flight tst crash
Fm: John M
To: Raymond C
>In that case, it makes me wonder if Airbus designed their cockpits a bit TOO differently from what pilots who flew 747's, DC10's and L1011's are used to.<<
It would be worth getting the list of pilots and pilot union groups who had a DIRECT input into the cockpit design for the A320/330/340 range. You might be very surprised to find that Airbus supplied the cockpit requested by fellow professionals after many years consultation. I can't help feel that the collective professional "We" have got no more than we asked for with this flight deck design. So perhaps we should take the time to learn to use it properly, thus avoiding unpleasant situations.
There is 1 Reply.
#: 637901 S4/SAFETY
13-Aug-94 14:33:31
Sb: #637883-#A330 flight tst crash
Fm: Howard B. Greene
To: John M(X)
John:
You've got the origin right; a lot of the A320 "wierdities" were the direct result of the early A320 customer input. Of course a different lot of them were initiated by AI as a "good idea". But the FMGC layout (which I abhor) was directly spec'd by the customer, the displays, fbw scheme, even the autothrottles, if not spec'd by the customer, were certainly blessed by them. The Unions of the world also had a big say, but different union groups give different answers, hard for any manufacturer to sort out and make everybody absolutely happy. First priority generally goes to the customer technical pilots (they're the ones who carry the bags full of money around), second to union groups.
Of all the controversies, the biggest two that remain controversial even to
this day (with pretty strong pilot opinion polarization) are:
hard limiting;
non-moving autothrottles.
Of all the accidents, the autothrottles have been a piece of only a couple, hard limiting in none, although the 330 accident got close; limiting was active in the last stages of recovery, but too late to have an impact on outcome.
Berk
____________________________________________________________ _
FYI, Howard B. Greene's (Berk) was from his own autobiography:
"Walked right into the 767 program, managing the FAA's swallowing of the new "two crew" concept. In the succeeding years I've been the chief FAA project pilot for flight test and certification of the 737-300, 747-400, EMB-120, ATR-42, and now for the 777 and A330/340."
Very unfortunately, Berk passed away about two months after the above post.
____________________________________________________________ _
#: 637883 S4/SAFETY
13-Aug-94 13:54:56
Sb: #637565-#A330 flight tst crash
Fm: John M
To: Raymond C
>In that case, it makes me wonder if Airbus designed their cockpits a bit TOO differently from what pilots who flew 747's, DC10's and L1011's are used to.<<
It would be worth getting the list of pilots and pilot union groups who had a DIRECT input into the cockpit design for the A320/330/340 range. You might be very surprised to find that Airbus supplied the cockpit requested by fellow professionals after many years consultation. I can't help feel that the collective professional "We" have got no more than we asked for with this flight deck design. So perhaps we should take the time to learn to use it properly, thus avoiding unpleasant situations.
There is 1 Reply.
#: 637901 S4/SAFETY
13-Aug-94 14:33:31
Sb: #637883-#A330 flight tst crash
Fm: Howard B. Greene
To: John M(X)
John:
You've got the origin right; a lot of the A320 "wierdities" were the direct result of the early A320 customer input. Of course a different lot of them were initiated by AI as a "good idea". But the FMGC layout (which I abhor) was directly spec'd by the customer, the displays, fbw scheme, even the autothrottles, if not spec'd by the customer, were certainly blessed by them. The Unions of the world also had a big say, but different union groups give different answers, hard for any manufacturer to sort out and make everybody absolutely happy. First priority generally goes to the customer technical pilots (they're the ones who carry the bags full of money around), second to union groups.
Of all the controversies, the biggest two that remain controversial even to
this day (with pretty strong pilot opinion polarization) are:
hard limiting;
non-moving autothrottles.
Of all the accidents, the autothrottles have been a piece of only a couple, hard limiting in none, although the 330 accident got close; limiting was active in the last stages of recovery, but too late to have an impact on outcome.
Berk
____________________________________________________________ _
FYI, Howard B. Greene's (Berk) was from his own autobiography:
"Walked right into the 767 program, managing the FAA's swallowing of the new "two crew" concept. In the succeeding years I've been the chief FAA project pilot for flight test and certification of the 737-300, 747-400, EMB-120, ATR-42, and now for the 777 and A330/340."
Very unfortunately, Berk passed away about two months after the above post.
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PJ2,
Thank you for the interesting article on C* control.
From the small amount of information that BEA released, it still seems likely to me that difficulty in controlling the lateral channel was at the root of the loss of control in pitch.
If lateral control was difficult enough, that could take all the PF's attention.
I wounder if it is practical to split duties between pitch and roll channels between the two seats as an emergency means of control in a high workload situation? Very un-Airbus of course.
Your stick force diagram posted a couple of days ago showed much higher forces for the lateral channel of control than the A320 information I re-posted from an old 2004 posting on the subject. The higher the stick force gradient, the greater the lateral control difficulty with a "wing heavy" condition.
Thank you for the interesting article on C* control.
From the small amount of information that BEA released, it still seems likely to me that difficulty in controlling the lateral channel was at the root of the loss of control in pitch.
If lateral control was difficult enough, that could take all the PF's attention.
I wounder if it is practical to split duties between pitch and roll channels between the two seats as an emergency means of control in a high workload situation? Very un-Airbus of course.
Your stick force diagram posted a couple of days ago showed much higher forces for the lateral channel of control than the A320 information I re-posted from an old 2004 posting on the subject. The higher the stick force gradient, the greater the lateral control difficulty with a "wing heavy" condition.
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A lot of very technical, knowledgeable and insightful posts on the various systems and dynamics involved in these aircraft.
For the want of covering old ground, and not being nearly as knowledgeable as some others, there remains one question in my mind. I know that in alternate law there is a power + attitude configuration that will restore stable cruise (I believe 82% thrust and 5deg nose up...or something similar). I'm also led to believe that this is well versed in training / sim time. So why were they deviating from the 'norm' with different AoA and power settings? Why not revert to what is taught, and stabilise the aircraft within the known parameters? Surely setting power and attitude to known values is quite easy?
Seems the logical thing to do, for my part, and when I flew aircraft and everything turned to jelly I reverted to basics, and it saved my skin more than once!
For the want of covering old ground, and not being nearly as knowledgeable as some others, there remains one question in my mind. I know that in alternate law there is a power + attitude configuration that will restore stable cruise (I believe 82% thrust and 5deg nose up...or something similar). I'm also led to believe that this is well versed in training / sim time. So why were they deviating from the 'norm' with different AoA and power settings? Why not revert to what is taught, and stabilise the aircraft within the known parameters? Surely setting power and attitude to known values is quite easy?
Seems the logical thing to do, for my part, and when I flew aircraft and everything turned to jelly I reverted to basics, and it saved my skin more than once!
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Re: POB Fall sensation
RR_NDB, you might consider what I said about feeling acceleration and not velocity. They might have felt the plane shuddering and all that. They'd not feel weightless all the way down.
The simplest way to ram this "you do not feel velocity" concept down people's disbelief is simply noting that we are rotating around the axis of the Earth at speeds Jetliners can roughly equal even when sitting in our chairs. Then the Earth is whizzing around the Sun in its orbit. The Sun is orbiting within the Milky Way galaxy. And ... Well, if they don't get it by then they're hopeless.
The velocity may have changed, that cannot happen without acceleration. So any changes in velocity that took place with enough acceleration to be detected by humans would be noted. But it might not be noted and interpreted correctly.
The simplest way to ram this "you do not feel velocity" concept down people's disbelief is simply noting that we are rotating around the axis of the Earth at speeds Jetliners can roughly equal even when sitting in our chairs. Then the Earth is whizzing around the Sun in its orbit. The Sun is orbiting within the Milky Way galaxy. And ... Well, if they don't get it by then they're hopeless.
The velocity may have changed, that cannot happen without acceleration. So any changes in velocity that took place with enough acceleration to be detected by humans would be noted. But it might not be noted and interpreted correctly.
Machinbird;
Your welcome.
I know you and a few here will understand the processes behind the imagination and design capability required to create a reliable, predictable fly-by-wire control system. But there are misgivings about fbw sufficiently expressed here that I thought a comment or two was needed. I say this because I don't believe for one moment that "flight control laws" are in any way associated with this accident. This is not eliminating AFS matters, nor by virtue of exclusion does this now automatically point to "flight crew". The matter is subtle; Like others, I think we are going to learn lots. I think the cost of such learning is, however, unbelievably tragic and frankly intolerable.
I need to say that the document to which I referred should not be taken as "current" and that no attributions connecting the document with AF 447 should be made. While I do not claim to understand the mathematics presented, it is very clear from the thoroughness of the document that "flight control laws" in civil aircraft are not unexplored areas with many unknowns and, as stated, this paper was presented in 1993, thirty years after fbw technology was initiated. There is plenty of experience here.
The author states in this document that the B747 isn't a suitable platform for validating the C* parameter because "it was too stable a platform", (given the original military origin of C* and the desire to control neutrally-stable fighters)! Quite a back-handed complement in my view. Nevertheless I understood enough in the document to know that there was much more behind every stick movement I made on the A330, than I had appreciated.
The intent of posting the paper wasn't to bolster any view; it was to convey to some contributors on the thread who I believe don't fly, or have never flown an Airbus or specifically the A330 and who seem to believe that flight control laws have something to do with this accident, and that a perceived "rudimentary" nature of the A330 flight control laws threatens to leave pilots in the lurch at the most critical moment, and that these laws are squirrelly (even roguish), and that they do unpredictable things that are beyond the control of the pilot, and that they were not vetted thoroughly when designed, and that because they are subject to continuous improvement is proof that at one time they were "dangerous", and, the most humorous remark of all, that there are "too many laws", which brings to mind Emperor Joseph II's remark to Mozart upon hearing The Marriage of Figaro for the first time that there were "too many notes". Too many servos?...too much wiring?... I think the point is gently made. That said, the C* Law is not about protections, it is a non-numerical parameter which defines how fbw interfaces with flight controls. "Protections", came with digital capability. In this, we need to ask not about too many laws, but are such requirements for a sophisticated AFS sufficiently transparent to the user? I think that is a legitimate question.
I think flight crews here will concur that there is no mistaking what an A330 pointed up a 16deg pitch attitude at 37,500ft, at 215 kts with a rapidly decreasing airspeed, is going very shortly to do.
Yet, according to the BEA Report, back stick was held throughout. Further, though THS was continuously, though manually available, no one rolled the trim wheel forward.
The answer to the pitch up and loss of control is not to be found in the minutae of flight control laws even though as a result of flight well beyond the boundaries of controlled flight, the flight control laws responded "as designed".
I believe that an answer may be formed by a deeper understanding not of flight control laws but of what, and why it made sense to the PF to maintain back stick pressure when for about three minutes after the start of the stall at 38,000ft, the descent rate could not be arrested.
For sure this isn't second guessing the PF or hindsight bias. The BEA Report is clear on the back-stick position throughout the recording.
gums and NoD are completely correct to observe first, that overspeeding the airplane, (which, in practise would never occur or for that matter even be necessary), is far preferable even to the maximum dive speed plus, than moving the stick back which would make a loss of energy inevitable. At least on the way to an overspeed, recovery would occur at some point, and second, to NoD's point, at what juncture must the designer continue to intervene through software and, in the present case, provide stall warnings below a chosen speed, here, 60kts? Besides the stall warnings which did operate, the aircraft provided the same cues as any other in such circumstances...buffet and an unstoppable descent.
At what point may we claim "too many laws", while saying that such interventions also have the characteristic of impairing and inhibiting pilot awareness, response and control? Who is capable of determining that point to the satisfaction of those who actually did the work. Peter Mellor's change of heart was cited earlier in the thread by Dozy. A re-read is indicated.
A fuel imbalance, an unintended stick input, an asymmetric thrust/rudder input are all possible factors in the right-roll. The event that was behind the Perpignan accident, and cause of the A340 AIRPROX event are almost certainly not behind the pitch-up after the AFS/AT disconnected.
I have said many times that I do not "defend" the A330 so much desire that any criticism come not from a personal bias against autoflight or other areas which are not widely claimed by others, but if one is going to claim something, I think it is fair to say it should come from a keen knowledge of systems, (if this is to really be a Tech Forum). That approach is a very different matter than claiming that the A330 is pristine and without fault.
Your welcome.
I know you and a few here will understand the processes behind the imagination and design capability required to create a reliable, predictable fly-by-wire control system. But there are misgivings about fbw sufficiently expressed here that I thought a comment or two was needed. I say this because I don't believe for one moment that "flight control laws" are in any way associated with this accident. This is not eliminating AFS matters, nor by virtue of exclusion does this now automatically point to "flight crew". The matter is subtle; Like others, I think we are going to learn lots. I think the cost of such learning is, however, unbelievably tragic and frankly intolerable.
I need to say that the document to which I referred should not be taken as "current" and that no attributions connecting the document with AF 447 should be made. While I do not claim to understand the mathematics presented, it is very clear from the thoroughness of the document that "flight control laws" in civil aircraft are not unexplored areas with many unknowns and, as stated, this paper was presented in 1993, thirty years after fbw technology was initiated. There is plenty of experience here.
The author states in this document that the B747 isn't a suitable platform for validating the C* parameter because "it was too stable a platform", (given the original military origin of C* and the desire to control neutrally-stable fighters)! Quite a back-handed complement in my view. Nevertheless I understood enough in the document to know that there was much more behind every stick movement I made on the A330, than I had appreciated.
The intent of posting the paper wasn't to bolster any view; it was to convey to some contributors on the thread who I believe don't fly, or have never flown an Airbus or specifically the A330 and who seem to believe that flight control laws have something to do with this accident, and that a perceived "rudimentary" nature of the A330 flight control laws threatens to leave pilots in the lurch at the most critical moment, and that these laws are squirrelly (even roguish), and that they do unpredictable things that are beyond the control of the pilot, and that they were not vetted thoroughly when designed, and that because they are subject to continuous improvement is proof that at one time they were "dangerous", and, the most humorous remark of all, that there are "too many laws", which brings to mind Emperor Joseph II's remark to Mozart upon hearing The Marriage of Figaro for the first time that there were "too many notes". Too many servos?...too much wiring?... I think the point is gently made. That said, the C* Law is not about protections, it is a non-numerical parameter which defines how fbw interfaces with flight controls. "Protections", came with digital capability. In this, we need to ask not about too many laws, but are such requirements for a sophisticated AFS sufficiently transparent to the user? I think that is a legitimate question.
I think flight crews here will concur that there is no mistaking what an A330 pointed up a 16deg pitch attitude at 37,500ft, at 215 kts with a rapidly decreasing airspeed, is going very shortly to do.
Yet, according to the BEA Report, back stick was held throughout. Further, though THS was continuously, though manually available, no one rolled the trim wheel forward.
The answer to the pitch up and loss of control is not to be found in the minutae of flight control laws even though as a result of flight well beyond the boundaries of controlled flight, the flight control laws responded "as designed".
I believe that an answer may be formed by a deeper understanding not of flight control laws but of what, and why it made sense to the PF to maintain back stick pressure when for about three minutes after the start of the stall at 38,000ft, the descent rate could not be arrested.
For sure this isn't second guessing the PF or hindsight bias. The BEA Report is clear on the back-stick position throughout the recording.
gums and NoD are completely correct to observe first, that overspeeding the airplane, (which, in practise would never occur or for that matter even be necessary), is far preferable even to the maximum dive speed plus, than moving the stick back which would make a loss of energy inevitable. At least on the way to an overspeed, recovery would occur at some point, and second, to NoD's point, at what juncture must the designer continue to intervene through software and, in the present case, provide stall warnings below a chosen speed, here, 60kts? Besides the stall warnings which did operate, the aircraft provided the same cues as any other in such circumstances...buffet and an unstoppable descent.
At what point may we claim "too many laws", while saying that such interventions also have the characteristic of impairing and inhibiting pilot awareness, response and control? Who is capable of determining that point to the satisfaction of those who actually did the work. Peter Mellor's change of heart was cited earlier in the thread by Dozy. A re-read is indicated.
A fuel imbalance, an unintended stick input, an asymmetric thrust/rudder input are all possible factors in the right-roll. The event that was behind the Perpignan accident, and cause of the A340 AIRPROX event are almost certainly not behind the pitch-up after the AFS/AT disconnected.
I have said many times that I do not "defend" the A330 so much desire that any criticism come not from a personal bias against autoflight or other areas which are not widely claimed by others, but if one is going to claim something, I think it is fair to say it should come from a keen knowledge of systems, (if this is to really be a Tech Forum). That approach is a very different matter than claiming that the A330 is pristine and without fault.
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Originally Posted by MurphyWasRight
On the other hand there is a huge difference between inhibiting initiating a stall warning at <60Kts and arbitrarily -stopping- an existing stall warning when the (sensed) speed goes from 60 to 59 Kts and then even worse-resuming- the alarm just as corrective action is taken (nose down) and the sensed speed rises above 59Kts.
The airframe / software designers have to work to some parameters. Knocking off AoA interpretation below 60K IAS seems valid enough to me - and also at that sort of airspeed you are not just "stalled" in the normal sense of the word - you are a falling leaf.
It is still a stall in that the surfaces are producing no meaningful lift. It's one that probably requires breaking the aircraft's symmetry - full thrust RHS and idle LHS might convert it into something that could then be converted into a airspeed restoring dive. It'd be risky. It'd be better than dying. (And the farking trim should be zeroed on the basis of Otto telling pilot, "Let's try this one again, Fred." Give the poor plane a CHANCE of recovery, please!)
AF447 was placed 3950 meters under water by an amazing cascade of cockpit errors (mostly due to training), conditions that designers never thought possible, friendly controls that suddenly turned well meaningly hostile, and incredibly bad instruments only conditions that made visual recovery impossible.
It might be instructive for somebody more aware than I to go through the report as we have it now and delineate items that if changed the plane would have landed somewhere safely. Start at, perhaps, 01:35 during the flight.
For example had communications existed the pilots might have requested a wider deviation. (But then, maybe not. They never were heard squawking on VHF.)
Or had a radar existed that could spot the fuzzy frozen water ahead of the plane they'd have use that radio then deviated around that nasty stuff.
Had stall warning (really, impending stall warning) training not stressed potentially spurious warnings and recommended recovery as apply power and nose up (I still don't grok that) they'd have stayed level and made it the rest of the way through that murk with a modest anecdote for their troubles.
And so forth. Locate places the scenario could have been broken. Then let's start looking at them for improvements we can suggest. That's pretty close to what the BEA would be doing. They have better data. We have more heads. Advantage - who? Hoped for result - another digit following the "99.x%" reliability figures.
I don't have a relative or friend who died in this. (That was an other event.) So I've no real use for "fault" or "blame" as in "somebody to....". Let's make flying safer for both pilots and packages of Self Loading Ambulatory Meat.
A systematic search might be more productive than the one person shot in the dark like Av Leak produced. (Heh, I used to work for a sister publication at one time as a moonlighting adventure. Press passes to COMDEX were FUN.)