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 it´s 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.

Let´s 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 doesn´t 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.

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? :confused: (If I have that arse backwards, apologies)

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.

It is possible you still have the first version of the BEA note, the second was different :

"The airplane’s 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?

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.

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.

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.

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.

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

Except for a damaged AoA probe, no stall warning in Normal Law.

Actually, from what I remember that is pretty much what they were for the lunar landing; Aldrin's job in particular was primarily to monitor the computer and other systems while Armstrong concentrated on landing safely. I believe true manual control of the LEM was tried and found to be impossible due to lag between control input and response; the 'manual' mode as implemented basically gave the crew the ability to select a point on the surface near the programmed landing site and tell the LEM computer to land there.

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.

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.

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.

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.

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!

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.

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.

(mumble-mumble) Nigel remarked With that in mind Murphy, I am willing to have the stall warning go away at 60kts if you are say 1000' above ground level and not replace it with something else. Above 1000' AGL I'd expect the stall warning to change as airspeed drops below 60 knots, perhaps to a shrill woman screaming, "You're gonna die!" or something else equally attention getting.

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.)

Rush To Judgement
 

So it has begun in earnest. Such a rush is ALWAYS an attempt to cover up real failings by blaming those who cannot be present and have no meaningful advocate.

If anything I'd side with bearfoil on his screaming about the pitot tubes being defective before I'd blame the pilots. I'd also document very carefully what they WERE trained to do rather than what they did. There are many holes visible to this non-pilot cyberunit all of which could have facilitated the crash with what appears to be their normal actions in a situation outside the rational range for those normal actions.

NOW, jcjeant, I am willing to entertain the notion of a coverup. It will be MOST important to read the report and pry loose the raw data for independent review.

Meanwhile hold those authorities', such as this French Transport Minister, feet to the fire until they tell you in detail WHY they make their assertion. On what scientific basis does he make that report? I can't believe the BEA has already reached a conclusion and signed off on it. (And any court that ignores the BEA report is criminal, itself.)

Indeed, unless they suddenly magically entered a serious breeze, 100 knots or more, the AoA would not have changed. So why did those stall warnings take place? The warning after the climb was right on. Why did the PF see that caused him to pull up and not push down as is proper for a stall? That climb cost them a considerable amount of their 1/2 m v^2 energy.

OK, that brings to mind an interesting question together with the original stall warnngs. I presume the vanes can freeze and collect frozen "stuff" at the same time the pitot's iced. Slight scratches on one surface and not the other could leave it unbalanced leading to a bad indication. That seems far fetched. I don't know if it is impossible.

16deg pitch attitude at 37,500ft, at 215 kts
 

Right on Sir !:ok:

You miss my point! The Stall warning, quite correctly, demands immediate, and usually unquestioning, reaction from the crew - and that reaction may endanger the aircraft and occupants - see NTSB Report L1011 JFK

The Stall Warning is essentially driven by a wind driven vane - that vane needs a certain airspeed to produce an acceptable level of accuracy. 60K seems to me, bearing in mind the types I fly / have flown, a typical sort of value where such an instrument will be unreliable below, and in a modern type, it's outputs inhibited.

Other types might use WoW to "isolate" the Stall Warning. Maybe a good idea - however, if you have ever experienced a "false" WoW indication, in a modern electric jet it also is very confusing, and you now have no Stall Warning either.

Now, we can argue forever whether in this, fairly incomprehensible, accident (in which we yet know little) whether a much more complex system that somehow determined the lack of IAS was incorrect / unrealistic and so, on this occasion only, the Stall Warning should have continued, we shall see.

Hi NoD,

On previous aircraft we had this logic:
The stall warning is inhibited on the ground AND with IAS less than 60 kts (to prevent false stall warnings during the take off roll).
If airspeed is > 60kts or aircraft in flight then stall warning was activated.

If AB used the same logic, maybe the Captain would have diagnosed the problem correctly rather than the "Wind Shear GA" (???) procedure the crew seem to have been performing.

Your suggestion might be valid if the certification authorities / manufacturer / design process determined:

1. It is worth giving a Stall Warning from a vane that requires, let us say, 60K to give a valid output, yet IAS appears to be <60K (i.e. we want to give the warning regardless of being outside the vanes limits?)
2. That it is worth giving the warning regardless at <60K WoffW since if we are <60K we are stalled (or if not, shortly will be!)
3. That the various fault sequences / trees are followed

I think we are firstly jumping to fairly firm conclusions from a short interim statement from the BEA, but more importantly, zeroing in on one unusual accident, trying to close percevied design loopholes from that, and not looking at the wider picture.

CONFiture

Fair enough, my last sentence should have said that Vsw in Alt Law occurs at a lower AoA than Alpha Prot in Normal Law. I say that clearly enough elsewhere but it was late and as I mention the Safety First article explains it all much better.

No evidence to suggest the AoA prob/s were damaged though and I would still tend to believe, at this point, that they were providing accurate info.


I would definately agree with those who point to training and how well we are prepared. I have a-lot of military time so upsets and stall recovery procedures were day in day out bread and butter stuff but in a very different aircraft to the Bus. The training I recieved on stall recovery in the airlines was fairly minimal by comparison and certainly I have learnt a-lot of new stuff from background reading as a result of this but should it not have been covered before?

I have had false stall warnings before in large aircraft, one in particular just after take off on a dark, v windy, rainswept in a heavy aircraft was a little stressfull but that was the only symptom, the config was correct as was the speed on all 3 speedo's and the attitude on all the AI's so it was a case of applying full power for comfort, taking it as spurious and at a safe alt getting the eng to shut it up.

Perhaps because :
In alternate or direct law, the angle-of-attack protections are no longer available but a stall warning is triggered when the greatest of the valid angle-of-attack values exceeds a certain threshold. In clean configuration, this threshold depends, in particular, on the Mach value in such a way that it decreases when the Mach increases. It is the highest of the valid Mach values that is used to determine the threshold. If none of the three Mach values is valid, a Mach value close to zero is used. For example, it is of the order of 10° at Mach 0.3 and of 4° at Mach 0.8.
(BEA interim report #2)
Or was the Mach dependency also rejected because of ADR DISAGREE ?

JD-EE

Read the safety first article, it will explain, as I have tried to do, why the stall warning may have gone off.

Ah yes, Nigel, then why did the plane transition from Mach 0.8 into a stall merely because the reported airspeed became incoherent and went to a very low number?

If it is stall warning is solely vane controlled methinks something went REALLY sour in weather system for that to happen. simultaneously something makes the air speed indicators drop AND the angle of attack to appear to change significantly.

Now, speed and altitude MUST figure into the stall warning as the stall speed at 1000' and 250 knots and a 2 degree AoA is not quite the same as at 35000' and a 3 degree AoA (picking numbers of of my nose not for exactitude.)

As a lay person who does understand "systems" concepts this stall warning without the plane changing velocity, moments after the AP and AT cut out and the air speed drops to 60 knots MUST have some screwy roots somewhere in the system. Could the plane have hit a weather phenomenon that could mess up the pitot tubes and simultaneously subject the plane to a high speed down draft strong enough to move the AoA indicator into a stall configuration? If this can happen would there be any way to save the plane? If it speeds up out of the stall and comes out the other side into the matching up draft things could get exciting in a Mach sort of way, couldn't it?

If it can, would it vindicate my smart assed remark, "Don't just do something, sit there!" (Meaning of course, think first. Think THEN Avigate, Navigate, and Communicate.)

What sets of events could that report from 2 h 10 min 05 really mean?

safety first article Jan 2011
 

I think, you reference to that part of it.

Ah, NoD, I am willing to suggest an A330-200 at 1000' AGL going 60 knots is in deep deep fertilizer. You seem to be saying that the warning about this is unwarranted and should be turned off. I'm silly, I guess. I don't understand the logic.

RetiredF4, this needs definition, "at or close to the maximum recommended FL".

Some discussion here has suggested it was not particularly near maximum recommended FL and had a considerable speed margin both above and below the speed at which they were operating. So in context I wonder what "close" would mean.

If it was "close" and the stall warnings were transient, that suggests the PF should have done nothing for long enough to see if the warning would go away. Near the ground an instant reaction is needed. At 35000' it seems there is a lot of time to spend thinking before action becomes critical. And it would take you out of any transient condition that would suggest stall.

JD-EE

according the reference the warning could have been caused by turbulence in combination with the FL, roll to the right, reduced mach 0.8 and turbulence, correct?

At least i think it´s more probable than any other discussed scenario.

Correct, however there was that roll, that probably motivated the PF to do something, at least counter the roll by opposite input. I dont know, wether the computers would assist the opposite roll with the apropriate rudder input or wether it would be inhibited by alt LAW and wether that started the whole outcome.

For the ice crystal encounter phenomena I don't think (IMO) the aero surfaces with streamline flow (vanes) are likely to be affected as they would be in freezing drizzle/rain.

The reason the probes and engine struts are sensitive is the bluntness internally (stagnation?)

happy to be corrected by an aero guy :)

JD-EE

Heh. Given that landing speeds of most transport category aircraft are well in excess of 60 knots (A330 looks to be 135-145kts, depending on weight and other conditions) an aircraft in that class with a stall warning at 60 kts below 1000 feet has been in deep manure for too many seconds, and has most likely been gettting stall warnings since long before that ...:cool:

Which makes me understand why clipping it at sixty might seem to make sense in the design process, but that seems to indicate a design assumption:

That stall will be triggered at low speed and low altitude? (I may not have successfully reverse engineered the thought process on that one ... )

GUMS-

(From several resources I have saved)
There is no 'feedback' to the pilot from the stick besides its physical position, its totally fly by wire and sitck deflections are monitored by microswitches, probably feels much like a standard computer joystick prehaps.


The Rotational Hand Controller (RHC) is an analog controller, not digital, so it uses potentiometers, not microswitches, to measure stick deflection. (Technically, there are microswitches in it but they are used for BFS engage, trim, PTT, etc, not flying). Like a PC joystick, the RHC has centering springs that provide increasing resistance to larger deflections: 1.45 in-lb per deg in pitch, 2.1 in-lb per degree in roll, and 0.7 in-lb per degree in yaw.

more here;

ROTATIONAL HAND CONTROLLER

JD-EE;
Lonewolf_50's post describes this very well.

If we must continue to reference airspeed when in fact it has nothing to do with the stall warning which is driven solely by AoA, the "screwy roots" are the fact that the engineers designing the system knew that the airspeed information was both unreliable and beyond design expectations (as a realistic flight regime) for use by downstream system users and was designed to not be "input" below 42kts, (AMM).

I think there is something to be pursued in an AoA display discussion but, as someone pointed out, there's another item to scan on a very busy PFD, so how, when, why do the engineers and pilots decide to display the AoA?

A "flight" regime which is below 60kts IS certainly unrealistic, ....isn't it? And the AoA was working all the way down...the parameter is there, in the data. But so was the buffeting and the inability to stop the extreme rate of descent.

If that set of circumstances is somehow "still confusing" and, as per calls for continued stall warnings when the airplane is falling vertically we need more warnings "just to be sure", where are we then, when it comes to aviating part of "aviate, navigate, communicate"?

You have to think of the designer's problem. You can imagine all kinds of screwy roots that engineers can conjure that pilots may tap but what do you design for? You make intelligent assessments of not only what is probable but what is possible within the realm practicality.

Do you as an engineer design for 10 ^-9 probability? Why all of a sudden is it an expectation here? Where is the engineering case for this specific case?

At some point the engineer assumes that others in the system know what they're doing, aren't ham-fisted and that they aren't handing a squirrelly system over to amateurs and gamers but handing a robust system which has been vetted by extremely capable engineering people over a long period of time.

That was the point of my longish post about the C* Law...these people actually do know what they're doing and even the harshest critics came to say so after actually taking a look at the A320 design more than twenty years ago.

Does a pilot really need a stall warning at <60kts/AoA > 35deg to tell him or her that the aircraft is stalled?!

We cannot have it both ways. The complaint has been "too many laws", too confusing an airplane! and whatnot, but wait... We need another law to ensure that the stall warning stays in play until...? When...a minus airspeed to cater to the foolish scenario that someone here posited ten-thousand posts ago that the airplane somehow "back-flipped" and that's the reason the spoilers were damaged the way they were?

At some point just a little credit must be offered the engineers who conceived of the designs, put these airplanes on the drawing boards and built them, if only to get some here off top-dead-center in their thinking to consider positing something approximating a marginally-possible scenario regarding this loss-of-control. Focussing narrowly on 'no stall warning below 60kts' is simple, ignorant folly which caters to an unbelievably low standard of professionalism in airline crews.

Your smart-assed remark about "don't just do something, sit there" was and remains spot on. DO NOTHING was the correct response and that is the action which would have "prevented" the loss of control. I pointed that out three threads ago because that is the way the checklist reads - the airplane was stable before the event and, doing nothing while the airspeeds return will keep the airplane stable. Move it, and you've moved the one "known" in the equation that you had and you've taken yourself into no-man's land with no way back, without a great piece of luck and skill.

In thirty-six other similar events no one did anything and it worked as intended, but no one here is ready to examine the record and talk about it and instead prefer to bash the airplane in ways that have nothing to do with the accident.

Why did the airplane climb? I have already posited a perfectly good reason and no one has examined or critiqued it probably because they can't but they can focus on "the automatics". I don't claim it as correct, just a possibility. I discussed hundreds of posts ago the possible inappropriate execution of the UAS memory items, which require an increase in pitch attitude. Silence about that, but not about the minutae of flight control laws and off-the-wall theories about AIRPROX events and stuck vanes. I won't even bother waking Mr. Occam.

Someone observed that "the thrust didn't increase so how could the airplane climb thus?" One must appreciate the energy/inertia resident in a 205T mass moving at 900 feet per second to understand that one could have pulled the thrust levers to IDLE and raised the nose just 5 degrees and still spectacularly achieved what was always going to be a momentary climb and a pathway to a stall if one didn't get one's act together in a real hurry, or, as you succinctly pointed out, a rapid reduction in their 1/2 m v^2.

Wait!....someone, jcjeant perhaps?, mentioned coverup...yes, yes!...there it is, in front of us all along and we missed the obvious.

More AOA sensor info!
 

- Operating Range 120º, limited by fixed stops.
- Balanced (= static it could take any position).
- Contains a dual purpose damping motor, damping the rotor in opposite direction of movement
created by Eddy Currents with a torque proportional to the speed of rotor movement, with a breakout force of 0.04 Nm.
- Damping motor can also be used on GND for test purposes (positions AOA sensor in a pre-determined test position)
- Internal heating element 115 VAC, 400 Hz; operating temp. 120 ºC.

For Audible Stall Warning I could find:

- Normal Law - AOA > 23º.
- ALT Law & Clean - AOA > Valued function of Mach number.
- ALT Law & SF - AOA> Valued function of SF position.

Just for a moment went into the B777 books too, for reference only not for pleasure :bored:
far less information but everything related seems to be unvalid if CAS< 30 Kts.

Thank you for the info, A33Zab. I found one additional piece on accuracy of the AoA sensors. The accuracy of the AoA sensors at 100kts is +/- 0.30 deg.

The FWC [Flight Warning Computer] activates the audible stall warning. All other aural warnings are inhibited if a stall warning is in progress.

The Boeing manuals indeed have far less information.