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AF 447 Thread No. 5

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AF 447 Thread No. 5

Old 14th Jul 2011, 02:50
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Reflecting on my miniscule PF time (c.f. PNF & SLF)

I do not think I would like being handed an aircraft in much turbulence of any kind, some way out of S&L wings level trim, with a FCS stick offering me (what I know as) neutral stability...
at night, when perhaps I'm not even too happy or in tune with > what has just occurred (perhaps a flight disturbance, small or large).

I do stand to be corrected that I have misread 12,543,298 posts and that the a/c does not feel to me through the stick as one with neutral stability... but that is (within reasonable pitch rates and 'g' load) what the concensus seems to be.

=============

In other words, I suppose I'm asking.. was this a good choice for cruise flight? Indeed, was it considered just a fallback for 'occasional usage' and thus not too important.. the normal reason for moving away for a goodly degree of static stability is 1) maneouvrability (wrong type of a/c) 2) ease of control over longer periods (not usually applicable with auto-trim)

So...

Last edited by HarryMann; 14th Jul 2011 at 03:02.
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Old 14th Jul 2011, 03:49
  #282 (permalink)  
 
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c.g. and longitudinal stability

And yes, TK, I have been here recently.

Good points Harry, no problem with you jumping in.

As the c.g. moves back to the center of aero pressure, or so called neutral point, the jet has less tendency to revert to the 'trimmed" condition that existed when a disturbence upsets the flight path. In short, a very statically stable jet tries to get back to the trimmed state real quick. In non-FBW systems this is very apparent, as you have to "hold" control pressures to keep the beast from returning to its happy, trimmed state. This all goes out the window with most FBW implementations.

Flying with an aft c.g. is like balancing on the point of a needle. Lottsa work, and you better be good. So your inputs might, indeed. be very effective for a second or less. Then you overshoot your desired attitude or AoA or gee and you can get into the classic PIO or worse.

If you get the c.g. sufficiently rearward, the plane will flop end over end. Think about an arrow. See where the center of aero pressure is compared to the weight of the arrowhead and shaft. Ever see an arrow flip end over end? Didn't think so, heh heh.

One way to experience the real static stability of a FBW jet is to go to something like the "direct law", where all the electrons are doing is commanding a control surface movement. Then you make a small stick movement, relax pressure and see what the jet does.

I would take the forward c.g., and I have a feeling that the AF447 folks knew there was a possibility of some turbulence and didn't move a lotta gas rearward for the improved trim drag and such. If I got close to a stall I would have basic aero laws helping me as much as HAL's laws.

As a point ( short war story), our little jet had a negative static stability margin until about M 0.9 to 0.95. Like the 'bus, the "auto trim" tried to maintain our "hands off" gee command. Due to the negative static margin, our HS was actually providing "up" force. Hence, our first complete loss of the confusers at about M 0.6 or so resulted in the jet going nose down and between 10 and 20 negative gees on the pilot. He got to the bar that night with two super shiners and had bloodshot eyes for a week, heh heh. Next one was a fatality, and estimated 22 negative gees ( too low for a good ejection).
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Old 14th Jul 2011, 03:55
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Chris S, gums, Turbine D, takata, airtren, this paper may have been referenced before but if not it may be of interest in the present discussion, and also in the ongoing discussion regarding "Why the pitch-up?": "The Effect of High Altitude and Center of Gravity on Handling Characteristics of Swept-wing commercial airplanes"

syseng68k;

Thank you for your response. Some of this has been discussed thoroughly before, but some has not - the following summary, part way through the Boeing paper referenced above, advances the point I was trying to make regarding training, knowledge of High Altitude flight and transport characteristics.

Believe me, these notions are not discussed in any groundschool I have ever had from start to retirement - "NTK" was, and is, taken seriously by cost-conscious beancounters, (as you say), but even in recurrent, none of this is discussed - it is assumed (self-study is fine, but materials the deal with the issues but which aren't academic papers, are scarce) - and in fact, accident reports and FOQA trends are never discussed either. Here's the relevent section, (my emphasis) - kindest regards, PJ2...
Summary
The use of wing sweep and stability augmentation on modern commercial airplanes makes them more fuel efficient. However, flight crews must understand the effects of CG and altitude on performance and handling qualities. For example, operating at an aft CG improves cruise performance, but moving the CG aft reduces static longitudinal and maneuvering stability. Many modern commercial airplanes employ some form of stability augmentation to compensate for relaxed stability. However, as long as the CG is in the allowable range, the handling qualities will be adequate with or without augmentation. An understanding of static and maneuvering longitudinal stability is an essential element of flight crew training.

Last edited by PJ2; 14th Jul 2011 at 04:08.
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Old 14th Jul 2011, 04:30
  #284 (permalink)  
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Longitudinal Stability -

gums' last post highlighted the relationship of stick force/stability/CG. That is carried on in the Boeing paper cited by PJ2.

Casting the Boeing words a little differently, it is worth keeping in mind that longitudinal static stability is all about what stick forces the pilot "feels" when the aircraft is not on speed so far as trim speed is concerned.

At the trim speed the stick force should be at or very near zero.

If the aircraft slows down (for whatever reason) and the pilot desires to hold that lower speed, acceptable static stability requires that the pilot MUST hold on a PULL force of some measure. Furthermore, the further away from the trim speed, the HIGHER the pull required

If the aircraft speeds up etc we are looking at a PUSH force etc

As the CG moves aft the slope of this line (stick force against off trim speed delta) reduces and the aircraft gets progressively twitchier.

In an ideal world, the line would be straight and extend well away from trim speed. In reality sometimes the line does strange things and we need additional add ons (such as SAS) to adjust forces so that the pilot is fooled into believing that all is well. In small aircraft, this might be something as simple as a fancy spring arrangement bolted onto the elevator control circuit - typically seen in GA piston to turboprop conversions.

As CG moves aft we eventually get to a point where the static stability is unacceptable and we enter the region of static instability. The aircraft is still flyable (if you know what you are doing). The technique is tiring and stressful and consists of making an input, freezing the controls, assessing the response, making an input .. etc.. etc. A pilot in the know might manage to get the aircraft around the circuit for a landing .. but an extended flight recovery is a big ask. Hence the need for fancy electronic stuff to do the hard work.

Further CG aft movement eventually gets into a region of dynamic instability and the aircraft is beyond the ability of the human pilot altogether ...

Whatever the AF guys had in front of them in the cockpit, they had their hands well and truly full ... one way or the other .. not a situation any of us would wish on any pilot.
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Old 14th Jul 2011, 06:13
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Thank you (in chron. order) HarryMann, gums, PJ2 and John T. for providing some very usefull informations about longitudinal stability (or instability) related to CG position.

My question was also aimed at this aircraft pitch authority (or lack of it) once stalled at such an AOA : If, after a while, a recovery was attempted, the issue faced by the PF could be that he could lack the necessary pitch down authority in order to regain some lift.

If I understand correctly, assuming that the elevators were not fully stalled, an aircraft with an aft CG would have to be pitched down to a lower angle-of-attack but would also have a better pitch rate than with a more forward CG. On the other hand, more forward CG doesn't help for stall recovery but it's better for avoiding to enter the stall at the first place.
Then AF447 had a less than optimum lift cruise CG that did not prevent it from stalling without being (in theory) as much helpful for a recovery attempt.
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Old 14th Jul 2011, 06:47
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Hi Takata,
If, after a while, a recovery was attempted, the issue faced by the PF could be that he could lack the necessary pitch down authority in order to regain some lift.
I suspect that Airbus have already done some wind tunnel tests to prove what would happen, and the answer most likely was that sufficient THS and elevator authority was available with 29% CG.

The abnormal airflow and drag conditions during this high and increasing angle of attack appears to have had a stabilizing affect. ND on the elevator would have increased the surface area facing the airflow and provided the initial lever to pitch the nose down. Provided the THS followed along, the area and angle to the airflow would have been maintained and eventually all the airfoils would have slipped into an unstalled regime.

There is some evidence that this was tried and unfortunately stopped.
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Old 14th Jul 2011, 07:22
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I think I have mentioned this before, but if an aircraft is pitching down after encountering a deep stall and has a significant pitch rate going while just getting into flying airspeed range, it is quite possible to overshoot the flying attitude and enter a negative g stall. If CG is aft & static stability is low, and with high enough pitch rate, you may not have enough control authority to stop the pitch down motion before stalling in the opposite direction.

Several military NATOPS manuals specifically address this point in stall and spin recovery.

Here is another situation where an AOA indicator gives you some warning before you actually float out of your seat that your AOA is approaching the normal range. Otherwise, when do you stop pushing the nose down?
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Old 14th Jul 2011, 08:23
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mm43 wrote:
There is some evidence that this was tried and unfortunately stopped.
The reference is to the ND input by PF.

The question that must be asked is, did PF think that he could recover given enough altitude, but that at that point realize that the A/C would auger in before completing the recovery? Did PF then opt to return to the stable stall with vertical component just over 100 mph in the hope that some would survive?

Well, pasteboard cabinets survived. Suppose the PAX had been in rearward facing seats, and instructed to recline the seats? What more in the way of energy-absorbing collapsable support struts would have been required? After all, there are miles of support struts under those seats already, so that any airline can have any passenger density in any area of the cabin it may desire, cost and complexity be hanged for that.

Yes, I am aware that bearfoil said that someone was going to say this was a survivable crash, way back when. And I read the exellent helicopter post on this subject. But the PAX here were on top of many feet of crumple zone.

Would someone want to run a parabolic trajectory at 2.5g (normal to wing surface) recovery from a 45 degree descent, starting from the altitude at which PF abandoned the ND effort, and the assumed 1.414 times vertical speed of 100+ mph? This overlooks the time required to effect ND, of course, just for simplicity, as well as speed increase as the ND effort gradually takes effect, but would include the speed increase from starting on a 45 degree slope. Just a first cut to get a ballpark. Thanks, OE.
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Old 14th Jul 2011, 08:27
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Not sure if it is the right table but the CG should have been here:
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Old 14th Jul 2011, 08:57
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As I have said before, once the crew found themselves with an 'unknown' ?60? degrees AOA at the top of the 'zoom', they were well outside any training environment we could contemplate. In terms of instantaneous manoeuvre, they would have needed to pitch around 50-60 degrees nose down to unstall - who outside an aeros/mil pilot would dream of putting the nose down 40 or more below?

If we, as pilots, are to expect 'cradle to grave' protection (with limited understanding of how it works) from our systems they have to PREVENT this position in the first place. IE

the system should either prevent or warn of excessive AOA
prevent or warn of excessive THS movement

in terms of the Mk1 FMCS (the pilot), he/she MUST be allowed to see a reliable attitude indication at the outset AND trained to maintain the correct one - before things go pear-shaped. Once IAS reaches the very low levels 447 saw, pitch attitude has little meaning in terms of performance

We still do not know whether 447's 'zoom climb' was initiated by the system. The AAIB appear to say TC-JDN's probably was due to the system dropping itself into alpha-prot. In both cases, it is probable that had crew reaction to the pitch changes been more effective and quicker, we would not be in this long thread. HOWEVER - this assumes that crews are aware of the pitfalls of the systems, do not lose attitude info AND retain some basic flying skills - and, of course - are 'permitted' by the FCS to make the necessary control inputs.

I regret to say (yet again) that either the fbw sytems AND/OR the training need to change. I watch in a state of disbelief as all our 'experts' jostle backwards and forwards, page after page, over this flow diagram or this servo circuit or this latching/voting logic and wonder how mr/miss average Mk1 FMCS is going to cope with it all in a failure state - in the very limited time available - when even the computers do not know what has happened. The same goes for 'relaxed' stability in the drive for economy. If the a/c is too statically unstable to allow the 'average' to fly it without the fbw systems, this also needs to be reviewed.

Several of us have re-iterated that had the crew been able to and had flown basic pitch/power at the outset, this would not have happened. We need to focus on the why. We should, in the short-term, forget all the millivolts etc. Someone knows. It is probably on the CVR.
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Old 14th Jul 2011, 09:07
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mm43
The abnormal airflow and drag conditions during this high and increasing angle of attack appears to have had a stabilizing affect. ND on the elevator would have increased the surface area facing the airflow and provided the initial lever to pitch the nose down. Provided the THS followed along, the area and angle to the airflow would have been maintained and eventually all the airfoils would have slipped into an unstalled regime.
There are other issues to be observed, and which could have favoured the establishing into this high AOA and prevented recovery.
As we know, airfoils produce lift also after CLmax is left behind, which means in the stall condition. We have two airfoils (wing and THS) and the complete fuselage producing lift at those stalled AOA areas. Prior to AF447 nobody would have believed that it would be possible to get such a big transport aircraft into an AOA above 35° and keep it there (disregard the protections, i mean just from the aerodynamic point of view). Everybody would have expected that there would be not enough THS / elevator authority to get it there and keep it there. This applies especially if the CG is that much forward like with AF447. So we have to ask ourself, are there factors in this post stall region which helped to stabilize AF447 in that high AOA besides the THS input?

As mentioned, both airfoils produce lift in stalled condition, and if the highly sophisticated main wing of AF447 produces more upward lift at a given stall AOA than the THS is able to counter with an upward lift (Full ND) as well, then we have a big problem to get the nose down. To get the nose moving down, the THS has to produce more upforce than the wings (we for sure have to take the leverage into the equation). Do we know, how the lift generating capabilities of those airfoils changed in this high AOA regions? No, we dont know. Do we know, how the lift will change, when we go f.e. from 50° AOA to 35° AOA by THS input (assuming it has enough authority at 50AOA)? No, we dont know.

Aditional lift producing parts would be the fuselage. Would the forward part produce more lift then the rear parts, or equal or less? We dont know. How would the lift change, when we change fuselage AOA from 50°AOA to 35° AOA? We dont know.

Travel of center of lift is another issue, we have not adressed yet. How far does center of lift move (aft or forward? In normal unstalled AOA conditions center of lift moves forward with increasing AOA?) with that high AOA? We dont know.

Maybe somebody has answers to those points, but its no use to disregard those issues when judging wether THS would be effective or not and only assume them to be a non issue.

Yes i know, We seem to have evidence that THS was effective. They did some nose down and AOA and speed changed....., but those changes might not be related to the ND input, or they might have not been effective enough (due to change of effectiveness in relation to different AOA), see my comment some posts before.


Edit: Center of lift = Center of pressure Cp

Last edited by Jetdriver; 14th Jul 2011 at 16:18.
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Old 14th Jul 2011, 11:40
  #292 (permalink)  
 
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RetiredF4;

Please have a look at Figure6, originally linked by
zumBeispiel and tell me what is so different about the A330?
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Old 14th Jul 2011, 12:01
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Hi PJ2,

Thanks for the link "The Effect of High Altitude and Center of Gravity on The Handling Characteristics of Swept-wing Commercial Airplanes"

"For a statically stable airplane the required column force, as speed varies from the trimmed condition, is less at an aft CG than it is at a forward CG. The minimum average gradient allowed by U.S. Federal Aviation Administration FAR Part 25 is one pound for each six knots."
Obviously AB FBW in ALT LAW does not meet that specification - and unfortunately the aircraft stalled. The known problem with the pitots should have been designed out much earlier.
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Old 14th Jul 2011, 12:13
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HazelNuts39

RetiredF4;
Please have a look at Figure6, originally linked by
zumBeispiel and tell me what is so different about the A330?
i dont know, but there should be data available which represent exactly the aircraft we are talking about and not any kind of airfoil to answer the question, how AF447 could get into this high AOA regime and how it could maintain it.

By the way, note the text associated with Figure 6 of your reference:

The pitch control remains effective throughout the angle of attack range but diminishes with increasing angle of attack, due initially to the immersion of the horizontal tail in the wing wake and ultimately due to flow separation on the horizontal tail itself at post- and deep-stall angles of attack. The maximum steady angle of attack with full nose-up elevator deflection is at α≈25˚, which is significantly higher than the stall region near α≈12˚ and this result indicates the potential for the airplane to enter upset conditions using normal pilot controls during un-accelerated flight.
There seem to be lot of differences, AF447 kept more then double the AOA on the way down.
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Old 14th Jul 2011, 12:48
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Assuming the PF could have got the nose down once he was falling, stalled at 100mph, what angle would he have had to go to?
I would guess that he would have gained even more downward velocity as the nose came down towards the wind (less drag). This would have meant the AOA increasing and him having to chase it.
Has anyone ever turned the nose of a big aircraft down to the 30 or 40 degrees?
How many seconds would it be before the wings came off?
Is stall recovery only possible in the very first moments, before the downward velocity gets too big?
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Old 14th Jul 2011, 13:12
  #296 (permalink)  
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Originally Posted by t54
what angle would he have had to go to?
- post #288?
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Old 14th Jul 2011, 13:21
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As a ‘newbie’ here I have been reluctant to say too much in the presenc e of so much expertise, but I do think that I may be able to make a positive contribution towards understanding the aerodynamics – at least I hope so! I should preface any remarks by explaining that I was once an aerodynamicist, but anything I write is based on general principles and standard methods, not any particular knowledge of the A330, although I have of course used such data as is publically available. JT gave a lucid explanation of how the stability changes as CG is moved back, but before anyone goes off down one of those typical thread tangents about how the A330 with relaxed stability must be dangerous, it should be recognised that the CG where one arrives at static instability on the A330 is way back around 45~50% mac (it depends on Mach Number). Compare that with Takata’s CG envelope and you will see that it is nowhere near static instability even at the aft CG limit.
If I have understood correctly, the A330 in Normal, Alt and Alt2 laws operates under some form of C* control, and it is only in Direct law that this would come into consideration anyway. Also, AI would have had to show, for certification, that the aircraft was flyable in all states, including Direct law, without requiring exceptional piloting skill.
The requirements BTW, say that a pull must be required to obtain and maintain speeds below the specified trim speed and a push for speeds above that. The speed (in cruise) must return to within 7.5% of the original trim speed when the control force is slowly released but it is acceptable for the aeroplane, without control forces, to stabilise on speeds above or below the desired trim speeds if “exceptional attention on the part of the pilot is not required to return to and maintain the desired trim speed and altitude”. Not my words, not AI words; JAR words.
Retired F4 posed a lot of questions, most of which he answered by “We don’t know”. In this of course he is entirely correct, but that doesn’t stop us making sensible engineering estimates.
I suspect that most peoples’ mental image of a stall is a sudden loss of lift and a nose down pitching moment, typical of the straight, unswept wings on which they learned to fly (I am not a pilot BTW). However, a modern swept airliner wing does not stall like that.
The sweep, taper, camber and twist of a typical modern design will result in the maximum local lift coefficient occurring first at somewhere between half and two thirds semispan. This is generally a little way aft of the CG, so the first effect will be a gentle pitch up. What happens next will depend on how the wing is twisted. If the outer wing is more heavily loaded there will be a steadily increasing, but still relatively modest pitch up. When the inner (and forward of the CG) part of the wing stalls there will be a compensating pitch down. Throughout all of this process the wing lift coefficient is departing from the linear relationship with AoA that it had before stall, and eventually it will end up as a fully stalled wing with a more or less constant normal force coefficient. The actual ‘lift’ and ‘drag’ will then be the component of this normal force resolved into axes parallel and normal to the incoming airflow (i.e. to the FPA usually). When it gets to this point I think the centre of lift/pressure will be fairly close to the centre of area of the exposed wing. For the A330, this is about 70% mac. This means that when fully stalled the pitching moment from the wing will be a nose down value which does not vary much with AoA since the PM will be the normal force coefficient times the moment arm of this 70% to the CG at 29%, and the normal force coefficient will be nearly constant.
Net result of all this is that the actual process of stalling can be quite a gentle affair as Gums has suggested from time to time.
A couple of pages ago I posted an explanation of the mechanics of aerodynamic equilibrium in deep stall conditions, and nobody has yet said they think this was nonsense, so I am sticking with it. When you look at this, it is clear that the question to be asked is not will there be enough down elevator power to give a ND recovery moment, but rather will there be enough elevator power to get and hold 60 deg AoA. To recover all that would be necessary would be to remove the up elevator, although of course some down elevator wouldn’t hurt – so long as you don’t stall the THS.
OK, is forward or aft CG better/worse for stall/recovery?
The wing will stall at the same AoA regardless of CG position. With a forward CG the aerodynamic moment about the CG when approaching stall will be more ND because of the increased moment arm. So more up elevator will be required to approach and maintain a given AoA, the negative tail lift will oppose the wing and the overall lift coefficient will be lower with forward CG than aft and the aircraft will stall at a higher airspeed – but the same AoA!.
When the wing is stalled, for the reasons described above, the moment (a ND moment remember) will be nearly constant, so any variability in speed of recover will come from the elevator’s ability to provide ND pitch acceleration. This is going to be bigger with the moment arm from a forward CG, so my vote goes to forward CG as being more favourable.
Although it has been said that we cannot know what the post stall conditions are, we can calculate them for at least one point where we have ground speed, attitude, and descent rate; namely the point of impact. Taking the quoted values as ‘gospel’, one can calculate the airspeed as about 151 kts, the AoA as about 61 deg and the FPA as -45 deg. In addition it is believed that the engines were at Flight Idle at this time. For the sort of engine we are looking at this is as near zero net thrust as makes no difference. Assuming (OK, it is an assumption) that this was a stabilised state, then you can calculate what the lift and drag coefficients would have had to be to match – both lift and drag coefficients at about 1.07.
Hope this does eventually help and not spread more confusion!
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Old 14th Jul 2011, 14:23
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[quote} There seem to be lot of differences, AF447 kept more then double the AOA on the way down. [/quote]

Not the least being that AF447 had the THS at -13 all the way down where this model was (probably) set to zero.
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Old 14th Jul 2011, 14:34
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Owain: wow.

Here is a summary of what I gleaned from your post. Please let me know what I have misunderstood.

Once stalled at altitude, and with CG as estimated, (be it 29% or 37%) the center of lift of the wings will provide a self-correcting nose down pitching moment proportional to such lift as the wing is still creating.

Per the FBD I just sketched on my napkin, the arm that the force acts through is of the length somewhere between 70-29 to 70-37, (as outer boundaries). Based on where the THS is, I'd guess its relative number for arm calculation is about 96 or 97. (Am I close?) as compared to the center of pressure on the wing. (@takata: thanks for the posted CoG chart).

Thought: IF THS and elevator (as a lift producing system) have lost control authority or were stalled, THEN there would initially be no force from the back end, or a very small force, countering the "self correcting" pitch down moment of the stalled wing.

Thought number 1:

As the nose attitude gets closer to level, wouldn't the C of P start to move forward from 70 towards a smaller value, and gradually reduce the length of the arm, and thus the moment, of the correcting tendency?

Thought number 2:

THS is an airfoil, so even if stalled, it produces some amount of lift and thus provides, through that longer arm, some counter to the correcting tendency of the wing whether or not it is stalled.

Your line of thought presents me with the provisional conclusion that the THS was not stalled, since the nose stayed up (per the BEA report) and didn't (as far as we know) oscillate up and down as it might if the THS were stalled.

What you described is a "natural" pitching down movement (??) of the stalled wing, which seems to have been countered by the longer arm being acted on by lift from an unstalled tail/THS.

Am I close?

If that's about right, it leaves me with a non trivial concern:

if the nose stayed up due an input or command other than pilot control inputs, the nose being held up by (THS lift) x (arm) prevented stall recovery for about 30 thousand feet worth of travel down to the surface.

As Retired F4 points out, "We don't know." In this case, ignorance is surely not bliss.

The above sort of reasserts a fairly obvious point: an ounce of stall prevention avoids about 200 tons of attempted cure.

From PJ2's linked article on high altitude handling:

3 Maneuvering Stability
An additional effect is that for a given attitude change, the change in rate of climb is proportional to the true airspeed. Thus, for an attitude change for 500 ft per minute (fpm) at 290 knots indicated air speed (kias) at sea level, the same change in attitude at 290 kias (490 knots true air speed) at 35,000 ft would be almost 900 fpm. This characteristic is essentially true for small attitude changes, such as the kind used to hold altitude. It is also why smooth and small control inputs are required at high altitude, particularly when disconnecting the autopilot.
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Old 14th Jul 2011, 14:49
  #300 (permalink)  
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BOAC
As I have said before, once the crew found themselves with an 'unknown' ?60? degrees AOA at the top of the 'zoom', they were well outside any training environment we could contemplate.
The AOA wouldn't have been high at the top of the zoom. It would have increased to its terminal, large value as the plane gained downward velocity, creating an upward component for the wind, so to speak.
I'm thinking they had a chance if they'd been able to act quickly, but once they were falling fast the dip needed for the nose might have been more than they were prepared to contemplate - or would have resulted in greater forces than the airframe could withstand.
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