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AF 447 Thread no. 4

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AF 447 Thread no. 4

Old 18th Jun 2011, 06:53
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Promotion recommendation

I move that jpete, Probationary PPRuNer, be promoted immediately to Newby 1st class, with Elephant stamp and Star. Seconders? Ah. I see Thread Boss has already done that in effect.

Thank you jpete, for a wonderful tool. I hadn’t grasped the full power of Googles’ ‘site:’ prefix, and was only using it simplisticly to search my counties’ sites.
____

Thanks also all you others, who responded to my ISUS search query (ICUS sic). I drove the first steam age A320s for a bare 100 hrs or so, and I see the pneumatics are as always. However, I was mostly interested to discover the degree of autonomy that the new digital S/B 'A/H' has.

I twice suffered loss of all driven flight instruments, (not on the 'bus!) once a total shut-down for 8-10 mins, and the other more insidiously with both the ADI & HSI freezing in place with no warnings at all, both in dark-night. I therefore used to be somewhat paranoid about finding out how long each type that I flew could last on battery power, after loss of all electrics (had once), and would develop an Apollo 13 like power-down procedure, until all that was left was the 3A (IIRC) S/B AH, a torch, and the SB pneumatic gauges, radio sched call each 30 mins, from the one (motorbike rated) 20 A.H. NiCad battery in the belly, just in case I lost all elects again. With 3 hours max to the nearest available, highly desirable. I also frequently practiced hand flying on cruise to 'freshen up,’ and was always very manually current.

This preamble is to say that, given the 'bus’s zero stick-feedback, zero throttle-feedback, and non-amplified vertical attitude tape response (as, say, the Concord had in cruise), the thought of hand flying the A320 on cruise was a bridge too far for me. To return to the thread: Even given that hand-flying is actually only providing joystick like CWS inputs in Airbus's FBW logic, and far too coarse for the delicate inputs needed at high altitude, I cannot see how any line pilot, and possible even any test-rated pilot could hand fly this extreme initial jet upset, the cause of which remains a mystery I understand.

My interest in the ISIS was actually related to the soundness of the attitude display, whether it’s attitude reference is integrally sourced as the original A320s were, or whether the ISIS is dependent on external reference in any way. The manufacturers specifications make no mention of this. If the ISIS *is totally independent in attitude, it does seem odd that there appears to have been no flight guidance reference made to it, at least discussed. In such an upset, I would have been glued to it, regardless of all other indications.

But then, who can discuss realistically the forces in the interior of a Cb, except those who have done so and survived, usually with hairy stories?

In regard to PJ2’s comment: “..never saw the "slight increase"!” (a pitch-up) PJ2, it certainly existed in the climb. In every previous type I flew when possible I used to hand-fly to TOC. Constant refresh of cp movement/mach tuck, etc. That went out on the 320, with the pitch up. It was approx 0.2 deg/sec, which required approx 10 g nose down force applied to the top curve of the side stick to neutralize. (A gentle little-finger pressure.) Somewhere on file I have copies of the two (typewritten) letters I wrote to Airbus directly, requesting an explanation for the apparent external force being applied in CWS, which could compound a situation if a pilot chose to rely on CWS in some situation. Seems it may have been the 1g at increasing altitude effect. I never did heard back from Airbus.

In the meantime I go back to exploring the implications of RetiredF4's eye opening post (his~her reference to Ostawiri & Naik’s post stall study of the NACA 44x airfoil lift curves specifically) which has chilling implications which I find quite disturbing.

In memory.
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Old 18th Jun 2011, 09:31
  #142 (permalink)  
 
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Originally Posted by RetiredF4
Lets start with the 46.5°AOA of the THS (60° Aircraft AOA minus 13.5° NU trim) , which gives us a CL of 1.2 and a CD of 1.2. Lets do some ND input by trimming the THS to 0° and let´s see what it does to our numbers. We end up at 60° AOA at the THS, a CL of 1.0 and a CD of 1.6! Our upforce on the THS needed for lowering the nose has decreased and the drag has increased, our ND trim has caused the opposite of our desired outcome.
First of all: Thanks a lot for the interesting reference.

However there is one thing where I have to admit I disagree a little bit with your conclusion.
The drag along the flightpath would apply a Nose down moment on the Aircraft of (Rho/2 Cd A V^2 * sin (Alpha)* distance from center of rotation (sin Alpha ~0,87 @60°) while lift would act with the cosine, being 0,5 @60°.
Overall moment at 60° would be 0,5 * 1,0 + 0,87 *1,6 = 1,885 times the constant rest.
at 46;5° it would be 0,72 * 1,2 + 0,69 *1,2 = 1,698 times the rest.

Therefore the increase in drag would have more impact than the reduction in Lift in the given example.
The overall ND moment would increase in your given example.
As I already mentioned: at these AoA Drag acting on the tail is a good thing. It supports Nose down moment.

Last edited by henra; 18th Jun 2011 at 09:53. Reason: Sin (46,5°) corrected
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Old 18th Jun 2011, 10:25
  #143 (permalink)  
 
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Re @ Henra

Re Henra

Lets face the fact, that finally it´is the sum of all moments, which lead to the desired outcome. Wether the CD produces some ND input and CL produces some NU input or vice versa, the effectiveness of it depends on the sum of the individual forces. And as stated before, the CD and CL change of the wing has its share in the equation as well.

If we agree, that it never was the designed specification of this THS to get the AC out of a 60° AOA but to keep the aircraft in balance under the normal flight conditions by creating less or more ANU forces (curvature to produce downward lift on the THS), then the regime we are talking about here is as distant as the moon from the sun.

To get the desired AND vector, the THS airfoil has to produce lift opposite to its original design ( like if the main wing would have to produce not upward lift, but downward lift like during extreme pushover). I´m not saying that that is not possible, there are airfoils with symetrical curvature working as well by just changing the AOA of the airfoil (like the F-104 Starfighter main wing), but those designs are specifically crafted for that aircraft and its desired performance, and despite that, it had been highly succeptable to stalls and creating high amount of drag once outside desired flight envelope. I´ve lost lots of friends in accidents with this otherwise wonderful aircraft.

Another point i´m still curious, but have no expierience in it, so just correct me. I get the impression, that some posters simplify the function between lift and drag to the region of the pre stall flight envelope, and thus setting a constant between drag and lift, meaning where there is drag on an airfoil, there also has to be equivalent lift. 35 years ago i learnt in my academics classes, that if the airfoil produces lift, it also causes drag called induced drag (but not vice versa) up to max CL is reached (nearly linear function of drag and lift), but after that point drag continuous to increase while lift decreases. In our post stall discussion there are no formulas available to compute lift from drag, because there is no constant behaviour anymore outside the wind tunnel.

It is also way off reality to disregard the huge amount of drag produced by that stalled airfoil in the context of recovery efforts. To get any usefull output of the ND movement, we have to get the wind flowing over the lifting devices by increasing speed (which is the ultimate reason to decrease the AOA). When we increase the drag and thus reduce the forward velocity vector even more, we increase the descent rate by a comparable amount, thus increasing the total AOA again.

So it is no sense in discussing single points of some more or some less lift if it is not changing the overall problem to get the aircraft out of the extreme AOA condition within the available altitude.

One thing i´m sure : The simple stick forward and wait, like some are saying here, would only have worked up to a defined value of AOA and speed, and i see that value of AOA well below 45°. The discussion what the crew did do wrong in recovering the aircraft once established in the descent (estimnated FL 300 downward ) is IMHO useless, because there was no "conventional method" available which would have brought AF447 out of this position.

The problem lies in the beginning with the extreme climb rate, the chance to recover the following upset ended somewhere between FL 370 and FL 300.

Last edited by RetiredF4; 18th Jun 2011 at 11:13.
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Old 18th Jun 2011, 11:56
  #144 (permalink)  
 
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Pitching moment at large AoA

@RetiredF4 and Henra;

Perhaps you should have another look at Figure 6 in the document linked by
zumBeispiel six weeks ago. Admittedly that is not an A330, but what makes it different?

we have to get the wind flowing over the lifting devices by increasing speed (which is the ultimate reason to decrease the AOA).
First you have to reduce AoA to unstall the wing, then you need to increase speed to not stall again.
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Old 18th Jun 2011, 12:26
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Originally Posted by RetiredF4
In our post stall discussion there are no formulas available to compute lift from drag, because there is no constant behaviour anymore outside the wind tunnel.
indeed there is no way to calculate Lift from drag as Lift is not a function of drag.
What we have are merely datapoints from experiments on given airfoils.

That is everything we have. That said I hope I wasn't unclear when referring to drag acting as lift. That does not mean a mixing of Cl and Cd. They are orthogonal to each other. The trick is the coordinate system. When the aircraft has a high AoA there is an angle between both Cl an Cd and the Airframe itself as Cl and Cd are measured along the stream of air, i.e.the flight path. Wwhereas the moment and its related arm act on the airframe which is angled against these forces.
Lets assume the plane is falling at 90°. In that case it would only be the drag component of the Tail that would lead to ND forces. The Cl compnent would work orthoginally to the flight path, i.e. forward, which would be Nulll in that case.
I hope it gets clearer now what I want to explain.
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Old 18th Jun 2011, 14:38
  #146 (permalink)  
 
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Wing Aero

In all these pages, I have seen no mention of the basic airfoil of the A330. It's probably some version of supercritical, correct? What is its stall behavior?
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Old 18th Jun 2011, 14:45
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RetiredF4
One thing i´m sure : The simple stick forward and wait, like some are saying here, would only have worked up to a defined value of AOA and speed, and i see that value of AOA well below 45°. The discussion what the crew did do wrong in recovering the aircraft once established in the descent (estimnated FL 300 downward ) is IMHO useless, because there was no "conventional method" available which would have brought AF447 out of this position.
I don't think this is correct. Sustained full forward sidestick is effectively a demand for minus 1g. If the initial elevator movement did not achieve this the THS would immediately move in a nose-down trim sense to assist - if necessary as far as the stop. The difficulty lies in recognising the requirement for the forward sidestick.
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Old 18th Jun 2011, 15:11
  #148 (permalink)  
 
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Hi Tyro,

"The maximum elevator deflection is 30° nose up, and 17° nose down. The maximum THS deflection is 13.5° nose up, and 4° nose down."

I agree with RetiredF4. If the side stick had been held fully forward, the stab trim could have changed the THS's angle of attack by about 17.5 degs. Unfortunately, all that would have achieved would have been to INCREASE its angle of attack. The Stab is permanently stalled in their situation.

Last edited by rudderrudderrat; 18th Jun 2011 at 15:13. Reason: syntax
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Old 18th Jun 2011, 15:53
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Greybeard;

On these airplanes the airfoil section (thickness, camber and 'incidence') varies considerably from wing root to tip. The stall behaviour is not simply a function of the airfoils, but involves all of the wing and even all of the airplane including the fuselage.
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Old 18th Jun 2011, 16:16
  #150 (permalink)  
 
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by Rudderrat

"The maximum elevator deflection is 30° nose up, and 17° nose down. The maximum THS deflection is 13.5° nose up, and 4° nose down."

I agree with RetiredF4. If the side stick had been held fully forward, the stab trim could have changed the THS's angle of attack by about 17.5 degs. Unfortunately, all that would have achieved would have been to INCREASE its angle of attack. The Stab is permanently stalled in their situation.
As others have mentioned, what matters is the NET pitching moment. If there is enough pitching moment nose down, rotation nose down will occur.
There is Lift and Drag after stall. Lift does not suddenly dissapear post stall but follows a variable path toward zero lift at ~ 90 degrees angle of attack (and becomes 100% drag there)
The path depends on:

Angle of attack-The Primary variable
Aspect Ratio,
Airfoil thickness ratio,
Reynolds Number that the wing is operating at.

Take some time to understand the following document regarding post stall aerodynamics.
http://home.comcast.net/~shademaker/...tStall44XX.pdf

Last edited by Machinbird; 18th Jun 2011 at 16:46. Reason: Correcting statement to the first lift coefficient zero point, not the second.
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Old 18th Jun 2011, 16:28
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Promotion reccomendation: K.I.S.S. "tool" for finding stuff in past threads*

Hi,

Probationary PPRuNer, be promoted immediately to Newby 1st class
(*) jpete 1st post

Perfect! And with the Simple Characteristics defended by Kelly Johnson, Einstein, St. Exupéry, Occam, Leonardo Da Vinci and (some) other designers / engineers.


Machaca told me (and he was right) to look all posts before the ones i made and was making on a/c recorders. And now i will be able to post, checking not only here (AF447) but in many other PPRuNe threads.

Albert Einstein's: "everything should be made as simple as possible, but no simpler."

Leonardo Da Vinci's: "Simplicity is the ultimate sophistication."

Antoine de Saint Exupéry's: "It seems that perfection is reached not when there is nothing left to add, but when there is nothing left to take away."

Harry Hawker's: "Simplify, and add lightness."

William of Ockham: "the simplest explanation is most likely the correct one."

Bertrand Russell: Whenever possible, substitute constructions out of known entities for inferences to unknown entities."

K.I.S.S. The acronym was first coined by Kelly Johnson, lead engineer at the Lockheed Skunk Works (creators of the Lockheed U-2 and SR-71 Blackbird spy planes, among many others).

When possible we must avoid K.I.C.S. designs. There are (many) examples...

Last edited by RR_NDB; 19th Jun 2011 at 10:16. Reason: Add important feedback (same rationale)
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Old 18th Jun 2011, 16:41
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Hi Machinbird,

Thanks for the link - but I think you mean "Lift does not suddenly dissapear post stall but follows a variable path toward zero lift at ~ 90 degrees angle of attack."

Previous posters have suggested that if the stab angle of attack is about 45 degs (around the second lower peak of CL), then changing the angle would merely reduces the lift on the tail (hence aircraft nose would rise).
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Old 18th Jun 2011, 16:42
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Re LI9nk

@Machinebird
Take some time to understand the following document regarding post stall aerodynamics.
Comcast
Machinebird, the link is known and my statement concerning the effectiveness of the THS for ND is based on it. .

See my post No. 35 and PickyPerkins post No. 100 in this thread.

Another thought to relation of THS and wing:
In the post-stall the main wing produce some ANU lift, the tail produces some AND lift. Due to the more sophisticated design of the wing it could be possible that the wing produces more ANU lift than the THS could counter with AND lift. Remember, in normal operation the THS only produces ANU lift, therfore the camber is on the downside of the THS.

And lets look at the drag, which is working as a turning force around the lateral axis: The drag of the forward part of the fuselage and of the forward parts of the wings woking ANU night well be greater than the AND force by the fuselage aft of the wings and the THS.
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Old 18th Jun 2011, 17:07
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Entire plane aero

Hi,

even all of the airplane including the fuselage


The "rectangular belly" is perhaps an "important part" of this?
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Old 18th Jun 2011, 17:43
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Originally Posted by RetiredF4
The drag of the forward part of the fuselage and of the forward parts of the wings woking ANU night well be greater than the AND force by the fuselage aft of the wings and the THS.
Fully agreed. That is the real unknown when it comes to the net effect at 60°.
It might indeed be the case that the forward fuselage creates equal or higher lift than the tail, although at least for the A330-200 with the relatively short forward fuselage I doubt it.
edit:
Where I do not fully agree is the hypothesis taht the main wing will create a net NU moment. In non stalled condition the Center of lift is close to the quarter chord line. At fully stalled condition the Center of lift will move to the mid chord line. (It is only in a transient phase when the tips start to stall and the root is not yet stalled where you might encounter a NU effect. But at 50°+ AoA that is defintiely not the case any more)
/edit

This assumption is to some extent supported by the cm curve in the link as it suggests a net reduciton of cm with increasing AoA, i.e. a ND tendency.

If this was the case with AF447 we will probably never know as it appears that the Pilots unfortunately never really tried it.
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Old 18th Jun 2011, 17:45
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Originally Posted by rudderrudderrat
Hi Tyro,
I agree with RetiredF4. If the side stick had been held fully forward, the stab trim could have changed the THS's angle of attack by about 17.5 degs. Unfortunately, all that would have achieved would have been to INCREASE its angle of attack.
hmm, somehow I'm completely at a loss how you know that for sure !?

Would you mind sharing your source ?
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Old 18th Jun 2011, 17:54
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Hi henra

post 147 first line is copied from FCOM.
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Old 18th Jun 2011, 17:55
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At 2 h 12 min 02, the PF said "I don’t have any more indications", and the PNF said "we have no valid indications". At that moment, the thrust levers were in the IDLE detent and the engines’ N1’s were at 55%. Around fifteen seconds later, the PF made pitch-down inputs. In the following moments, the angle of attack decreased, the speeds became valid again and the stall warning sounded again.
The elevator was effective in the correct sense as reported by BEA. It wasn't used enough to recover.
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Old 18th Jun 2011, 18:09
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Originally Posted by CONF iture
DozyWannabe, if tomorrow I don’t make it, I want my wife, children, best friends, to have a look TOO at MY data when my authorities will set their dirty move.
Hi CONF,

If the information is accurate, I wouldn't call it a "dirty move". It may not be pleasant to hear, but according to what the minister has been told, the pilots were supposedly trained to handle pitot icing/blockage incidents, mode reversion and all that comes inbetween in this incident.

Note that he's not saying the aircraft is blameless - he can't because the information is out that the pitots were probably the initiating factor. The BEA were the ones who released that information publicly when their initial interim report was released a couple of years back - if they really wanted to "stitch up" the pilots then they would never have released that information.

As I said earlier - the BEA can be said to have handled a single AF/Airbus incident "badly", though personally speaking I wouldn't have wanted any part in that political minefield. That incident was 23 years ago and they're still dealing with the damage that did their reputation today. It would be utter madness for them to try to do so again, so I say again, give them a chance.
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Old 18th Jun 2011, 18:25
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Pilot error?

DozyWannabee

“I ask again - why is it you're asking this of the BEA and no other accident investigation bureau/institution? Is one incident forever going to preclude acceptance of any finding of theirs that includes "pilot error" as a factor?

Also, you're a pilot, I'm an engineer - but I wouldn't trust myself to derive a correct conclusion from the raw data. What makes you so sure you could? “


Just for the record, the mission of EVERY investigating body, is to gather the facts and make a determination of Probable Cause and to recommend a fix, but NOT to affix blame, fault, or liability, which is left to legal venues.

As a pilot all I want is all the given facts and then I can make my own conclusion regarding what to do about it. In this case avoid thunderstorms at all costs. Period.

It is way too early to start figuring out how to recover from an out of control deep stall, when in fact they won’t even allow pilots to practise “hand flying” in the simulator at altitude, let alone in the airplane.

No one knows what the plane is going to do.
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