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New (2010) Stall Recovery's @ high altitudes

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New (2010) Stall Recovery's @ high altitudes

Old 13th Jun 2010, 17:19
  #21 (permalink)  
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Danger

great opinions everywhere and also experience of other aircraft,.. what i am currently flying is an md80. as where we have st.shaker and st.pusher.
where the shaker kicks is, you're approaching the stall, and the pusher kicks in where the wings stall.
also when i mentioned lift improvement devices,, that also 'slats' not flaps. these have certains speed limitations aswell, which i guess at the point of stall the current ias/mach will be below the flaps limitations, didnt double check that very accuratly in the sim, but at altitude, the simulator recreated a full stall between 160/170 KIAS (mach: didnt notice)

but i mean, it's awful,... you will get so many warnings b4 a stall, and still it happens.
my main reason for this thread.
what i could see in the simulator... the md88 (md80 series) will give you these warnings:
-airspeed drop (obvious)
-210KIAS -> Landing gear aural warning
-210KIAS -> Langing gear 3 red lights
-FMA-> Low SPD
-<200KIAS Buffeting starts very subtile, but noticeable and ever increasing
-stick shaker
-stick pusher

our demonstration we being conducted at FL370 (max md80) with AT off, clean config.

1.recovery at st.shaker keeping AP ON and firewall throttles took about 40seconds, 4000' alt loss

2.recovery at st.shaker w AP OFF trying to maintain ALT with throttles to firewall took also 40seconds, 2000' alt loss

3. recovery at st.shaker w AP OFF deliberately lowering nose to horizon immediately and THROTTLE REMAIN IDLE took 4seconds, alt loss was +/- 2000'

4. recovery at st.shaker w AP OFF, lowering nose to horizon Throttle to firewall took also 4 seconds, and alt loss was >1000'

----
all in all a great eye opener. we knew that the sim couldn't simulate a complete stall accurately but, for training purposes this was a very very great test ! ( i'm <1yr on jet)

OPINIONS pls.
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Old 13th Jun 2010, 17:27
  #22 (permalink)  
 
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One should read John Farley's book especially the chapters on stalling and stall recovery technique.
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Old 13th Jun 2010, 18:11
  #23 (permalink)  
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I find it hard to believe that anyone with an understanding of aerodynamics would disagree with anything I said, but it seems to have happened. Sorry.

If there is a pusher, the pusher will kick in at a determined point. You can take that, if you like, to be a definition of "stall", namely the point at which the engineers decided the pusher should activate. It is not necessarily the same thing as CLmax, as Mansfield suggested it was.

Mansfield also quotes the certification regulations, to show that flight tests are performed at various combinations of parameters to a defined point of stall. Well, yes indeed. Those have all been done during certification of the airplanes we are talking about.

So what data is it that a given manufacturer didn't have, that they would have gone back up to get because the regulators were worried again about LOC? Can anybody here say?

On to semantics. If an airplane is equipped with a stick pusher, then, granted if you will, a stick pusher activates at the point of defined "stall". That is, you get slower and slower, up to a specific airspeed at which stick pusher activates, and over you go. At what point in that entire process can you be said to be "stalled"? A suggestion: at no point, or maybe at just one. So, if I may be permitted to insist on accuracy, you are performing a manoeuvre which can be best described as recovery from approach to stall, not as recovery from a stalled state. The pusher does not let you get into a stalled state. Isn't this obvious?

An analogy. You walk up to the edge of a lake with shallow banks. You let your shoes go right up to the edge of the water. The water touches the very tips of your shoes, but no more. Are you in the lake? Are you wet? Have you been wetted? If you step back, are you recovering yourself to dry land from being in the lake? Answer as you will. I would say, you went up to the edge of the lake but you didn't recover from being in it. Similarly with a stick pusher and stall. You went up to the point of stall but you're not recovering a stall.

I am happy to agree that the current action is a general response to recent LOC accidents if that wording seems more appropriate to some.

PBL
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Old 13th Jun 2010, 20:53
  #24 (permalink)  
 
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PBL, this is from your post:

"You think people have been going out doing stalls in Airbuses to see what happens?

The aerodynamic characteristics of a modern commercial jet are determined in wind tunnels, mostly for certification purposes. The wind tunnel work determines how the wing behaves, not the whole airplane. It determines at what point lift drops off sharply, and so forth. There is a lot of data, but there is by no means everything. The point of "stall" is not necessarily defined as being where the lift drops off sharply; it is more often defined by a degree of buffet (I refer to the regs).

"Sim tests" cannot help anyone with stalls. Simulators are only veridical at most up to the point of defined-stall (which may be at lower AoA than lift-break). So anything you can do in a sim is an approach-to-stall procedure."

Yes indeed, Airbus in this case thought it was worthwhile for writing the FOT to conduct additional many stall tests on the aircraft and compare them to simulator behavior, not only rely on certification data. At the same time it has all the data required for its operators to simulate stall recovery in the simulator, this will not be sim tests as you state it but training on stall recovery.
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Old 14th Jun 2010, 03:24
  #25 (permalink)  
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Giuones and others,

I seem to have trouble explaining myself and getting my points across. Since my career is partly based on explaining myself and getting my points across, I find this situation somewhat puzzling. I have met it before, often, on PPRuNe, which is partly why I no longer contribute on a regular basis.

I have tried before the strategy of writing long, pedantic pieces to try to get my points across. It doesn't always work, but let me try it just once here. If this doesn't work, I'll just stop, for I don't know any other ways of trying.

s bakmeijer was talking about recovery from stall in an MD-80 series jet. Now, I don't know anyone who worked on the MD-80 wing, but I do believe I can guess how they thought and worked in general terms. I do work on a regular basis with an aerodynamicist who was responsible for the wings on some other well-known commercial jets. And I am not yet sure that everyone here is thinking the issue through. So, at the risk of being pedantic, let me tell a story which I think is very likely to be true of, say, the MD-80-series wing design process.

There you are, designing a wing, looking at the wind tunnel experiments, and you notice that, in particular circumstances, when the airstream separates from the upper surface and the lift drops off (it may do so suddenly or gradually; I don't know the story for the MD-80-series), the turbulent wash blanks out the tail (or flows over the place where the tail should be; it may be that your wind tunnel model doesn't have a tail attached).

Now, you and everyone else knows that you cannot certify the aircraft if this is allowed to happen in real life, for it is a certification requirement that it not do so.

So you put in a stick pusher. When you do so, you have to choose at what point you design it to activate. Do you design it to activate at the point at which the airstream separates? Well, that would be unwise, indeed useless, because at that point, as you have seen in the wind tunnel, the tail is blanked, and that is exactly what you are trying to avoid. You design it to activate *just before* that point, so that the separation does not occur, the tail is not in danger of being blanked, and you and your DI can go home and sleep comfortably.

Mostly, lift increases up to the range at which that potentially-blanking airflow separation occurs. You are triggering the stick pusher just before it gets to that point, so you are not triggering it "at" CL_max in physical terms, but just before that point.

And maybe you tell your certification agency: we aerodynamicists define the "point of stall" to be the point at which we trigger the pusher. Because there is some leeway allowed to you to do so if you wish. The stall phenomenon is actually a range of aerodynamic behavior over a range of airspeed and you can set the point for definitional purposes more or less wherever you choose to do so in this range.

Years later, some pilots, say, MD-80 pilots, say that stick pusher activates "at the stall" and call what happens "stall recovery", because that is what everyone else in their community says. So be it. But if you want to talk about aerodynamic phenomena, as I often do, this phraseology does not accurately represent what is going on.

Guiones, I know that flight tests were done and sims data was validated for this whole endeavor. But I believe that it misdescribes what was done to say that the airplane was stalled and recovered and the sims were modified to reflect that behavior. I think you will find that the airplane was taken up to the point at which unpleasant handling characteristics started to occur (maybe up to the engineering-department-defined "point of stall" - which may or may not be at the point of maximum lift, depending on your airplane - and maybe to just before, but not over that point, since going over it is possibly dangerous and people who do these tests generally do not want to kill themselves, and their bosses don't want them to do it either). And that simulator behavior was validated up to that point. But sim behavior may not be trusted after this break, for the simple reason that nobody has flown the airplane after this break to know what it really does then.

And you can ask an aerodynamicist responsible for a specific wing, about behavior post-stall: "what happens here?" And, depending on what you are asking about, the answer may very well be "I don't think anybody knows". I know this because it happens to me a lot.

Can we agree now? I am not going to belabor it further, because if this way of putting things does not suffice to explain where I am coming from, I don't know what I can say that will.

PBL
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Old 14th Jun 2010, 04:40
  #26 (permalink)  
 
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FAA trained or not, it is a fact that using rudder to lift a wing is commonly taught in flying schools
the use of the rudder to lift the wing is required if you are flying an airplane without boundary layer control non-frise-type ailerons or if you are an aerobatic student

YouTube - Falling Leaf - Instructor Demo

none of that applies to transport aircraft
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Old 14th Jun 2010, 12:43
  #27 (permalink)  
 
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let's not confuse folk here please! The use of rudder to lift a stalled wing is ESSENTIAL and correct and MUST be taught. Use of aileron on a fully stalled wing will merely increase the wing drop (basic aerodynamics?) and will probably cause a spin.
Current spam can training and also what I have been taught in turboprops is that rudder should be used to hold the current roll attitude. Then the wings rolled level again after the aircraft is unstalled, as per a stall in a turn. The rudder should be used to hold the wing from dropping further but its not to be used to lift it back to level.

I will standby to be corrected though.
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Old 14th Jun 2010, 12:59
  #28 (permalink)  
 
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Perhaps a better way to come at this is to refer to Mad (Flt) Scientist's excellent explanation of the pusher concept back in 2007:

http://www.pprune.org/tech-log/27056...ml#post3213867

PBL, I believe the disagreement lies not with the point at which a stick pusher is triggered, but with the premise that it be interpreted as an approach to stall. The manufacturers and certification authorities intend for the pilot to interpret the stick pusher as an indication that a stall has occurred. Now, aerodynamically, the pusher may operate before the actual stall is reached, however it is defined, but this seems to me to be a technical point and not a practical one from the pilot's viewpoint.

s_bakmeijer and all: for a more representative look at these types of events in service, take a look at the following reports from the NASA Aviation Safety Reporting System:

ASRS - Aviation Safety Reporting System

376201
418260
441448
663390
665350
672119
749437

I believe you'll find three MD-80 series aircraft, and one additional that is unidentified but most likely also a DC-9 or MD-80. These provide some insight into how these types of events take place.

The major difficulty with in service stalls is that at the point of occurrence, the dynamics of the actual event and the flight crew's mental model of the situation are not aligned. Further, the behavior of the airplane in the fully stalled condition may well not be what the simulator has modeled, as PBL has said; this is particularly true of premature stalls resulting from ice accretion. The event is usually a terrifying surprise, developing very rapidly and possibly becoming somewhat violent. Pilot reactions can be surprising.

Although the response of the captain in the Buffalo accident was tragic, it was not as atypical as the Board seemed to think. In the icing accident database that I maintain, I currently have sixteen events in which I am able to document control column pull forces following a stall warning. These include AA 903 at West Palm Beach, ABX N827AX at Narrows, Virginia and a number of EMB-120 and ATR events. I am certain that none of these pilots would have done this in the simulator; but then, what they encountered in the airplane was like nothing they had ever seen in the sim.
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Old 14th Jun 2010, 22:34
  #29 (permalink)  
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All simulator data packages contain NO valid data beyond the "g" break.

Depends on what the airframe and sim OEMs got up to in the commercial arrangements for the airframe data pack.

More importantly, the pilot does not know to what extent the sim responses are validated or extrapolated. Indeed, two apparently identical sims can be tweaked a little differently by two sim operators to give not quite the same experience to the pilot.

Underslung engines may create more pitch up at low speeds/high AOA than elevator can pitch down!

Caused by the low thrust line and nacelle lip normal force - both destabilising. If the stability characteristics are unacceptable, then one would expect to see an SAS of some sort incorporated to give the pilot the impression of stability. Excluding research aircraft and odd ball system failures, one should not see a pitch unstable aircraft out on the line.

using rudder to lift a wing is commonly taught in flying schools

Probably not good technique at/in the stall due to the risk of spin outcomes. A search of John Farley's comments on stalling are a useful read.

The use of rudder to lift a stalled wing is ESSENTIAL and correct and MUST be taught. Use of aileron on a fully stalled wing will merely increase the wing drop (basic aerodynamics?) and will probably cause a spin.

Very much a case of following the OEM's guidance for a specific Type. However, rudder mixed with stall is a recipe for potentially interesting results. For other than ancient Types, the aileron should be functional throughout .. this, naturally, will depend on wing section variations and whether all or part of the wing is stalled.
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Old 15th Jun 2010, 07:47
  #30 (permalink)  
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this, naturally, will depend on wing section variations and whether all or part of the wing is stalled.
- exactly the problem, John. Here we are all talking about 'stall' in different ways, from light buffet/stick shake through to fully stalled. I refer, as I said, to fully stalled.

Fully stalled and aileron will be a disaster, as will allowing the ensuing yaw to the dropped wing. Using aileron to pick up a stalled 737 wing at 80kts will simply make the wing drop faster.

At stick shake it should be fine

Some of our 'recent' low speed/extreme pitch accidents and near accidents will have been fully stalled, I suggest?

As expressed in posts #12 and #13, "Approach to stall recovery techniques are irrelevant to actual stall recovery". Full stall recovery has long been ignored in airline flying - largely because the training (rightly) is to avoid such. Sadly that seems to have failed.
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Old 15th Jun 2010, 21:21
  #31 (permalink)  
 
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PBL, this is why you have trouble explaining yourself, two contradictory statements in two posts, is not us that fail to understand, but you that contradict yourself:

"You think people have been going out doing stalls in Airbuses to see what happens?"

"Guiones, I know that flight tests were done and sims data was validated for this whole endeavor."

On the aerodynamic descriptions I do agree with you; but I am form the Pilot side and we must call it approach to stall and stall to get the difference across, it may not be perfect from your point of view, but we must make a difference between the two recovery procedures. And by the way, some people have flown past the break, most are just not around to tell us about it.


G
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Old 16th Jun 2010, 09:32
  #32 (permalink)  
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Oh, dear, here we go again.

Guiones, the advice I give to most people who are tempted to think I contradict myself is that it is most likely to be a clear indication to themselves that they have misunderstood.

In your case, you have misunderstood the concept "contradiction" as well as what I said. Let me explain.

First, in a very simple case, two assertions A and B are contradictory if their conjunction is logically guaranteed to be false.

In the two sentences you quote, there is only one assertion, not two. The first sentence contains a question, not an assertion. It was a question to you, indeed one which I believe you have not answered. When you give an answer, if it contradicts the second sentence (my assertion), then I will be contradicting you, not contradicting myself.

Enough of trivial explanations of logic, back to aero.

So, answer the question: say what you think those flight tests did. Then I'll see if I can find out what they did, and get back to you. It might take me a while.

PBL
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Old 16th Jun 2010, 14:29
  #33 (permalink)  
 
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PBL, with your question you challenged my assertion that Airbus was conducting flight tests; with your second statement you confirmed it, here in the US we call that a Clinton answer.

I already said what the tests did, compare aircraft behavior to sim behavior in order to write the FOT.

I do not need you to find out anything for me as far as the tests, I have spoken to one of the pilots involved.

G
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Old 16th Jun 2010, 15:29
  #34 (permalink)  
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Guiones,

I don't participate in these forums to engage in silly word games. I participate to engage in discussion about matters of mutual interest in aeronautics. So one last word from me on this to the (hopefully) wise:

Originally Posted by guiones
PBL, with your question you challenged my assertion that Airbus was conducting flight tests
You are mistaken. I asked

Originally Posted by PBL
You think people have been going out doing stalls in Airbuses to see what happens?
And you still haven't answered.

PBL
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Old 16th Jun 2010, 16:57
  #35 (permalink)  
 
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PBL:

To answer your question, again.

Yes, Airbus did conduct tests and this were done to validate aircraft behavior vs sim behavior for the FOT.

PM me if you ever come to Miami and we will discuss in person, I WILL BUY THE BEERS.

My last post on this matter, got to go out to fly.

G
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Old 19th Jun 2010, 09:11
  #36 (permalink)  
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To pick up on two very important points mentioned previously.

1. Stall has absolutely nothing to do with airspeed. It has everything to do with angle of attack. It is entirely possible to fly any aircraft at an airspeed of less than 10 Knots but not stalled - because the angle of attack is less than that at which the stall occurs.

2. Unless one is intending to do some aerobatics or aligning the aircraft with the centerline on landing, with all engines operating the rudder is only used to prevent yaw. Yes that is to prevent yaw. Or as some people like to say - keep the ball in the middle.

In order to "pick up a wing" using rudder - stalled, near the stall or at Vmo - one must yaw the aircraft. The only time I would expect anyone to intentionally yaw the aircraft at or close to the stall is when they want to experience the combination of stall and yaw = spin.

The stall recovery (or prevention) actions are basic and simple - reduce angle of attack and prevent yaw.

Height loss above 20,000 ft in a stall recovery (or avoidance) is not an issue that is worried about. If you are 20,000ft above the MSA then where is the problem in using some gravity to assist a prompt and effective recovery.

Finally, in a multi, it is not simply Vmca and engine failure that is an issue in stall recovery which involves accelerating to TOGA thrust / firewalling the levers but simple things like unequal acceleration times that can introduce unwanted yaw at the critical time when it is least desired.

Reduce AOA and prevent yaw.
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Old 19th Jun 2010, 11:28
  #37 (permalink)  
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It is entirely possible to fly any aircraft at an airspeed of less than 10 Knots but not stalled - because the angle of attack is less than that at which the stall occurs.
- a little 'pie in the sky', DFC - you need to add 'but not in any sort of controlled flight nor for very long', just as it is not possible to stall an aircraft at zero g, but again, not for long..
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Old 19th Jun 2010, 12:54
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I hope this adds something here. From my types you will see I haven't been hands on for a while.
What I was taught in engineering training is that the pusher in the 737 was called a stick nudger and is purely there to properly identify the stall by pushing the column forward gently. The stick shaker similarly is there to replicate the pre stall buffet for those aircraft whose characteristics need additional identification.
(Now showing my age)
The Trident and the BAC 1-11 had true stick pushers and were a recovery system. Being a T tail configuration they couldn't be allowed to stall fully as they wouldn't recover after the tail enters turbulent air from the wing as some poor souls found out in testing.
In those aircraft the stick pusher operates rapidly pushing the control columns forward while the tail is still in clean air. Pitch control is this case was taken away from your good selves as it wasnt possible to restrain the columns. In the case of the 1-11 accompanied by very loud klaxons.
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Old 19th Jun 2010, 14:13
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Height loss above 20,000 ft in a stall recovery (or avoidance) is not an issue that is worried about. If you are 20,000ft above the MSA then where is the problem in using some gravity to assist a prompt and effective recovery.
If the recovery is made correctly, then as you say, no problem. But if in IMC and particularly at night, the sticker shaker operating suddenly will almost certainly be an unexpected event catching a crew by surprise. It would then take only a few harsh control movements to cause serious overcontrolling and unless the pilot was on the ball with his basic instrument flying skill, an unusual atttitude can result. Now you have a deadly serious problem while in IMC. Realistic unusual attitude recovery training is rarely conducted in airline recurrency training. It is seen as unnecessary and a waste of simulator time. Even if this training is conducted it might only be once every couple of years. Not exactly effective recurrency.
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Old 19th Jun 2010, 14:48
  #40 (permalink)  
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Why, I wonder, didn't someone like BOAC pick up on this?

Originally Posted by DFC
Stall has absolutely nothing to do with airspeed.
That is so wrong. It must be corrected.

The angle of attack at which a modern commercial jet stalls at 0.8M is something around half of the angle of attack at which it stalls at Vs1g.

PBL
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