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AF447 Thread No. 3

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AF447 Thread No. 3

Old 9th Jun 2011, 06:56
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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.)
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Old 9th Jun 2011, 07:19
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Originally Posted by bearfoil
a/p loss: after the loss of autopilot, there were two Stall warnings. They cannot have been genuine, for the a/c commenced a climb without adding thrust, two things that would have caused a Stall post warning, if genuine, they were not.
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.

Originally Posted by CONFiture
Quote:
Originally Posted by NOD
When would you like the stall warning to go off? Even at airspeeds below which the vane can work?? These vanes are substantial pieces of metal that can withstand the best part of 500K/1.0M. They are not little feathers that will read reliably at 20K IAS
...
OK - even when the spec/manufacturer of the vane says the IAS is way below the validity of the vane's spec?


What are the vane's spec ?
Would you like to ref a manufacturer documentation ?
Because to me, by merely direct observation of those vanes, they are pieces of metal which behave like feather
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.
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Old 9th Jun 2011, 07:39
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16deg pitch attitude at 37,500ft, at 215 kts

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.
Right on Sir !
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Old 9th Jun 2011, 08:14
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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.
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.
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Old 9th Jun 2011, 08:38
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Hi NoD,

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.
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.
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Old 9th Jun 2011, 09:22
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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
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.
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Old 9th Jun 2011, 10:44
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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.
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Old 9th Jun 2011, 11:02
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Originally Posted by JD-EE
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?
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 ?

Last edited by AlphaZuluRomeo; 9th Jun 2011 at 17:02. Reason: typo
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Old 9th Jun 2011, 11:09
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JD-EE

Read the safety first article, it will explain, as I have tried to do, why the stall warning may have gone off.
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Old 9th Jun 2011, 13:16
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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?

Originally Posted by from the BEA report
From 2 h 10 min 05, the autopilot then auto-thrust disengaged and the PF said "I have the controls". The airplane began to roll to the right and the PF made a left nose-up input. The stall warning sounded twice in a row. The recorded parameters show a sharp fall from about 275 kt to 60 kt in the speed displayed on the left primary flight display (PFD), then a few moments later in the speed displayed on the integrated standby instrument system (ISIS).
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?
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Old 9th Jun 2011, 13:17
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safety first article Jan 2011

JD-EE

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

I think, you reference to that part of it.

Typically, in cruise at high Mach number and high altitude, at or close to the maximum recommended FL, there is a small margin between the actual cruise AoA and the AoA STALL. Hence, in ALTERNATE or DIRECT LAW, the margin with the AoA SW is even smaller.
The encounter of turbulence induces quick variations of the AoA. As a consequence, when the aircraft is flying close to the maximum recommended altitude, it is not unlikely that turbulence might induce temporary peaks of AoA going beyond the value of the AoA SW leading to intermittent onsetsof aural SW.
Equally, in similar high FL cruise conditions, in particular at turbulence speed, if the pilot makes significant longitudinal inputs, it is not unlikely that it reaches the AoA SW value. For those reasons, when in ALTERNATE or DIRECT LAW, it is recommended to fly at a cruise flight level lower than the maximum recommended.
A 4,000 ft margin is to be considered. Then, for the same cruise Mach number, the IAS will be higher, the AoA will be lower, and therefore the AoA margin towards AoA SW will be significantly increased.
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Old 9th Jun 2011, 13:20
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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.
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Old 9th Jun 2011, 13:29
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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.
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Old 9th Jun 2011, 13:39
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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 its more probable than any other discussed scenario.

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.
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.
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Old 9th Jun 2011, 14:04
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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
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Old 9th Jun 2011, 16:19
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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 ...

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 ... )
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Old 9th Jun 2011, 16:33
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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
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Old 9th Jun 2011, 16:38
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JD-EE;
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?
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.
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Old 9th Jun 2011, 16:44
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More AOA sensor info!

- Operating Range 120, 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 C.

For Audible Stall Warning I could find:

- Normal Law - AOA > 23.
- 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
far less information but everything related seems to be unvalid if CAS< 30 Kts.
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Old 9th Jun 2011, 17:07
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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.
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