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AF447 Thread No. 3

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AF447 Thread No. 3

Old 12th Jun 2011, 02:04
  #1821 (permalink)  
 
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PJ2, re # 1805, 1819, DJ77, #1793, et al.
As I understand, there is a step change in the value of stall warning at the transition between normal and alternate laws (upper vs lower tables in # 1818) and it is also possible to infringe the lower, alternate boundary, in turbulence or gentle maneuvering at high altitude.
Both values of stall warning are a function of Mach as aerodynamic stall AOA decreases with increasing mach No, and alternate law has no stall protection; thus at higher speeds the warning is at a lower AOA. The details are explained in What is a stall.
Also at higher Mach No, trim (THS) is biased to counteract the Mach induced nose up pitch moment (Mach trim in conventional aircraft); I assume that this moves the THS slightly nose down.

At the moment of changing law (airspeed mismatch / failure), the control response is identical except for the stall warning. In addition, the aircraft aerodynamics and the control surfaces, THS/elevators, are matched - the aircraft is 'in trim'.

“Hypothesis”. If we consider a slow blockage of the temperature sensor (cf Vh-EBA ~ 30secs), at some time before the pitot blockage, and that this resulted in an increase in computed Mach (cf VH-EBA); then the Mach Trim function could be incorrectly biased nose down. In auto flight, this would probably be countered by an up elevator demand (all small amounts), but the aircraft would be technically mistrimmed as the wing aerodynamics and the control surfaces, would not be matched, i.e. the wing was not creating a nose up moment – no real Mach increase, but the trim was set to a real nose down position due to a false Mach increase.

When changing to Alt Law, - the point of pitot blockage and autopilot disconnect, the aircraft would be mis-trimmed for level flight. If the trim aspects (THS, control stick – elevator) were not zeroed out electronically (and I can’t see how this might done at this time), then the resultant effect might offset the 1g level flight demand such that nose up control input was always required, i.e. the zero control position did not result in 1 g flight – in this instance level flight. An obvious question is what exactly does the FDR record as ‘control demand’?

Following on, the PF would require a nose up movement just to fly level, but at some point, this should command auto trim (THS) – nose up, but what the null trim datum would be, either the stick centre (no demand), or offset THS datum due to previous false Mach, is unknown.
Again, the values might be small, but sufficient such that the PF would have to control a mistrimmed aircraft, perhaps with increased sensitivity and tendency to over control which could trigger the earlier stall warning.
Add to this any control activity with regaining altitude; cf VH-EBA, -300ft (more nose up demand and thus THS nose up), and any latent perception of increasing Mach (false value), again nose up attitude demand to reduce speed.

This is a very complex technical scenario dependant on where the trim datum (null control) might be positioned when changing to alternate law with erroneous Mach. The elements which trigger the mismatch are highly likely to be present; the details in the control law are unknown.
However, the hypothesis might provide a reason for the propensity for nose up control and trim movement (FDR), the early stall warning, and in the extreme (and perhaps most important), the mis trimmed interaction with normal control in alternate law might provide a reason why the trim ran fully nose up and ‘apparently’ did not move in the opposite direction, as the THS never achieved its (false) null position.
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Old 12th Jun 2011, 03:04
  #1822 (permalink)  
 
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AoA sensor experiment

I had a Giannini Controls Corp, vane type AOA sensor in my hands today and took it for a ride in my car. Wind was still, so no wind correction needed. Movement of the vane was slightly damped, but I noticed the vane come alive at 15 mph on the speedometer.

I also accelerated the unit perpendicular to its axis at around >1 g and did observe the vane moving, but smoothly and without slamming. When turned so its axis is horizontal (normal aircraft mounting position), the vane stayed where it was left and did not fall "down" under gravity.

This vane is typical of those used on Airline aircraft in the 1970's and 1980's (and the A-4 Skyhawk). Physical characteristics of this vane are assumed to be comparable to the A320 AOA vanes.
There appears to be no valid vane function reason for invalidating AOA indications at 60 knots.

The military AOA systems I flew were fully functional on the ground and in the air and at any speed. When we flew a bit of a tail slide maneuver in the A-4, the indications were useful in preventing post stall gyrations.
I avoided tail slides in the F-4 since it had some flat spin modes that were unrecoverable, but at very high AOA, it always told you what you had to do with the controls to recover normal flight, even if you were well below level flight stall speed.

Last edited by Machinbird; 12th Jun 2011 at 04:09.
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Old 12th Jun 2011, 03:52
  #1823 (permalink)  
 
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Sciolistes

If an accident investigation is manipulating the release of information in order to manage the debate on a particular accident, then by definition there is is an intention to subvert the message. This is quite clearly apparent in this BEA report and the subsequent news coverage. Therefore, in effect the effect of the report is to create speculation, in this case speculation over the implied sub-standard performance of the pilots.

If there were no interim or update reports of this nature, then there would not be any, meaningful speculation in the media as the public would be aware the final and most complete assessment practically possible will be released in due course.

It is extremely concerning that people justify the release of that BEA update, clearly worded to divert attention away from a sub-standard aircraft configuration ahead of a major sales event! I just don't understand how this can be acceptable.
Very interesting reading of the tea leaves floating at the top.

For extra credit could you interpret for us why no additional information has been released.

Do you suppose they are now sitting on a bombshell that changes their direction? Or are they, like this forum, struggling to understand who commanded what and why in the flight of the aircraft ?
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Old 12th Jun 2011, 04:01
  #1824 (permalink)  
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Originally Posted by safetypee
However, the hypothesis might provide a reason for the propensity for nose up control and trim movement (FDR), the early stall warning, and in the extreme (and perhaps most important), the mis trimmed interaction with normal control in alternate law might provide a reason why the trim ran fully nose up and ‘apparently’ did not move in the opposite direction, as the THS never achieved its (false) null position.
In normal conditions, to achieve +7000ft/min from FL350 on a single NU input, you effectively need a lot of speed initially, or a quite aggressive input, or ...
Be aware that the BEA did not mention that the trim moved during the initial climb from 350 to 375.
Please, would you have again a link for VH-EBA.
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Old 12th Jun 2011, 04:30
  #1825 (permalink)  
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CONF iture...this should do it:

Investigation: AO-2009-065 - Unreliable airspeed indication - 710 km south of Guam, 28 October 2009, VH-EBA, Airbus A330 202
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Old 12th Jun 2011, 08:09
  #1826 (permalink)  
 
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lomapaseo,
Do you suppose they are now sitting on a bombshell that changes their direction?
I don't suppose anything other than why?
Or are they, like this forum, struggling to understand who commanded what and why in the flight of the aircraft ?
They are struggling to understand it less than us, I know this because it is clear that they released a report that contains less information than they are obviously aware of?
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Old 12th Jun 2011, 10:00
  #1827 (permalink)  
 
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Just re-read the BEA report looking to see if the word 'stall' was reported at any time as having been used. If the pilots had recognised a stall, surely even a very summary report would state that at x time the pilot recognised the stall. Anyway,

First off, would the normal response to: :
At 2 h 06 min 04, the PF called the cabin crew, telling them that "in two minutes we should enter an area where it’ll move about a bit more than at the moment, you should watch out" and he added "I’ll call you back as soon as we’re out of it".

be to secure the cabin ?

Second:
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 theengines’ 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.
At 2 h 13 min 32, the PF said "we’re going to arrive at level one hundred". About fifteen seconds later, simultaneous inputs by both pilots on the sidesticks were recorded and the PF said "go ahead you have the controls".


Is this all the information released ? That statement leaves a period of one and a half minutes empty of data, that is a big fraction of the whole incident time. And these simultaeneous inputs, no information on what they were, eg NU ?

I thought I saw other information but can't find it so apologies if I am behind the curve.
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Old 12th Jun 2011, 11:50
  #1828 (permalink)  
 
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Cool

Hi,

First off, would the normal response to: :
At 2 h 06 min 04, the PF called the cabin crew, telling them that "in two minutes we should enter an area where it’ll move about a bit more than at the moment, you should watch out" and he added "I’ll call you back as soon as we’re out of it".

be to secure the cabin ?
I assume your question has to relate with the many victims found on the surface and thus apparently were not sitting in their seats or did not have their seatbelts closed ?
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Old 12th Jun 2011, 13:24
  #1829 (permalink)  
 
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Originally Posted by Mr. Optimistic
Just re-read the BEA report looking to see if the word 'stall' was reported at any time as having been used. If the pilots had recognised a stall, surely even a very summary report would state that at x time the pilot recognised the stall. Anyway,
I believe the report was released to show that the plane did everything the pilots told it to do so that AirBus could use that at an air show. The plane did what it was told to do and what published documents declare it was programmed to do. I don't believe it was done to give any additional useful data.

Do you see anything to make you think otherwise?
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Old 12th Jun 2011, 13:29
  #1830 (permalink)  
 
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Jcj, indeed so, especially as the message was I'll let you know when it is ok again. Doesn't seem like the dynamics were sufficiently severe to prevent taking a seat, as the forces would have produced some recognition and action from the FD, other than what seems like incomprehension. So why weren't they sitting down - were they also unaware through the nature of the forces that something was so wrong ? If the instruction from the FD was such that it was a command to secure, then all this musing by me is irrelevant.

Despite all the thoughts about control laws, aoa indicators, what should have been done and so on, I hope the first step is to understand why these capable pilots were not presented with the crucial defining information to let them understand the situation in a timely way. Hence my unproductive search to see if 'stall' was said at any stage.

Edit: JD-EE, well it does make you wonder I agree.
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Old 12th Jun 2011, 14:11
  #1831 (permalink)  
 
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Stalled tilplane

mm43
The 13° NU position of the THS will most likely have helped in stabilizing the descent - see post #1393 by PickyPerkins and post #1395 by grity.
Thanks for that picture, mm43. I tried in a previous post by numbers, but your picture is excellent and shows, that the tail was stalled as well , and that trimming the THS ND would have increased the AOA of the THS and thus probably would have had no effect in getting the nose down. Hell, at 60° AOA that THS is nothing more than a big door producing no lift but only drag.

Might be, that the NU input by the PF causing a reduction of THS AOA produced some response (also not the desired one, probably smoothing the ride down), the ND input causing negative response symptoms due to complete stalling of the THS.

I´m familiear what happens when a wing profile stalls (relative airflow from the underside of the profile), however in the case of the THS the relative wind hits the top of the profile, as the THS has the camber on the downside (producing downforce).

Could somebody explain the behavior of an airfoil like in that case? At what AOA would it loose it´s effecftiveness and could even a kind of "reversal" take place?

Last edited by RetiredF4; 13th Jun 2011 at 10:16. Reason: misleading title
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Old 12th Jun 2011, 14:23
  #1832 (permalink)  
 
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safetypee,

Mach number calculation depends only on impact pressure and static pressure. Possible TAT probe anomaly (due to icing) is no factor.
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Old 12th Jun 2011, 15:18
  #1833 (permalink)  
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Wie gehts? What do you make of the initial STALLSTALL?

At Mach.82, it takes very little NU to produce the (cricketStall)?

IF PF input of NU caused the alert, could it in some way be related to an unloading of the a/c such that the ensuing climb was negative "g"? Actual Stall?

That would make the climb negative g, and ballistic in character. If g = < 1 wouldn't the PF maintain pull? Likewise, would not the a/c follow his input with THS (Leading Edge) DOWN?

If negative, the pilot would feel a profound "falling" sensation, as would the airframe? Hence his pull throughout in concert with THS TRIM? He had no visual reference, and his instruments would not be helping, "No valid indications". He would not feel deceleration (It would also be felt as -g), and at "over the top" the negative would increase rapidly as the a/c started to "fall" having run out of energy? This could explain his reversal to ND at the top, the additional neg. g he may have taken for the Nose falling through at "Stall".

Was the game over at a/p loss and first "STALL". This is a heavy a/c with beaucoups energy to sustain a short climb of 3k feet?

For this, the a/c would not have fallen off the Nose, so was the THS stalled, and Center of Gravity aft enough to prevent the Nose from falling through?

When I trained, we practised "maintaining Stall". Instead of releasing the HS and Pushing, we held it back, sufficient to continue the Stall, the effect was a MUSH, a way to lose gobs of altitude in short order.

There was a trainer available that was rumored to have "Inadvertent IMC" recovery solved by initiating this >MUSH< descent through the TStorm, and break out beneath to recover. It was said this a/c could actually "land" in this configuration, and slowly enough to prevent serious injury.

Every pilot knows it is possible to "fall" UP.

Last edited by bearfoil; 12th Jun 2011 at 15:43.
 
Old 12th Jun 2011, 15:42
  #1834 (permalink)  
 
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Stallstall

@Mb:

There appears to be no valid vane function reason for invalidating AOA indications at 60 knots.
Maybe, not sure! this lower CAS speed limit is introduced for maintenance. Sometimes we need to jack (WoW) the A/C for a longer period of time and don't like to introduce spurious Stall Warnings because its highest priority (#1) will prevent other aural attention getters to be noticed.

Why would one jack the A/C in a 60 Knots violent storm? We surely don't!!But for certain tests we have to simulate a - as close as possible flight situation- and therefore can select all kinds of WoW situations. (Nose, LH or and RH main gear) in CMC and that's allowed with all kinds of wind speeds.

Pulling CB of FWC's -to silence the STALLSTALL- for maintenance isn't an option because will also prevent other alerts and warnings to be shown.

I think 60 Kts was initial a compromise, as said before they have changed that with the BUSS mod.
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Old 12th Jun 2011, 15:51
  #1835 (permalink)  
 
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Hi, and eventually sorry by advance if this have allready been said.

It seems to me somewhat indredible the aircraft computers did not give any warning: as they were registred by the "black boxes", the vertical speed AND AoA were "known" by the computers. Even if there was some abnormalities about air speed , those datas had to trigger some warning: they, alone, were the sign the aircraft was stalled and falling.

About the NU inputs: somebody has told they can be the result of PF thinking of windshear. It would be very interesting to know about the two or three last simulator trainings of the PF (and other two crew). If one or more of those trainings were about windshear, maybe he (they) had (have) a tendancy to reproduce this case. It is a phenomenon well known, in medical domain for example: if you read some litterature about a rare pathology with relatively common symptoms, you have a tendancy to think about this one if your patient has those symptoms, even if it is not the most probable, statistically speaking.
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Old 12th Jun 2011, 16:30
  #1836 (permalink)  
 
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Originally Posted by Sciolistes View Post
It is extremely concerning that people justify the release of that BEA update, clearly worded to divert attention away from a sub-standard aircraft configuration ahead of a major sales event! I just don't understand how this can be acceptable.
I think it is not acceptable to post this kind of rubbish about what you call a sub-standard aircraft configuration based on the limited amount of data we have at the moment.
There was a really good discussion going on about the technical aspects.
Can we please refrain from spoiling this with this kind of drivel ?!
Thanks !
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Old 12th Jun 2011, 16:30
  #1837 (permalink)  
 
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Historical Info

What is heartbreaking all the way around is the mode of failure. One moment all is fine, and the next the speed is unknown/unavailable/kaput. There was no opportunity to take note of the situation immediately prior to failure. Had the PF had his eye on speed at the time of pitot icing, he would have known the stall warning was spurius. Is that true?

Would it help to have a display of a minute or two of historical data, something like an FDR trace on a display with a few parameters such as speed, altitude, AoA available for a situation like AF447? It would have been immediately obvious that the speed was fine 5 seconds ago. Maybe there is a way to display this information in a more helpful way. Or, would that just contribute to the confusion?
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Old 12th Jun 2011, 16:53
  #1838 (permalink)  
 
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Question Altimeter display?

From an ignorant newby.
I have found no mention of altimeter display either here or in the BEA report. You mentioned the aircraft falling vertically. Is it assumed that the pilots knew this? If they did they would also have seen the initial climb, wouldn't they.
Please be gentle, I almost did not dare to post.
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Old 12th Jun 2011, 16:54
  #1839 (permalink)  
 
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Originally Posted by RetiredF4 View Post

I´m familiear what happens when a wing profile stalls (relative airflow from the underside of the profile), however in the case of the THS the relative wind hits the top of the profile, as the THS has the camber on the downside (producing downforce).

Could somebody explain the behavior of an airfoil like in that case? At what AOA would it loose it´s effecftiveness and could even a kind of "reversal" take place?

franzl
I would expect that NU input would not unstall the THS as the AoA at the leading edge remains unchanged. And that is where the separation occurs at a stall.What happens downstream won't change that.

NU input will only reduce drag and effective area, so reduces drag in the direction of movement, thereby reducing Pitch Down forces. At an AoA of 61 ° drag acts also as lift (Factor 0,87) .
ND Elevator on the other hand would increase drag and effective area, thereby increasing Pitch Down force.
Albeit probably not by much.
But I would not expect any inversion effects in this case.
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Old 12th Jun 2011, 16:55
  #1840 (permalink)  
 
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Why AOA is disabled <60 kts?

A33Zab
Maybe, not sure! this lower CAS speed limit is introduced for maintenance.
That would be an unfortunate tail wagging the dog approach to a minor engineering problem.

With a computerized aircraft you no doubt have maintenance modes you can enter (that are locked out during flight operations). Besides, how do you test the stall warning system on the ground then?

The 60 knot limit below which AOA is disabled more than likely results from another corner of the envelope. Something such as flow impingement from thrust reversers at low speed (on the ground of course).
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