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AF447 Thread No. 3

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AF447 Thread No. 3

Old 11th Jun 2011, 00:39
  #1781 (permalink)  
 
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Thanks A33Zab and Lonewolf 50:

Ok, so to compute Mach speed, which is a conditioning value for the Stall Warning, we need dynamic pressure which we know (believe) was lost (for some duration) due to pitot icing. So the Stall Warning would have done what? Defaulted to the low speed limit? That doesn't seem smart.... this whole issue has been whirling around my head for a few days. Can anyone shed some light on what the heck the Stall Warning system does once the speed input becomes unreliable? Agreed the system would recover with the return of the dynamic pressure.
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Old 11th Jun 2011, 02:29
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A33Zab, so there could be Mach problems (amongst others) with a blocked pitot probe, but what if TAT rose erroneously to zero coincidently or even before the pitot problem – due to TAT probe blocking – a feature of ice crystal encounters?
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Old 11th Jun 2011, 03:46
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Ah, Machinbird, I'd separated the roll and the pull up as two different problems. The roll correction should be automatic. The pull up was wrong. So I suppose "do nothing at all" is probably wrong. Correct roll once you recognize it is real and then do nothing about speed.

And, of course, doing nothing about communications with DAKAR or Brazil trying to reach the plane without using selcal is also the wrong thing.

All that said, if the roll was not dramatic, why not wait a half dozen or dozen seconds to see what happens? The ride for the SLF might be a little rougher. But, they might have lived through it. Diagnose then do. "Don't just do something (reaction), sit there( think), then do (action)."

It will be interesting to find out how fast the plane rolled when the AP kicked out. Was it urgent? Was it slow enough to ask why then act? That much tonnage seems like it'd take quite awhile to develop a really sincere roll or pitch change absent strong control surface actions. Is my intuition here right?
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Old 11th Jun 2011, 04:15
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Aviate, navigate, communicate, comes to mind. How did they aviate to a 10 degree + nose up attitude at FL350? Did it have something to do with the Airbus can not stall teachings?
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Old 11th Jun 2011, 07:27
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Thing of it is, according to BEA, the a/c rotated progressively up to more than ten degrees NU, and "then started to climb." There's a real poser, eh? It takes 10 degrees NU to initiate Climb? There's a sluggo. From 0.80M ?
 
Old 11th Jun 2011, 07:33
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Bearfoil,

Isn't that the whole problem with that damn stupid report. Essentially hundreds of pages of posts that still haven't actually understood what that repoort is actually reporting. Why such a vague description of the events? What nonesense is this
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Old 11th Jun 2011, 09:18
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Originally Posted by etudiant
As SLC, that is a very frightening perspective.
You are in cruise, albeit in bumpy weather.
Depending on the breaks, you are 11 seconds from an unrecoverable stall.
That is the message that bearfoil is articulating in message 1747. ( any subliminal Seattle affect??).
Is he wrong? if so, why?
He is probably referring to a different accident.
In this one it took more than 1 minute to deeply stall the aircraft (Alpha >>10°) and there is no indication it was unrecoverable.

Last edited by henra; 11th Jun 2011 at 09:41.
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Old 11th Jun 2011, 09:52
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Hi bubbers44,
Did it have something to do with the Airbus can not stall teachings?
I think so. Maybe it is over "confidence" in FBW to the exclusion of intelligent flying technique? e.g. the QRH Windshear procedure has two references to the use of "full backstick".

"Airborne, initial climb or landing
Thrust levers ... TOGA
...
SRS orders ...follow
This includes the use of full backstick if demanded.
Note2. If the F/D bars are not available, use an intitial pitch attitude of up to 17.5 degs, with full back stick if necessary."

If they mistakenly diagnosed their problem as one of CB "windshear", their actions could be explained by performing that particular QRH procedure. After the stall warning ceased, maybe they were just waiting to get out of the "downdraft".
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Old 11th Jun 2011, 10:26
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Originally Posted by Sciolistes
Bearfoil,
Isn't that the whole problem with that damn stupid report. Essentially hundreds of pages of posts that still haven't actually understood what that repoort is actually reporting. Why such a vague description of the events? What nonesense is this
It was a note (not a report, to be published before the end of july) released under the pressure of leaks, speculations, and of Airbus (which wanted the preliminary data to be published before the Bourget air show). Even if uncomplete, we now know who was the PF/PNF in the cockpit, what were the crew reactions, and how the A/C stalled. Now we have to understand why these multiple NU inputs in the presence of stall warnings (in many of the past Pitot incidents with early stall warnings, ND inputs put the A/C in descent. None of the crew attempted to set a CLB/5°). Here:

-In a first sequence (from the Pitot signature at 2 h 10 min 05 to 2 h 10 min 50), the A/C climbed very rapidly to FL 375 but was not yet stalled (AoA~4°). Whether this climb was voluntary or not is not clear, even if the PF expressed regrets during the brieffing for not being able to climb due to the temperature profile, there were NU inputs as soon as the PF got the controls, even before the 1st stall alarms, then ND orders to stabilize the trajectory at the FL 375.

-The following 50 sec sequence between 2 h 10 min 50 and 2 h 11 min 40 (CPT reentered the cockpit), beginning with stall warnings, and then TO/GA + maintained NU inputs, effectivelly stalled the A/C (AoA=pitch=16° @ FL380). I am puzzled by these maintained NU orders by the PF (~800 hrs on A330 type) when stall warnings sounded for the 2nd time, but even more by the fact that the PNF (~4500 hrs on A330 type) did not react to the increasing pitch 2.5°->4°->10°->16° at such a flight level (over FL 375), leading to the stall. The BEA note says that during this phase the PNF tried several times to call the Captain back into the cockpit: was his attention devoted to help aviate/navigate or to communicate with the cabin crew ?

Last edited by Hyperveloce; 11th Jun 2011 at 10:43. Reason: correcting my poor english
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Old 11th Jun 2011, 11:13
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Originally Posted by bearfoil
..." There's a real poser, eh? It takes 10 degrees NU to initiate Climb? There's a sluggo. From 0.80M ?
bear - do take another look at the A340 AIRPROX zoom-climb Appendix B FDR plots, there is a 10 second lag between pitch up and establishment of climb. There's even a hint of the classic "non-minimum phase" control response - up elevator pushes the tail and the whole a/c down slightly before the lift/thrust vectors and Newton's second law build vertical velocity.

[NB no UAS different laws/protections active in the A340 vs AF447, not too clear what commanded the pitch up, lots of elevator action with A/P off controlling G? Is pilot sidestick trace inverted? Who/what adjusting throttle with A/T off?]
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Old 11th Jun 2011, 11:32
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I'm sorry but if I induce a 10deg NU input on an aircraft flying M.8 the climb rate will come on pretty much as the input is applied. We simply don't know enough to ascertain (data wise) to make anything sensible out of that statement by the BEA. For all I know it can just be lost in translation.

The only reason why I can see an experienced Captain/Crew maintaining a NU input with stall warning going off is that they believed that the aircraft could not be stalling (it's an Airbus right?) and therefore deduced that the warning must be erroneous.

Mode and Law confusion are the most common things that pop up in the simulator:
'Wtf is it doing now'' seems to be a common phrase.
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Old 11th Jun 2011, 13:01
  #1792 (permalink)  
 
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What Garage Years has been asking for a while now is the value of the critical AoA (AoAc) which is used by the confusers to trigger the stall alarm in ALT law when airspeed (including Mach number) is unreliable.

Unfortunately, it is not documented in the A330 FCOM. Only those who have access to technical documents not publicly available could answer the query. Others and I can only speculate hopefully trying to use sound logic.

The Air-Caraibe incident report mentioned a critical AoA of 4.2 deg. Cross-checking with the data just posted by A33Zab, this corresponds exactly to M 0.80, the recommended turbulence Mach number i.e. the speed at which the ACA A330 was supposed to fly at the time. It don’t see here a confirmation that it was the AoAc really used but just an assumption.

Obviously, confusers cannot just cancel stall warnings on the basis that Mach number is unreliable. They have to know what to do in this case but different solutions are possible like:

- Use the last Mach value considered valid.
- Use the highest current indicated Mach among the 3 sources.
- Use a combination of the above.
- Use MMO instead.
- Your choice …

None of these possible solutions are ideal.
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Old 11th Jun 2011, 13:24
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No it doesn't take 10 degrees nose up to initiate a climb but if you applied a large aggressive input with less aerodynamic damping (at high alt) then you would reach 10 degrees very quickly. There's a-lot of momentum to overcome before the climb begins and I would say that the BEA comment tells us about how the controls were apllied and the rate of change of attitude that was achieved as a consequence.
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Old 11th Jun 2011, 13:45
  #1794 (permalink)  
 
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DJ77:
Obviously, confusers cannot just cancel stall warnings on the basis that Mach number is unreliable.
Well the stall warning -is- cancelled if the sensed speed goes below 60Kts and then comes back when > 60Kts so maybe not so obvious to the designers that cancelling an in progress stall warning is suboptimal.

This snip from the report, well into the descent with the plane undeniably stalled, is the second instance the stall warning coming and going with invaled speeds could have further confused the pilots:

Around fifteen seconds later, the PF made pitch-down inputs. Inthe following moments, the angle of attack decreased, the speeds became valid again and the stall warning sounded again.
The report then mentions dual sidestick inputs at that point but frustratingly does not specify if they match or in what direction.
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Old 11th Jun 2011, 14:15
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Murphy:

The dual sidestick input(s) are bracketed time-wise with Prim 1 fail or switch off, and Sec 1 fail or switch off. Where are the switches situated to switch off/isolate/disable these units? I recall somewhere that dual SS use has to be enabled to allow summing of stick movement. Could the movement be the result of a hand finding it's way back to the SS?

Gums, if you are around: when this thread is all wrapped up and shifted to the dusty vaults, don't wander back to your Sipping Whiskey and fishing pole- we need a memoir writing!
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Old 11th Jun 2011, 14:33
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Minor request :
With all the quibbling about words like "assister", and 'around', I wanted to check back to the original French, but, with well over 150 pages of threads, I've now lost the link to the original of the latest BEA note (in French).

Could somebody post it again? Thanks in advance!
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Old 11th Jun 2011, 14:40
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I think this is what you are asking for:

http://www.bea.aero/fr/enquetes/vol....mai2011.fr.pdf
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Old 11th Jun 2011, 14:57
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Weeds round the prop,

What you recollect and are asking for can be found here, Pages, 35,36 & 37

A330 Flight Controls
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Old 11th Jun 2011, 15:01
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Turbine D,
That's it. Thanks!
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Old 11th Jun 2011, 16:09
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I believe the 30 second time grab with the a/p disconnect square in the middle would give it up. The Captain has said a climb is not available (I think they were cleared for an eventual Cruise @ FL370). Warm Air. He has also claimed the a/c will (in two minutes time), enter increased turbulence. He is not making small talk. Someone wants a left turn. With an aft load of Trim fuel, one expects at least a slight THS ND to load the HS to lessen Drag (oa). But the THS at handoff is 3 degrees NU.

Warm Air, sink, probably ND (descent)? No big, the Trim is correct, until, Pitots supply discrepant Pressures to AD(s). The autopilot gets out of whack, perhaps attempting a renew of Trim (airframe), and in doing so can't keep up, and for whatever reason, it is disconnected. Now it isn't that important how the Pitots started to read in disagreement, let's assume ICE, though that is not necessary, imo. ICE is patent in UAS, so we go with it. How much has the nose dropped since the beginning of the Temperature change? Unknown, but why would it be less? The a/p (and a/thr) are about trim, so let's say increase in Thrust and a notch more of NU.

The a/p quits, Cavalry Charge, and Master Caution. "I have the controls" says PF, and so he does, for he (right away) inputs NU and Left ROLL, one assumes from what he sees on the FD? Now this is a big a/c, and it is Nose Down, accelerating, and may have excess energy, even too much Thrust. At handoff, does the a/c remain at current Trim? That is my understanding, so if the Pilot has to correct, the a/p was unable anyway. So far so good. We don't know if the a/p had a chance to trim speed back to .80M so may be the PF has to think about that at this point also. He just got the stick, and he hasn't hand flown for awhile, and perhaps never after an emergency loss of Autopilot. He's a little aggressive, but quickly relearns a soft hand. Fighting this soft hand might be indicated, though, for he needs to make some corrections to regain the Plan. Did he overshoot Pitch? Was the THS quick enough to retrim the Tail?

If PF maintained NUtrim to slow, recapture altitude 'lost' in the temperature exchange and possible variable and substantial airstream fluctuations (vertical), then to me he's got it right, and the a/c will catch up.

"What's it doing now"? I don't give this sa remark much credence, and I never did. It is, like Dozy said, almost certainly doing what it is programmed to do.
How often is this type exposed to the combination of factors present for this upset? Computers don't 'forget', and that is a good thing. Neither do they 'think'.

The last thing on Earth anyone wants is to not understand the a/c, maybe worse is to misunderstand the aircraft. At least in confusion lies inaction?

For me, I'd like some more data, especially what happened in those thirty seconds. I think the Pilot turned left, and the aircraft turned right. (Metaphorically).
 

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