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AF447 Thread No. 3

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AF447 Thread No. 3

Old 30th May 2011, 21:27
  #781 (permalink)  
 
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Thirdly, I couldn't get your experiment to work - would the fact that I am left-handed be a problem there?
Try the mirror image experiment.
The idea is that to exert force toward the body's center, you are probably going to use the wrist joint to do this if the thumb is pointed up.
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Old 30th May 2011, 21:30
  #782 (permalink)  
 
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Secondly, I can see how it would be a handful, but the force we're talking about here is not an accidental tweak in the wrong direction. To get the THS up to 13 degrees requires full back pressure for several seconds. This is the problem that is not going to be solved until at least an interim report is out, and anything we suggest is ultimately pointless.
The THS movement was in response to the sustained nose up input as described here:

At 2 h 10 min 51, the stall warning was triggered again. The thrust levers were positioned in the TO/GA detent and the PF maintained nose-up inputs. The recorded angle of attack, of around 6 degrees at the triggering of the stall warning, continued to increase. The trimmable horizontal stabilizer (THS) passed from 3 to 13 degrees nose-up in about 1 minute and remained in the latter position until the end of the flight.
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Old 30th May 2011, 21:32
  #783 (permalink)  
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Bigdave599

The initial storm climb
Although not an airline pilot, I have read every post on this thread. There are a couple of things as yet unexplained:
1) there seems to be an assumption that the first (2) stall warnings (just after 2h 10m 05) were spurious, possibly related to the IAS error. Firstly, there is nothing in the BEA statement to suggest they were spurious; and secondly, as the stall warner is fed from the AOA indicators and not the pitots, the IAS error would surely not cause a stall warning
2) the "storm climb" at 7000fpm for 2500 feet is not explained. The PF made "a left nose-up input" not, as he is later stated to be making, "sustained" nose-up input. And there is nothing in the text to suggest that he pitched the aircraft up to the extent needed to climb at this rate on cruise power settings - surely that would have required 20-30 degree pitch angles that would surely have been mentioned? There is also a mention of nose-down inputs in this phase

On the other hand, a big updraft would both lift the aircraft without big pitch-up, and cause the AOA to increase, perhaps setting off the stall warner?

Where this leaves the explanation of the crash I don't know, but we do not yet seem to have satisfactorily explained this first 45 seconds.

The time frame from 15 seconds prior to, and 30 seconds after a/p drop are the focus. The rest is marbles and chalk. In your post is a gob of important thought.

Short some one who is bilingual and conversant with Physics and aero, the majority of 'aha' is here. Energy( including ambient), Flight Path, and Input by both a/c and PF.
 
Old 30th May 2011, 21:35
  #784 (permalink)  
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I'm not 'anti airbus' - I've never flown one. But...

It strikes me that the bus flies quite unlike other aircraft even when everythings working as it should, which is practically always. I'm talking about the way it processes roll and pitch inputs, with the a/c staying rolled or pitched when pilot input ceases.

But when the autos have gone off-line and instruments have stopped indicating normally, its behaviour is going to change more than a non-airbus would. ie it changes back into what a non-airbus would be like with the same failures.

Someone mentioned having to roll the wing level by hand. The pilot of a non-airbus would not be surprised at suddenly having to do this. An airbus pilot, with probably very small actual 'stick time' (never mind stick time in IMC on partial panel) would probably be very surprised to fiind that his 'bus had stopped helping him in this way.
Also, unless given the chance to practice in this degraded flight condition, is it really surprising the pilots struggled?

The trouble with these autos is that the better they get, the more confused you are when they fail.

All I'm saying is that a highly automated Airbus makes a pilot highly dependant on its autoflight system and 'easy handling'.

Maybe a bit too dependant.
 
Old 30th May 2011, 21:35
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Originally Posted by Machinbird
JD-EE

JD-EE and others.
Most of you may not recognize what sensor_validation is doing with his analysis, but if anyone is familiar with the communication analysis techniques of Sidney Dekker in (for example) "The Field Guide to Understanding Human Error" (a highly recommended read) they will recognize the process.

Sensor_validation gets a big attaboy for those initial efforts.
There is a lot that can be learned from this type of analysis process.
Gotta go and read that ref now - but we are just given a glimpse into the mindset of the the BEA investigators who can't say what they are not certain about, or AF / AB could object to - but sure they will by now have more potentially controversial 'facts' and theories. I hope they can recreate what speed indication was on the RHS display.

On consulting mon interprète there's little doubt "réveille" means "wake", but perhaps there's nothing in the captain "assiste" rather than just "attend" the handover, and the PNF "éventuellement" translated to "maybe" could have been "possibly" and not be passive in context - but why did BEA highlight that but not whose decision to reduce speed. Maybe they wanted to make the point he was fresh and alert after "power napping"?

By the way - if you study the A340 airprox Appendix B zoom climb it appears (as has been pointed out before in this thread) pitch-down commands are plotted as positive which is somewhat counter-intuitive isn't it?

Last edited by sensor_validation; 30th May 2011 at 21:52.
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Old 30th May 2011, 21:45
  #786 (permalink)  
 
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Firstly, was the aircraft right-wing heavy?
Circumstantial evidence only at this point. All turns were made to the right following the initial problem. We will have to wait for BEA to confirm.

Secondly, I can see how it would be a handful, but the force we're talking about here is not an accidental tweak in the wrong direction. To get the THS up to 13 degrees requires full back pressure for several seconds. This is the problem that is not going to be solved until at least an interim report is out, and anything we suggest is ultimately pointless.
Pointless? Not necessarily. Once we have an idea of how this might have happened, we can explore other implications. The number of pathways we have to explore are far fewer than before AF447 was found.
It takes a while for the light to come on with some regarding what the crew faced. The more lights that come on, the easier it is to discuss other factors. At the moment, it seems no one on the thread is volunteering any of their experiences with the ALT2 flight control law. Or maybe they only did it with the aircraft in lateral trim (due to a kind simulator instructor) Maybe no one trained for it with lateral imbalance. That would be significant.
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Old 30th May 2011, 21:45
  #787 (permalink)  

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a big updraft would both lift the aircraft without big pitch-up, and cause the AOA to increase
AoA would decrease.
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Old 30th May 2011, 21:48
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L337. Other poster's correct. Increase.
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Old 30th May 2011, 21:55
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Folks

May I point you to this Spiegel online article with the following

The BEA report, in its current form, only provides the angle of the stabilizer but provides no explanation as to why. The report merely indicates that it was at this moment that Captain Marc Dubois re-entered the cockpit.

Exactly what orders he issued are not part of last Friday's report. But sources close to the investigation are saying that he said: "This is a stall. Reduce power and nose down!"
I have no idea about the veracity of this quote but I would tend to say that this is generally a serious publication. If confirmed this would obviously paint a very different picture.
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Old 30th May 2011, 21:59
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Originally Posted by BarbiesBoyfriend
It strikes me that the bus flies quite unlike other aircraft even when everythings working as it should, which is practically always. I'm talking about the way it processes roll and pitch inputs, with the a/c staying rolled or pitched when pilot input ceases.
I distinctly remember when doing my AEF in a Chipmunk that moving the stick to the left would put me in a left turn and returning it to neutral would hold me at that angle of bank. How is a FBW Airbus in Alt2 or Direct Law any different? You're not positioned as precisely as you would be in Normal Law, and the computers can't hold you there if you're being buffeted - but that's a pretty basic flying skill surely? Isn't that what recurrent training is for?
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Old 30th May 2011, 22:06
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L337

Originally Posted by L337
a big updraft would both lift the aircraft without big pitch-up, and cause the AOA to increase
AoA would decrease.
How's that then ?
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Old 30th May 2011, 22:14
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little known facts

Well, well, well.

As someone asked the 'bus drivers some posts ago, how many realized that AoA was considered invalid if airspeed below 60 knots? Oh!!! Sheesh, the airspeed is deemed invalid. What now, kemo sabe? Where's my AoA "protection"? Oh! There it is again, but I am stalling and maybe getting a speed warning. I am confused.

So "attaboy" to mm43 and Hoppy.

I then ask all to look at the "reversion" sequences in the manual and all the "protections" lost at each stage. Having trouble with capturing the page, so check back later or look at previous ref by 'bird, et al.

To Doze:

We luv ya, man.

I want you to code my next hypersonic space vehicle's flight control system and guarantee the system will work exactly as SPECIFIED, not DESIGNED. Jez kidding, and I feel your pain ( not really, but we're in the stag bar now and I can be honest, heh heh).

Jocularity aside.

The concern over weather seems overdone. From the BEA report, it appears climbs and dives were not done with fairly level flight attitudes as we would see in a severe updraft/downdraft or mountain wave or microburst. They seem to be directly related to pitch attitude and later, a flight path vector in a fully-developed stall. Sorry, but that's my story, and I'm stickin' to it.

Secondly, unlike several here who wish to have a "conventional" control mode, I can't see it. Even in the older jets there are dampers and limiters on the control deflections and deflection rates. Some are based on dynamic pressure, some on body rates. Last jet I flew with zero of those was in 1971. So the best I can see is a FBW backup mode that emulates the older jets we are used to as the Boeings and original Airbus.

And we don't have it!!

My biggest problem with the laws for the FBW 'bus is they try too hard to "protect" basic attitude and roll angle without emphasizing AoA. Then there are all those connections with the autopilot. I really like some of the law changes in "TOGA", wow, couldn't go around without all those, huh?

On the good side, the gee command for pitch is really neat ( versus AoA command like a normal plane), and allows for the so-called autotrim. Actually, all it does is maintain 1 gee, even in a bank up to certain limits. My jet didn't do that, so we needed to click back a notch of trim in a turn. Big deal.

On the bad side, a reliable backup law for the 'bus might consider primarily the gee and AoA for limits, while still applying body rates and maybe "standby gains' for dynamic pressure, and allow airspeed to take a lesser role ( especially unreliable airspeed).

I see far too many "notes' in the manual and combinations of failure modes that are not clearly explained unless you read all the fine print.
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Old 30th May 2011, 22:28
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L337 - perhaps I need some clarification. If the air flow vector at the front of the plane changes from horizontal towards the tail into something with a vertical upwards component towards the tail and upwards would that not look to the plane as if it had just had its nose planted upwards at a higher angle of attack?
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Old 30th May 2011, 22:28
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Originally Posted by Dozy Wannabe
To get the THS up to 13 degrees requires full back pressure for several seconds.
Are you sure? The BEA Update says after 2:10:51:
The trimmable horizontal stabilizer (THS) passes from 3 to 13 degrees nose-up in about 1 minute ...
I believe to have read somewhere that full back pressure demands 1.5 g. I further believe that the 7000 ft/min was reached after approx. 20 - 25 seconds, which would correspond to 1.2 g or less, i.e. 40% of stick travel. Stand to be corrected, of course.
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Old 30th May 2011, 22:34
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When stationary the AoA vane, which is pivoted, will be at an angle to the ground. The vane requires airflow over it to give accurate readings, I would therefore humbly suggest that the actual airspeed required for an accurate reading is 60kts.
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Old 30th May 2011, 22:37
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HN39:
At 2 h 10 min 51, the stall warning was triggered again. The thrust levers were positioned in the TO/GA detent and the PF maintained nose-up inputs. The recorded angle of attack, of around 6 degrees at the triggering of the stall warning, continued to increase. The trimmable horizontal stabilizer (THS) passed from 3 to 13 degrees nose-up in about 1 minute and remained in the latter position until the end of the flight.
The PF maintained nose-up inputs. So the question is whether a nose-up input not to the wall will activate autotrim or not. I suspect it will simply because the ride for the SLF would be kind of rough if autotrim only responded to a full forward or backwards stick motion.

In that case maintaining the nose-up stick input will bring THS upwards to its limits over time.
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Old 30th May 2011, 22:45
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L337
Quote:
Originally Posted by L337
Quote:
a big updraft would both lift the aircraft without big pitch-up, and cause the AOA to increase
AoA would decrease.
How's that then ?
An updraft would initially increase the angle of attack, and the airplane, being statically stable (one hopes) would react by pitching down in order to regain its trimmed angle of attack. What happens next depends on whether the pilot (or autopilot) attempts to hold altitude against the updraft. It is not correct, however, that an updraft ipso facto would decrease the airplane's angle of attack.

WRT the Spiegel article, the leak regarding the pilot's statement is extremely interesting, and it is hard to see why BEA would have withheld it from the interim report. Some statements in the article are incorrect, however, for instance the claim that the pax were held in their seats only by their seatbelts. At a steady rate of descent, the pax would experience 1 G.

It is unlikely that a recovery could have been made without manually trimming the THS. At high alpha the elevator is not very powerful compared with the stabilizer. That this was not done suggests that the crew were not aware of the -13 degree THS setting. Doesn't it seem unlikely that if they were aware of it, and had identified the stall, they would have been deterred from using it by their simulator training? They were about to die. Or did I overlook something in the report?
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Old 30th May 2011, 22:45
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JD-EE : I have it on good authority that it does. If the limit of elevator authority is reached and the pilot is still commanding full back-stick, then autotrim will move the THS to comply with what is being asked.
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Old 30th May 2011, 22:50
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..

As a professional translator, assiste in Portuguese means to watch, as in watch TV, watch a football game etc.
I am almost 99% sure it is the same in French
It does not mean help or actually assist in any way
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Old 30th May 2011, 22:52
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Just a little more of the CVR would help ! Presumably the CVR will show what the intent was and for some reason, legal or otherwise, someone is uncomfortable with divulging too much information. Doesn't it rather look like the initial climb was intentional: perhaps to get out of an bad icing level (with the storm below being unwelcoming) ?
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