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AF 447 Thread no. 4

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AF 447 Thread no. 4

Old 16th Jun 2011, 23:44
  #81 (permalink)  
bearfoil
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IF789

If the descent was transient, and chronic, why shouldn't the THS react with 13 degrees nose up?

Transient as in intermittent, and Chronic as in trend. Both. Pilots do this often, tweak, wait, tweak, wait ad ab. Another term could be: Hunting.

BEA report the THS moved from 3 - 13 degrees. Three degrees could mean sometime before the a/p is lost. This fits in with one excursion only, and could include both a/p and PF inputs, separated by the drop.

"...So they were trained not only not to touch the trim wheel, but also that if they concentrated hard enough, it would actually disappear ?

To me, some of the scariest comments on these threads have been on training practices. Was PFs only training for stall warning "pull up" (to minimize alt loss) ? That is all it would take..."

The pullup at STALL may mimic an artifact in training (not changed til after 447 went in) where the "Recovery" is w/o excessive altitude loss.Where was the added Thrust?

Turbulence? There is nothing in this sequence (sic) to base a conclusion of PF chronic NU. BEA say ".......A INPUT....."
you say

and indeed they don't specify a duration, however the next mention of inputs is nose down after the (first) climb.

During that first climb PF gets stall warning. At 37500ft he gets stall warning again and "maintains" NU input in response. If the response to the SW is consistent, then the conclusion is chronic PF NU.

If the THS had some excess NU, and PF added more NU at handover, what is to say he was not attempting his trained recovery. At STALLSTALL he would NOT immediately input ND but per SOP he would maintain "pull" to minimize altitude loss, not knowing his meager ND, (relaxed back pressure) was way insufficient to keep from Stalling.

He may have actually input Nose down, but it was not beyond neutral and was "READ" as NU by the FMC? He may have been trying to fly with elevators "Only", (not knowing NU/ND would react completely different in response) that would be his set from Stall Training? He certainly didn't train approach to STALL with 13 degrees of THS NU.

At this point, would he become a little unnerved at the reaction to seemingly "appropriate inputs. He would be getting large (and variable and delayed) actions from his hybrid elevator, Tail Plane mongrel. Once again, was his continuous back pressure on the way down his stubborn insistence on his reaction to the a/c at this point (Pull Back, Nose drops, Push, Nose raise?)

If so, at what point was the "feel" of the a/c so foreign to him that he acted in "unpredictable" ways?

thanks for the response
 
Old 16th Jun 2011, 23:48
  #82 (permalink)  
 
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Originally Posted by bearfoil View Post
Or more to the point, what was the THS' deflection at a/p drop? BEA don't say. A/P will hang in until 13 degrees Pitch UP before it quits.
You're getting things a little muddled, bear. A/P disconnect appears to be caused by the loss of air data, not the position of the THS.

Read the article that PJ2 linked to, it contains some very useful information - including the fact that the ADR disagree lasted for 45 seconds, which means that return to Normal Law is unlikely to have happened (Svarin should find that interesting).
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Old 17th Jun 2011, 00:01
  #83 (permalink)  
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You're getting things a little muddled, bear. A/P disconnect appears to be caused by the loss of air data, not the position of the THS.

No. A/P disconnected via unreliable air data OR it limited out. Not loss. It is the cause of the discrepancies that is important, and unknown. The a/c was being trimmed at handover. It was descending and rolling OUT of A/P. This suggests to me that the cause may have been severe turbulence AND/OR an autopilot that could not keep up.

Either way, these are two distinct possibilities. With Ice, we are stuck with only one. The timing of the faults suggests a/p dropped before the reads became unreliable. This means it is possible the a/c remained in Normal Law for a short term, until the discrepant reads instigated the Law change. I have not seen a proof against this. Only some "assurances".
 
Old 17th Jun 2011, 00:04
  #84 (permalink)  
 
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Zorin_75:

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.
Full text of the paragraph from the BEA.

Your version:

According to BEA, the THS passed from 3 to 13 degrees nose-up in about 1 minute at 2:10:51, 46 seconds after a/p disconnect. Even applying the least plausible interpretation to the report, that it was already arriving at 13 deg at 2:10:51, THS would just have started moving when the a/p disengaged.
Not saying the same thing are they. You are interpreting a version that assumes completion of THS movement at 2:10:51 (which is only 46 seconds and quite short of "about 1 minute" and since they have exact times, why not state 46 seconds?), my read is that starting around 2:10:51 the THS moved from 3 degrees NU to achieve 13 degrees about minute later.

You pick? Either is possible from the given text. Note the PF NU inputs (maintained) at and beyond 2:10:51... what would they do to the THS?
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Old 17th Jun 2011, 00:10
  #85 (permalink)  
 
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@bear - The article also states that the turbulence was moderate, not severe. The A/P is likely to have disengaged at the first sign of pitot blockage, which was some seconds before the computers confirmed ADR DISAGREE.
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Old 17th Jun 2011, 00:15
  #86 (permalink)  
 
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That initial pitch-up

I am still not convinced by most theories posted here on why there was a nose-up input as soon as the autopilot dropped out due to unreliable speed data. The most rational so far:
  1. it was an involuntary action, an unwanted side effect of putting in a left roll demand to counteract a right wing low tendency, or
  2. It was the response to a false indicated speed increase,

I find (1) unconvincing because it was one hell of an input, sufficient to cause a 7000 fpm climb and, apparently, two stall warnings, sounds sort of deliberate to me.

similarly (2) sounds wrong because, while we don't know for sure what the indications were on the PF's panel, due to the lack of recorded data, we do know that the other two systems showed abrupt speed decreases, and the previous high altitude pitot icing incidents, including the Air Caraibes precursor, also only registered speed decreases. So it would be extraordinary for the unforunate AF co-pilot to be the only one to have been presented with the opposite.

That same Air Caraibes report also said that at the time of the beginning of the anomalous air data, the indicated altitude rapidly dropped 300 ft (presumably due the lack of a vaild Mach number to correct the static pressure (? ) the BEA has already told us that with no vaild M, the stall warning system uses a value near zero...) Maybe the PF was reacting to that, admittedly 300 ft isn't much, but I suspect it looks a lot on the tape, and it would have been rapid. Perhaps this is the trigger we've been looking for.
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Old 17th Jun 2011, 00:18
  #87 (permalink)  
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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.
BEA. The first two statements, by BEA, describe a recovery from APPROACH TO STALL.

The next Statement describes a good picture of the THS after its position at a/p loss plus PF's additive, created the climb. If PF was unaware of THS postion at handover, he is figuring, hmmm. three degrees. His attempted recovery from APPROACH TO STALL, instead of being from three degrees, was from SIX, which he appears to have held.

If he had captured TOGA, (such as it was), the absence of the STALLWARNING means he is a success. Unfortunately, the TRIM is helping him maintain back pressure up to thirteen degrees.

SO. Two recoveries from APPROACH TO STALL. Holding back pressure all the way. With the eager help of THS.

Last edited by bearfoil; 17th Jun 2011 at 00:32.
 
Old 17th Jun 2011, 00:24
  #88 (permalink)  
 
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garageyears:
my read is that starting around 2:10:51 the THS moved from 3 degrees NU to achieve 13 degrees about minute later.
Actually I absolutely agree with you. Please read again, what I was saying is that even when read that unreasonable way, you can't get the report to align with bear's theory of the a/p disconnecting as the THS reached its end stop (which is also completely ignoring that we have already quite a good reason for the a/p disconnecting in the UAS).
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Old 17th Jun 2011, 00:26
  #89 (permalink)  
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Your turn to read again. I say no such thing. THS at 3 degrees, at drop. With the a/c descending and rolling right. AT STALLSTALL, PF selects back pressure to escape the Warning this is not the same as NOSE UP. It is agee thing, to which the THS responds. The only training he had re: APPROACH TO STALL was certainly without the THS involved? PITCH is protected in A1 and NL.

yep.
 
Old 17th Jun 2011, 00:34
  #90 (permalink)  
 
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Your turn to read again. I say no such thing. THS at 3 degrees, at drop
Uhm, I did:

Originally Posted by bearfoil
Ten degrees nose UP? That tells us nothing about the AoA, or the position of the THS. Was it already at 13.2? Did pilot cease his initial NU and begin his repeated ND inputs? The 7000fpm suggests the climb was (initially) very rapid, and there is no reason to think the THS was not "UP" having corrected for a chronic descent?
Originally Posted by bearfoil
A/P will hang in until 13 degrees Pitch UP before it quits. If the descent was transient, and chronic, why shouldn't the THS react with 13 degrees nose up?
Originally Posted by bearfoil
This means that the THS may have been responsible for the A/P quitting. 13.2 degrees exceeds its Limit for TRIMMING.
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Old 17th Jun 2011, 00:43
  #91 (permalink)  
 
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@bear - in anything other than Normal Law, protections can be overridden with sufficient control input. If you're now suggesting that he pulled hard backstick expecting the protections to have his back, then he either failed to hear the call "Alternate Law" from the PNF or there was something wrong with his training. All this supposes that your theory is correct - for which, of course, we have no evidence.
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Old 17th Jun 2011, 00:44
  #92 (permalink)  
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The THS "reaching its stops or limits at "2deg (ND) or -14deg (NU)" will not cause the a/p to disengage.
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Old 17th Jun 2011, 00:58
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I doubt if there is overly much difference between the initial FDR printouts of the Jetstar VH-EBA UAS incident and those recorded by AF447. Have a close look at what happened to the TAT, SAT and CAS traces and similarly to the altitude when the Mach correction wasn't available.

Probably worthwhile bearing those traces in mind when discussing the initial upset. How LOC came about is another matter.
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Old 17th Jun 2011, 01:01
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PJ2;

A nagging thought has been, why wouldn't/didn't the THS move the additional degree to 14 NU with what appears to have been fairly constant NU inputs?
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Old 17th Jun 2011, 01:15
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bear:

Where do YOU get this stuff from? Nowhere does anything state the aircraft was DESCENDING at the start of event, except in your posts - I honestly think the BEA might have mentioned such a condition. And now you are imagining some kind of AP drop off at THS authority limit.... This is bunkum and the worst kind of dramatic imaginings, solely it seems an attempt to fit some other version of reality to the BEA framework in a twisted fashion.
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Old 17th Jun 2011, 01:21
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Zorin_75:

(re-read your post... got it third time round )
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Old 17th Jun 2011, 03:01
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stalls, OODA, THS "laws"

Salute!

- First of all, there are "stalls". Then "deep stalls". Then "deep" stalls. So despite my posts describing a true "deep stall" in another jet, I can't say for sure if the Bus will do the same thing. I brought it up because the other jet was FBW and it was not a T-tail ( the config normally associated with a "deep stall"). My only valid comparison is the FBW system tried to do its best to no avail. In our case we had a large aft c.g. and a demonstrated pitch moment at a certain AoA that allowed the jet to settle into a true "deep stall".

Then there are "deep" stalls. Way beyond C-sub-ell max, but basic static stability and nose down stab/elevator usually allows recovery.

- I can't think of any pilot that does not employ the OODA principle, especially when making an instrument approach. Further, if it had not been for Boyd, Moody Suter, Sprey, et al, I would never had the chance to fly the first operational 100% FBW jet ( and no trim wheel for the THS, just our last-ditch manual pitch override doofer).

- PJ and another one or two agreed with my view of the THS trim law, but another pilot has implied that under certain conditions or reversions that the pitch control trim becomes more of an "attitude" command versus a gee command, and then an AoA command when certain limits are reached.

My point is that from the manuals' description, the HS is trimmed so that a stick command is always a gee when the gear is up or some AoA limits are reached or .... Gets confusing, don't it? So pull back a bit, then relax and the jet climbs at 1 gee corrected for pitch. So at a 30 deg attitude, it would trim the jet hands-off for a 0.87 gee normal force on your body, at 45 degrees 0.707, and the beat goes on. This makes sense for a heavy, as a continuos 1 gee would result in ever-increasing pitch, wouldn't it?

So the jet might appear to be in an attitude control mode, but damned if I can find that in the descriptions.

The potential problem that may deserve attention in this accident and with respect to the Airbus control laws is what does the THS do if the pilot has a small, but constant back stick when the jet is already at 1 gee , or even a bit less ???

more later.....
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Old 17th Jun 2011, 03:07
  #98 (permalink)  
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mm43;

I believe the slightly higher limits are the mechanical trim stop limits.

gums, when I first checked out on the A320, a really sharp friend (also doing the same) observed that the one-gee law would actually cause a slight increase in the rate of climb as the aircraft got further away from earth... ;-)

Last edited by PJ2; 17th Jun 2011 at 03:19.
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Old 17th Jun 2011, 03:57
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Awwww maaaan.....

Salute!

Gee, PJ, I thought I was losing it....

the one-gee law would actually cause a slight increase in the rate of climb as the aircraft got further away from earth... ;-)
Unlike our little jet with different operational requirements, that's what we saw when we relaxed pressure on the stick in a slight climb. Hmmmmm......

So I was pleasantly surprised when I read the Airbus pitch correction for the "gee input" by the stick. Made sense to me for a heavy.

One of my dreams is to have all the heavy pilots fly a system that commands gee until the jet reaches a certain AoA, then follows the AoA limit ( I don't like the term "protection"). Upon releasing the stick the system tries to establish the last gee command, whether a trim gee by the confusers or a manually set gee. I know for a fact what my jet's laws did.

I have a bad feeling that the Bus may not re-trim the stab real quick after a prolonged maneuver at the AoA limit, then just relaxing on the stick.

respectfully,
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Old 17th Jun 2011, 04:58
  #100 (permalink)  
PJ2
 
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Funny what goes around when no one knows...never saw the "slight increase"! Funny what still goes around...

I think the term "protections" conveys a prophylactic sense which speaks to some airline folk who want to reduce training, hire cheap and claim that commonality means one-for-all training. You can always tell a salesman but not very much.

The A332 trim rate is 0.2deg/sec on AP and I believe the rate is the same for manual flight using the sidestick. The same 3 electric motors, (only one operating at a time, IIRC) drive the THS. Manual/mechanical trim is meant to be used and employs two hydraulic motors. The mechanical stops are part of both systems. A pawl and ratchet system prevents back-movement under air loads. From what I have been able to learn, I don't believe that the THS would return to a previous setting after a prolonged manoeuvre if the stick is merely relaxed, (neutral, no orders to the FMGEC > FCPCs). I believe the THS would remain "as is" in manual flight and the airplane would "settle" into the setting just as any airplane would. In autoflight it would of course, trim.
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