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AF447 Thread No. 3

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AF447 Thread No. 3

Old 27th May 2011, 16:19
  #101 (permalink)  
 
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A question for the AB pilots:

Is there an elevator offload to the stab? By this I mean does the aircraft auto trim in the direction of input to centralise the elevator? and would a prolonged nose up pitch input lead to the stab auto trimming itself nose up?
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Old 27th May 2011, 16:21
  #102 (permalink)  
 
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studi wrote :

Am I the only one here thinking that the climb was not induced by the pilots but by some sort of ill functioning protection?
No.
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Old 27th May 2011, 16:33
  #103 (permalink)  
 
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Was the stall recoverable?

Neglecting for the moment how the aircraft entered the stall, do the A330 pilots here believe such a stall was recoverable, allowing for the THS trim (correctable? ), cruise aft CG, and aircraft weight and altitude?

Possibly recoverable, or routinely recoverable?

Is this ever practiced in sim? ( guessing not, hoping it is )
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Old 27th May 2011, 16:41
  #104 (permalink)  
 
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Am I the only one here thinking that the climb was not induced by the pilots but by some sort of ill functioning protection?

moi aussi.


But the question is what were his instruments telling him?

If the static was blocked he would not have known that he was climbing.

If there was some pitot pressure and he climbed with blocked static then that would have indicated a reduction in IAS.

But it is still hard to believe with a correct horizon that he would pitch up.


Having declared a mayday with a hitherto unknown flight control malfunction on a classic aircraft I have a lot of sympathy for those who have to deal with modern aircraft - just spent two hours trying to get my computer to recognize my printer - unsuccessfully!

Despite AF resent history my money is on more than icing, pilot training and pilot error.
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Old 27th May 2011, 16:49
  #105 (permalink)  
 
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Manual pitch trim (3)

takata wrote :

Right, of course: it is like having a "direct law" with only manual THS trim available, with a message poping up and saying "Manual pitch trim only"... WTF!!! I won't ever use this [email protected] or I'll be fired! Your position doesn't make sense: it is not "per-design", it is not "forbidden": it is always better not to have to use it (like being in normal law instead of direct)... up to the point that you need it! Now, how training is performed is a different issue.
Regarding manual pitch trim, the design and associated training is thus :

Normal law : auto-trim. Dont touch it or fail the test and get fired. 99.9999% of the time for airline pilots. Protections have ultimate authority on control surfaces (hard protections). Eliminated at the start of the accident event sequence.

Alternate law (1 & 2) : auto-trim, which means dont touch it either. No specific PFD, ECAM, whatever, indication regarding manual pitch trim. Rarely trained for. Some protections remain, with a less assertive authority (soft protections). According to factual report, this was how the airplane flew its last minutes. Requires at least one working PRIM.

Direct law : PFD amber warning USE MAN PITCH TRIM. Similar to conventional airplane, direct stick position to control surface position. According to factual information, this law was not triggered. No protection. Rarely if ever trained for, except initial training towards type rating (box-ticking mentality). Requires at least one working SEC.

Mechanical backup : PFD red warning MAN PITCH TRIM ONLY. Basically, sidestick becomes useless. Remaining only manual pitch trim wheel and mechanical rudder, which could give you a reasonable fighting chance depending on actual circumstances. According to factual information, not triggered. Trained once, upon type rating training process (box-ticking mentality). Main condition is all five FCCs lost (3 PRIMs, 2 SECs, all off. Did not happen.)

For the sake of almost completeness :
Other laws : abnormal attitude law, flare law, ground law, ground/air blending laws... These (apart from abnormal attitudes) are quiet transitions, appear every day, and would be perhaps considered subsets of Normal law. No cockpit indication that these sub-laws are triggered. Conditions that activate them can be found in the FCOM. Probably irrelevant to our current matter, apart perhaps from abnormal law but this was not reported.

Assuming that such a significant event as any additional flight law change would have been reported by BEA. Subject to revision as more information becomes available.

Last edited by Svarin; 27th May 2011 at 16:50. Reason: added "no protection" to Direct law
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Old 27th May 2011, 16:53
  #106 (permalink)  
 
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I thought it was all going to make sense today...

I was wrong. I can't understand how a pilot could pitch his a/c up 2500 feet without realising it. He knew he had UAS but surely he didn't think he had unreliable altitude? 7000 feet per minute upwards after you get a stall warning? What happened in the 11 seconds between the AP disconnect and the pilot's realisation that he had UAS? Did he pitch up, get a stall warning or did he get a stall warning and then pitch up? What was the engine speed at this time? What BEA has done here is worse than useless. They have tried to provide details to quash rumours but by providing less information than is necessary to properly understand what happened. I think they should have either given out everything or nothing.
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Old 27th May 2011, 16:59
  #107 (permalink)  
 
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Originally Posted by studi View Post
Am I the only one here thinking that the climb was not induced by the pilots but by some sort of ill functioning protection?
The only protection I know of that would do anything like that is one that applies to the autopilot when the aircraft makes a sudden vertical maneouvre opposite the direction commanded. If the autopilot was already disconnected by that point then it's unlikely that protection mode would have been commanded.
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Old 27th May 2011, 17:12
  #108 (permalink)  
 
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Is there an elevator offload to the stab? By this I mean does the aircraft auto trim in the direction of input to centralise the elevator? and would a prolonged nose up pitch input lead to the stab auto trimming itself nose up?
Of course - that's how trim, and auto trimming works. A constant input on the stick (or from the autopilot) is trimmed out using the Trimmable Horizontal Stab.

Neglecting for the moment how the aircraft entered the stall, do the A330 pilots here believe such a stall was recoverable, allowing for the THS trim (correctable? ), cruise aft CG, and aircraft weight and altitude?

Possibly recoverable, or routinely recoverable?
The aircraft (with the possible exception of the pitot tubes) was fine - the situation should have been routinely recoverable.

If there was some pitot pressure and he climbed with blocked static then that would have indicated a reduction in IAS.
Climbing with blocked pitot results in the airspeed displayed increasing from the speed displayed when the blockage occurred, not decreasing.

In the Perpignon disaster, from the final report:

Originally Posted by page 28

When the aeroplane exceeds certain attitude thresholds, roll, angle of attack or speed, the system uses a specific law [abnormal attitudes law]. The display on the PFD is identical to that for alternate law. This abnormal attitudes law breaks down into two phases:

ˆ For the first phase, the law used in pitch corresponds to the alternate law without auto-trim and with only the load factor protection. In roll, a direct law with maximum authority is used. The yaw is controlled mechanically. This first phase should make it easier for the crew to return to more usual attitudes.
Originally Posted by page 86

When the stall warning sounded, the Captain reacted by placing the thrust levers in the TO/GA detent and by pitching the aeroplane down, in accordance with procedures. The nose-down input was not however sufficient for the automatic compensation system to vary the position of the horizontal stabilizer, which had been progressively deflected to the pitch-up stop by this system during the deceleration. The Captain controlled a left roll movement, caused by the stall. The aeroplane’s high angle of attack and the roll movements generated asymmetry, and a speed variation between ADR 1 and 2 appeared. This increasing divergence caused a rejection of the three ADRs by the FAC then the ELAC. The flight control system then passed into direct law. [i.e. No auto trimming in Direct Law] It is likely that the crew did not notice this due to the emergency situation and the aural stall warning that covered the warning of a change of flight control laws. The Air New Zealand pilot, by saying “alpha floor, we’re in manual” likely considered that the alpha floor function had triggered and that in fact the autopilot had disconnected.
Originally Posted by page 87

The aeroplane attitude increased sharply and its speed dropped to the point that rendered it practically uncontrollable, the flight control surfaces becoming ineffective due to the low speed and the high angle of attack. The aeroplane stalled again, this time irrecoverably, bearing in mind the aeroplane’s altitude and without any crew inputs on the trim wheel and the thrust levers.

The loss of control was thus caused by a thrust increase performed with a full pitch-up horizontal stabilizer position. This position and the engine thrust made pitch down control impossible. It should be noted that the PF made no inputs on the horizontal stabilizer nor reduced the thrust and that the PNF did not intervene. This seems to indicate that none of them were aware that the automatic trim system, which relieves the pilot of any actions to trim the aeroplane, was no longer available.
Originally Posted by page 93

On approach to stall and taking into account the dynamic of the flight and of the complexity of the displays, the automatic changes in the control laws can fail to be perceived and their consequences can sometimes be misunderstood by pilots. In this case, the passage to direct law rendered the auto-trim function inoperative. Even if the amber USE MAN PITCH TRIM flag was displayed on the two PFD artificial horizons, the crew did not notice the position of the stabilizer and did not command the trim wheel manually during the twenty-five seconds in direct law between 15 h 45 min 15 s and 15 h 45 min 40 s. From this time on and for the rest off the flight, as a result of passing into abnormal attitudes law, the amber USE MAN PITCH TRIM flag was no longer displayed. The systems thus functioned in a degraded manner, without the real overall situation of the aeroplane being known by the crew.
[emphasis and notes mine]

Last edited by Checkboard; 27th May 2011 at 17:30.
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Old 27th May 2011, 17:13
  #109 (permalink)  
 
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Am I the only one here thinking that the climb was not induced by the pilots but by some sort of ill functioning protection?
No - you yourself provide the quote from the BEA that indicate it was initiated by the crew (my bold):

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).
Note 1: Only the speeds displayed on the left PFD and the ISIS are recorded on the FDR; the speed
displayed on the right side is not recorded.
Note 2: Autopilot and auto-thrust remained disengaged for the rest of the flight.
At 2 h 10 min 16, the PNF said "so, we’ve lost the speeds" then "alternate law […]".
Note 1: The angle of attack is the angle between the airflow and longitudinal axis of the airplane. This information is not presented to pilots.
Note 2 : 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.
The airplane’s pitch attitude increased progressively beyond 10 degrees and the plane started to climb. The PF made nose-down control inputs and alternately left and right roll inputs. The vertical speed, which had reached 7,000 ft/min, dropped to 700 ft/min and the roll varied between 12 degrees right and 10 degrees left. The speed displayed on the left side increased
sharply to 215 kt (Mach 0.68). The airplane was then at an altitude of about 37,500 ft and the recorded angle of attack was around 4 degrees.
From 2 h 10 min 50, the PNF tried several times to call the Captain back.
From my read of the above, the pitch up command pre-dates the Stall Warning (though we do not have a well defined timeline to understand exactly the second by second sequencing).

We can see that the initial Pitch Up demand led to the climb, ultimately reading 7000ft/min, prior to the Pitch Down command which slowed the climb rate to 700ft/min, however, it seems by then significant airspeed was lost.

The text that follows in the report seems to be the most perplexing:

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.

Around fifteen seconds later, the speed displayed on the ISIS increased sharply towards 185 kt; it was then consistent with the other recorded speed. The PF continued to make nose-up inputs. The airplane’s altitude reached its maximum of about 38,000 ft, its pitch attitude and angle of attack being 16 degrees.
At this point we've seen a half-heart attempt to use pitch-down, which may just have been the solution. But that appears to have been abandoned. Pitch up demands then remain, until very late in the sequence, at which point the vertical speed was already at -10,000ft/min with an angle of attack stated to be +40degrees - the stall was full developed and they ran out of altitude.

Re-reading the report is a sad and sobering task. I simply do not understand the initial pitch-up demand. All that seemed warranted was a correction due to the initial right roll? However there is no mention of any throttle demand at the point where the first pitch up command was provided, so presumably airspeed is bleeding off from this moment on. TO/GA thrust was eventually selected 46 seconds after AP and AT disengagement. Too late.
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Old 27th May 2011, 17:16
  #110 (permalink)  
 
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@ studi, Svarin, blind pew and those who think "that the climb was not induced by the pilots but by some sort of ill functioning protection":

May I kindly ask:
- what protection you refer to / could you elaborate on why/how it's been triggered (with AP&ATHR OFF since 2 h 10 min 05, and ALT2 law active since 2 h 10 min 16 at the latest)?
- why/how do you seem do discart the followings?
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.
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Old 27th May 2011, 17:17
  #111 (permalink)  
 
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Of course - that's how trim, and auto trimming works. A constant input on the stick (or from the autopilot) is trimmed out using the Trimmable Horizontal Stab.
I prefer not to guess how other aircraft systems work, hence the question.

From this we can deduce that the THS movement was in response to the pilots input and not a cause of the initial climb or final stall event.
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Old 27th May 2011, 17:20
  #112 (permalink)  
 
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Quote:
Originally Posted by eireoflot82
""Could the physical sensation of a rapid descent combined with doubt about the veracity of the flight information and the lack of outside visual references simply trick the brain into misreading the situation they were in?

You don't feel descent, only acceleration. You will feel the initial downwards acceleration but once travelling at a steady speed there is nothing to feel.""
You don't feel acceleration but might be sensing the lack of it :

Descending at ~10.000fpm isn't it an aproximate rate of free-fall?
Could you relate this very situation with a 0-G maneuver (on the A300-0G)?

The report says 40° of angle of attack at some point even if pilots are trained to recognize a stall situation (on light aircrafts)
Wouldn't that be disturbing when flying in the "goo" with no airspeed, a pitch up indication, possibly a left bank angle but heading increasing ?

One of the copilot even says to the captain :
"We do not have any valid information..."

With all the workcharge, failures and warnings and wrong infos (speeds) one might become sceptical and "untrusty" over his Attitude indicator.
Moreover, nobody knows what the RH PFD was showing. Even if I tend to think they were both showing more or less identical values
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Old 27th May 2011, 17:27
  #113 (permalink)  
 
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Descending at ~10.000fpm isn't it an aproximate rate of free-fall?
Could you relate this very situation with a 0-G maneuver (on the A300-0G).
Actually, it's a bit quicker than a parachutist at terminal velocity in free fall - but a parachutist at terminal velocity is experiencing 1 G, not 0 G, (they are effectively lying on a bed of air - but internally feeling the same as lying on a real bed.)
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Old 27th May 2011, 17:28
  #114 (permalink)  
 
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Extraction of actions from narrative

An attempt at extracting a logical flow from the narrative. Ellipses (...)
indicate portions I removed for clarity, items such as inline explanations,times and displayed speeds. <comment> are my thoughts.
Bold indicate possible action/reaction chains.

...
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 airplane's pitch attitude <corrected from AOA> increased progressively beyond 10 degrees and the plane started to climb.

The PF made nose-down control inputs and alternately left and right roll inputs.
The vertical speed, which had reached 7,000 ft/min, dropped to 700 ft/min and the roll varied between 12 degrees right and 10 degrees left.
The airplane was then at an altitude of about 37,500 ft and the recorded angle of attack was around 4 degrees.
<almost regained control?>
...
the stall warning was triggered again. < by the AOA greater 6 degrees?, mentioned later>

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. < as result of sustanined nose up input?>
...
The PF continued to make nose-up inputs. The airplane's altitude reached its maximum of about 38,000 ft, its pitch attitude and
angle of attack being 16 degrees.
...
During the following seconds, all of the recorded speeds became invalid and the stall warning stopped.

<possible "well that worked" reaction by PF who was ignoring displayed speeds at this point anyway? >
...
The altitude was then about 35,000 ft, the angle of attack exceeded 40 degrees and the vertical speed was about -10,000 ft/min.
...
The airplane was subject to roll oscillations that sometimes reached 40 degrees.
<falling like a leaf but only sensing side to side not down since steady state,not trusting displayed vertical rate?>

The PF made an input on the sidestick to the left and nose-up stops, which lasted about 30 seconds.
...
(both said) "we have no valid indications". At that moment, the thrust levers were in the IDLE detent
...
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.

< return of stall warning was confusing since it coincided with actual improvement? Perhap led to nose up command? >
...
About fifteen seconds later, simultaneous inputs by both pilots on the sidesticks were recorded
<would be usefull to know the inputs and if they matched>
and the PF said "go ahead you have the controls".
The angle of attack, when it was valid, always remained above 35 degrees.

Last edited by MurphyWasRight; 27th May 2011 at 17:46. Reason: Chagnge AOA to pitch attitude Wozo points to /BEA correction
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Old 27th May 2011, 17:33
  #115 (permalink)  
 
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From the BEA report: about 15 sec after 2:10:51 (ie about 2:11:06), the two recorded speeds (LHS and ISIS) are once again the same, and remain the same for the remainder of the flight. To me, this sounds like the pitot probe problem cleared (ice melted or was dislodged?) at that point, and did not recur. The speeds then are noted as becoming invalid again (eg at 2:11:40 just after the captain arrives, until 2:12:02 + about 15 sec), but the report pointedly notes that they would do so whenever they fall below 60KT.

That's over three whole minutes in which they apparently had functioning speed indicators (a little after 2:11 up to impact at 2:14:28). They also had plenty of altitude, as this is about the same time they reached their maximum altitude of 38000.

By then the THS is trimmed nose up 13 degrees. Between 2:11:40 (when we are told N1 is almost 100%) and 2:12:02 (when we are told throttles are already at idle and N1 is 55%), the throttles were retarded.

At about 2:12:02+15, the PF commands pitch down, pitch is reduced, and speeds become valid again (ie climbing above 30KT).

Impact didn't occur until over whole two minutes after pitch down was commanded. Apparently not enough pitch down as the aircraft remained nose high (AOA at least 35 degrees), descending very rapidly. Did the PF stop pitching down? Or was he continuing to pitch down, but failing to drop the nose because of the up trim on the THS? The report says nothing about pitch commands after 2:12:02+15 except to note that, in general, pitch commands were "mostly up" after AP disengagement.

To my mind, it's those last three minutes, and particularly the last two, that need to be explained. There was an initial upset, and a reaction to that which led to a stall, but by about 2:11 the speed indications were apparently valid.

Was the crew confused or fixated on the wrong problem? If we knew more about what they were saying, it might be possible to figure it out. I can't imagine that nothing was said in these whole of the last two minutes beyond the brief exchange noted at 2:13:32.

Did the resumption of the stall horn once the speeds once again become valid after commanding pitch down (at 2:12:02+15) confuse the PF into thinking he was doing the wrong thing?

Perhaps it might also be worth noting that by 2:11:40, the aircraft was already dropping at -10000fpm, and that it would have almost exactly this vertical velocity when it hit, some three minutes later. This suggests there may have been little vertical acceleration during this period, perhaps lulling the pilots into thinking they were in stable flight? Yet they did note that they were dropping through FL100.

Lots of things are just not adding up.

[edited for grammar and to correct error on min speed at which speed is considered invalid; 60KT, not 30KT (see BEA report, note at bottom of p.2 of French version]

Last edited by spagiola; 27th May 2011 at 20:58.
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Old 27th May 2011, 17:35
  #116 (permalink)  
 
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Murphy, as you broke that down, you end up part way through the sequence with this progression:

"I push the stick forward, I am now stalled again, I need to pull the stick back" which is a valid response in inverted flight.



In re roll oscillations: is that characteristic of an AB in a stall, or a symptom of the usual auto trim/auto correct dampening functions dropping off, or a combination of the latter and a pilot seeking a stable condition?

(Recalling SAS off flying and how sensitive it is to inputs ... )
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Old 27th May 2011, 17:40
  #117 (permalink)  
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AoA/Pitch

Originally Posted by MurphyWasRight View Post
(...)The airplane's angle of attack increased progressively beyond 10 degrees and the plane started to climb.
Corrected by BEA to "The airplane’s pitch attitude increased progressively beyond 10 degrees (...)" (See current PDF)
 
Old 27th May 2011, 17:42
  #118 (permalink)  
 
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LW50:
Murphy, as you broke that down, you end up part way through the sequence with this progression:

"I put the nose down, I am now stalled again, I need to bring the nose back up" which is a valid response in inverted flight.
Interesting point, could the pilots have been -that- disoriented?
(I have no knowledge/opinion either way.)

If I parsed the narrative correctly (would much rather look at the DFR data) the stall warning returned when the speeds were again deemed valid by the system.
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Old 27th May 2011, 17:43
  #119 (permalink)  
 
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I am not a pilot, but have some knowledge of vectors and physics.

It seems to me that the critical fact is that the plane was pitched up, but the pilot input was to command nose up. It it possible that this incongruous situation was the result of a huge discrepancy between pitch (which is relative to the horizon) and AoA (which is relative to the apparent wind vector)?

If, for example, the plane was caught in a draft of wind from below (which might explain some of the altitude gain), wouldn't the AoA indication be negative -- and the pilot's response be to command nose down -- even while the plane's pitch was positive? Or is my thinking garbage? Don't worry about being harsh with me, I don't claim to be an expert.
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Old 27th May 2011, 17:49
  #120 (permalink)  
 
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Quick question: on what basis does the BEA reports that the speed measurement became valid again at 2:12:21 ?
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