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AF447 Thread No. 3

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AF447 Thread No. 3

Old 27th May 2011, 19:02
  #141 (permalink)  
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It notes no further disagreements between the two recorded speeds for the remainder of the flight; rather, it notes that at various points that "all recorded speeds became invalid" (presumably when they fell below 30KT) or became valid (climbing above 30KT), indicating that they remained consistent with each other.
BEA states that the stall warning system AOA sensor is considered invalid below 60KT.

When stall warning sounds there are 2 actions that can cause the warning to stop:

A: Nose down, gain speed apply power warning stops

B: Nose up, bleed speed to below 60KT warning stops.

Given what has been written about low altitude (approach to?) stall Airbus training (must loose minumum altitude) ending up with option B is not as strange as it might appear on paper.

Especially so since the indicated speeds had very recently been unreliable the stall warning may have been suspect as well.
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Old 27th May 2011, 19:05
  #142 (permalink)  
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Originally Posted by atakacs View Post
Quick question: on what basis does the BEA reports that the speed measurement became valid again at 2:12:21 ?

The reply to that question is probably given in that part of the BEA report:
Note: When the measured speeds are below 60 kt, the measured angle of attack values are considered
invalid and are not taken into account by the systems. When they are below 30 kt, the speed values

themselves are considered invalid.

My explanation:
After the AoA exceeded 40° the airstream to the pitot probably totally collapsed/stalled, leading to an IAS below 30kts.
After reducing AoA by the PF the airstream probably partly unstalled and IAS in the pitot exceeded that threshold again.

That is what probably led to the confusing result of pitch-down leading to the stall warning going off again.
IMHO that is not a good solution. AI should seriously rethink that.

The only strange thing is that speed being declared invalid and stall warning stopping suppoesedly happened at the same time as the stall warning should disappear below 60kts and the speed indication below 30kts.

/Edit: parallel post to MurphyWasRight
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Old 27th May 2011, 19:06
  #143 (permalink)  
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In all the shaking, rolling, whistles and gongs, could the Pilot Monitoring possibly not realize the PF was holding aft stick? Could the PM, if in the right seat, override or at least neutralize the PF holding aft stick if he did know what the PF was doing?
To clarify ... if I am in RH seat, I move side stick, LH seat side stick does not move. Do I have that right, or wrong? Block diagram (old) suggests to me that would be the case.
That is correct. In the aircraft you cannot see what stick command the other pilot is giving, and the other stick does not move in any way (unless you move it yourself). The only way to cross check is by monitoring the performance of the aircraft.
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Old 27th May 2011, 19:08
  #144 (permalink)  
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A useful reminder of basic aero behavior

Upset Recovery Training - Pitch Angle versus Angle of Attack when Thrust-limited | APS Emergency Maneuver Training

Found this explanation, which would seem to be useful to those less well versed with the basic mechanics being described in the thread.
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Old 27th May 2011, 19:12
  #145 (permalink)  
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And this is one of the reasons why Airbus cockpit philosophy has, and will continue, to be subject to criticism from pilots worldwide:

The lack of cues that we are so used to having since day one of pilot training. Yokes (or control sticks) that MOVE when the other pilot moves them. Throttles that MOVE when the engine power increases/decreases. Systems that allow the PILOT to be in control, and not 385 bloody on board computers!!

Not saying that the pilots would have possibly been able to maintain control had this been a Boeing - an airspeed/PFD disagree/unreliable at altitude, in IMC, around Tstorms, at night is a handful no matter what aircraft you are flying. And we have seen two tragedies in the past - Aero Peru 603 and Birgenair 301, where unreliable airspeed led to loss of control in a conventional airliner, the 757.

But the lack of traditional cues in the A330 probably did not help them one bit.
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Old 27th May 2011, 19:13
  #146 (permalink)  
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The aircraft encountered unusual weather conditions (not in a Cb) which resulted in erroneous airspeed.
The crew instruments used this data (thus the abnormal indications), and other systems including the flight control computers, which interpreted some values as invalid (out of tolerance), and thus reverted to an alternative control law.

The FDR recorded the erroneous values of airspeed, which were not the actual aircraft speed, nor necessarily even after the ‘icing’ encounter with apparently ‘real’ airspeed values – ice blockage still clearing etc.
It is assumed that these same erroneous data were displayed on the PFD / ISIS, if at all, due to out of tolerance blanking.

In alternate law, the aircraft commenced a climb; probably a combination of trimmed state – residual A/P trim before disconnect and ‘inadvertent’ crew input – lack of control force feedback.
Question; does the THS automatically follow up a nose up command in alternate law?

In alternate law, there appears to be some high speed protection. Thus even with increasing AOA (and subsequent stall warning), if the ‘sensed’ erroneous airspeed was high ‘over-speed’ (“sharp rise in ‘recorded’ airspeed 215 kts”), then an automatic nose up command would be provided to reduce speed. ( In the small print / notes – alternate law.)
Question; does the THS automatically follow up in this condition?

A change in yaw / roll control law and/or loss of yaw damping in abnormal alternate law could have contributed to the apparent roll control activity. Not discounting turbulence.

The aircraft decelerated in the climb, approaching the stall, but not necessarily stalling immediately (note nose down pitch control inputs, 7000 ft/min – 700 ft/min). The crew, without airspeed indication (or at least viable display values), experienced conflicting information; stall warning from the AOA vanes, but nose up movement (and trim?) from a potential over-speed condition (erroneous airspeed – as above).

Nose down control corrective action had minimum effect due to nose up THS position (and TOGA contribution). The aircraft apparently stalled with nose up trim, but subsequently, the crew at times had apparently ‘reasonable’ airspeed indications and no stall warning (AOA inhibited at very low speed).

The aircraft remained stalled, the crew had conflicting / minimum information, the trim remained nose up, and the crew had no control force feedback of the aircraft’s trimmed state.

Question; does the stall recovery action, or any other low speed flight, require the crew to check the trim (THS) position?

Question: at any time in this scenario, did the display of aircraft attitude fail?
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Old 27th May 2011, 19:17
  #147 (permalink)  
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aa73, on the other hand, with the system as it is in A330, you don't get the other pilot "riding the controls" on you. (Haven't we all run into that once or twice in our lives? )


You raise an interesting question beyond the three questions in re THS. :

At what point, after the interval (of unknown duration) during which airspeed wasn't reliable -- a period during which FDR recording is of dubious accuracy -- would the airspeed return to reliable state and so that one can again be confident of the FDR's recorded airspeed data?

Aside: It seems that at some point in the descent, the airspeed indication returned to reliable state. It might be dangerous to assume that when the stall indication returned, A/S was once again reliable. What appears to be true is that the airspeed data, erroneous or true, met the threshold of acceptance in the system. That is not necessarily identical to the airspeed measured by the system being what the airframe was experiencing in the airmass.

The crux of some tough analysis BEA will be doing in the near future seems to me to hinge upon the answer to that question.

Last edited by Lonewolf_50; 27th May 2011 at 19:33.
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Old 27th May 2011, 19:19
  #148 (permalink)  
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Question; does the stall recovery action, or any other low speed flight, require the crew to check the trim (THS) position?
Not in any of the aircraft that I have flown - Boeing, Airbus, BAe etc etc.
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Old 27th May 2011, 19:22
  #149 (permalink)  
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Why would the software maintain the THS at its maximum nose-up position for the last 3.5 minutes of the flight?
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Old 27th May 2011, 19:23
  #150 (permalink)  
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Firstly, hello! from a newbie. I have been following this thread with interest as a controls engineer, but in the automotive world rather than aero (more 2d than 3d, but many more things to hit.......)

So, please forgive me if this is made up rubbish, but:

assumption 1) no spatial awareness by the pilots (nighttime, possbly in turbulence etc)

assumption 2) they are aware of unreliable airspeed and possibly other instrumentation faults

assumption 3) They have no reference to quickly decide if the stall warning gongs are valid, or just due to unreliable airspeed

assumption 4) they have no way of knowing that in fact, the aircraft has not suffered some major structural or systems failure

assumption 5) Scaning the flight displays, the one number that might stand out, would be the altimeter winding down at a massive rate, and maybe with other signals, such as cabin pressure changes, and possible abnormal wind noise etc (due to the extreme AOA), helping push that "fact" as "True"

assumption 6) Without other stimulus, i could see how one could assume that in order to be loosing 10k f/min altitude, the plane "must" be in a steep dive (negative pitch, nose down) (I'm not sure if the thought of a "deep stall" would have even occured?, esp in an aircraft that is "impossible to stall" (not true obviously, but i can again imagine that after 10k hrs stick time, you might formed an option of the aircrafts correct and accurate control response to most unusual situations

assumption 7) Pilots cannot trust the autopilot with degraded information (i.e. the phantom steep dive "could" have been caused by the automatic flight controls pushing the nose down to maintain speed, except the pilots know they can't trust that speed, so by default, the actions of the autopilot might not be trusted either)

I these cases (and i admit it is a complex series of scenarios) then throttling back, and pulling "up" could possibly seem like a valid response. (especially if the lack of pitch authority might have seemed like the result of a mechanical systems failure (instead of the deep stall) ??

Unfortunately, in the limited time availible, they were unable to work out what was actually happening to the aircraft. (it would be a brave move to go full throttle and stick forward if you genuinely "believed" you were in a steep fast dive would it not)

Appologies for the lack of technical jargon, but if you assume that the pilots were trying to "pull up" the entire time, then something must have caused them to carry out that action?
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Old 27th May 2011, 19:25
  #151 (permalink)  
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I would like to know what was going on between 2 h 12 min 02 - 2 h 13 min 32.

The BEA release is quite devoid of any facts for that long 90 seconds or so where AF447 went from FL3xx to FL100.

No cvr information, no sidestick input information, no flight parameter information nothing.
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Old 27th May 2011, 19:26
  #152 (permalink)  
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Originally Posted by Rob21 View Post
Not what I see. I see pilots struggling to fly an uncontrolable airplane, with no attitude indication, at night with no horizon.
Ohhh, nooooo!
Not again!!

Where oh where does this rubbish with the lost horizon come from????

It has been mentioned time and again that there is not the slightest indication that happened.
And the fact that they always managed to keep the wings reasonably level supports the fact they had attitude information.
In IMC without attitude information your landing gear will point straight up at one point or the other.
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Old 27th May 2011, 19:26
  #153 (permalink)  
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Stupid question?

How would the airplane behave if its fin came off in the zoom? Wouldn't it start Dutch rolling?
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Old 27th May 2011, 19:32
  #154 (permalink)  
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PEI 3721

There is no requirement to check the THS.

Airbus have changed their stall recovery procedures to seperate the nose down input from the power up. You now only smoothly increase power when the aircraft is unstalled so the nose down input comes first. There is a note that you may even have to reduce power for this nose down input to be effective.

It also mentions that, below 20,000', if your clean deploy Flap 1.

One thought. As they lost the stall warnings due to low IAS, 30/60 kts when the aircraft stops them, maybe they got confused when they lowered the nose and the warnings returned when the speed increased. I.E. back pressure and the warnings go forward pressure and they return. As I said just a thought and I may have misread the report.
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Old 27th May 2011, 19:33
  #155 (permalink)  
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Hi grity,
Originally Posted by grity
pull zoomclimb stall chrash....
A340 TC-JDN:
the first elevator moving to +4 deg, 4 sec after the autopilot self-disengaged as the aircraft exceeded the speed limit, +4 deg only for 2 sec was enough to start the zoom climb for the A340 TC-JDN (en-route from Istanbul to New York) and was defenitiv not triggerd by the sidestick of the pilot (the first sidestick pitch was 20 sec later).....
AP/ATHR kicking off in this case was obviously the direct consequence of an high speed prot, but without switching her to ALT. Those few seconds lagg seems to be due to turbulence action at the same point. But those A340 pilots applied also full thrust when autothrust disengaged, right after its auto-reduction from turbulences... and then, she goes from less than 70% N1 to 100%. Guess what, she climbed well! (with four engines TOGA). But, here also, nobody applied any stick imput until the top of it (about 30 seconds without a single imput).

Originally Posted by grity
2 h 10 min 05 autopilot self-disengaged
2 h 10 min 15 "alternate law"
what is with the g-protection in the time between ???
Pilots are quoted talking about Alternate law less than 10 sec. after AP & ATHR kicked off... where is the lagg?
This is not like recorded DFDR data that will confirm that it was simultaneous with those airspeed rejection (and, at least, the BEA should know it without any doubt at this point).
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Old 27th May 2011, 19:33
  #156 (permalink)  
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Per BEA initial report - in structured form for clarity.

01:35:15 Radio:TO ATLANTICO - @ INTOL
01:35:46 Radio:FROM ATLANTICO - Maintain FL350...eta TASIL?
01:55:xx Captain awakens #2 co-pilot
01:59:32-02:01:46 Turbulence & WX Briefing
02:06:04 PF notifies CC turbulence ahead
02:08:07 PNF "Go left a little"

11 Seconds elapsed:
AP then AT disconnects.
PF I have the controls"
A/C rolls to right.
PF inputs left and nose-up
Stall warning 2X
A/S "sharp fall" from 275 > 60kts Captain's PFD
Same indications on ISIS "few moments later"

34 Seconds elapsed:
PNF "...we've lost the speeds" "...alternate law"
A/C attitude goes beyond 10 pitch up
A/C V/S indicates positive rate of climb
PF nose down inputs. L/R inputs.
V/S goes from 7000 fpm to 700 fpm
Roll angle recorded between 12 deg R and 10 deg L
Captain's A/S rises sharply to 215kts
ALT 37,500.
AOA 4 degrees

02:10:50 PNF calls Captain to cockpit

49 Seconds elapsed:
Stall warning
AOA 6 degrees
PF "maintained" pitch up commands
AOA 6 degrees and rising
THS 3 degrees to 13 degrees nose-up in ~1 minute and remained there
"15 seconds later" - ISIS records sharp increase to 185kts and agrees with captain's A/S afterwards
PF nose-up "inputs" continues
ALT 38,000
AOA 16 degrees

22 Seconds elapsed:
Captain returns
Speeds invalid
Stall warning stops
ALT 35,000
AOA > 40 degrees
V/S ~ -10,000 fpm
Pitch maxed @ 15 degrees
N1 ~100%
Roll angles approached 40 degrees
PF input: full left and full nose-up 30 seconds

15 Seconds elapsed:
PF "I don't have any more indications"
PNF "We have no valid indications"
TLA @ idle
N1 55%

75 Seconds elapsed:
02:12:17 (approx from BEA)
PF commanded pitch down
AOA decreased
Stall warning (upon valid speed inputs-BEA)

56 Seconds elapsed:
PF "...arrive at level 100" (10,000ft)
02:13:47 (approx from BEA)
Both sidesticks recieved "inputs"
AOA "when valid" remained above 35 degrees

02:14:28 End of recording
V/S -10,912fpm
GS 107kts
Pitch 16.2 degrees nose-up
Roll angle 5.3 degrees L
Heading 270 degrees

Last edited by vapilot2004; 27th May 2011 at 21:55. Reason: corrected minor typos
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Old 27th May 2011, 19:35
  #157 (permalink)  
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Originally Posted by kilomikedelta View Post
Why would the software maintain the THS at its maximum nose-up position for the last 3.5 minutes of the flight?
That would be the natural reaction of auto-trim to a nose-up command by the PF.
That seems to match the description by BEA that stick-command was predominantly nose-up. Auto-trim wiould try to follow suit in order to allow centralising of stick.

Had the stick command been constantly nose-down, Auto-trim would have reduced THS angle consequently.
Sadly that continuous Nose-down command was apparently not apllied.

CVR might have some cues as to the why.
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Old 27th May 2011, 19:36
  #158 (permalink)  
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I would like to know what was going on between 2 h 12 min 02 - 2 h 13 min 32.

The BEA release is quite devoid of any facts for that long 90 seconds or so where AF447 went from FL3xx to FL100.

No cvr information, no sidestick input information, no flight parameter information nothing
Vertical speed -10,000 ft/min...

You're not quite right though, there is some info about that time period:

At around 2 h 11 min 40 <snip>
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’s pitch attitude did not exceed 15 degrees and the engines’ N1’s were close to 100%. The airplane was subject to roll oscillations that sometimes reached 40 degrees. The PF made an input on the sidestick to the left and nose-up stops, which lasted about 30 seconds.
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 the engines’ 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".
The initial altitude (FL350) and 2 minutes @ 10K/min gets you close to FL100
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Old 27th May 2011, 19:48
  #159 (permalink)  
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Our training is always to honour the stall warning with unreliable airspeed indications as it is based on AoA.
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Old 27th May 2011, 19:53
  #160 (permalink)  
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Henra, I am talking about pitch, not only roll.
As far as I know, an attitude indicator indicates (or should) pitch and roll.

This is not the primary instrument you look when flying instruments by hand?
AI, airspeed, altitude, climb, AI, lost airspeed, altitude, climb, AI, lost airspeed, altitude, climb, AI, AI, AI (attitude indicator).

Ok, I'm sorry.
BEA did not say clearly that the pilots did wrong. But the lack of FACTS on this report, like the absence of dialogs between 2:10:16 until 2:12:02 is kind of strange. Why only open to the general public the parts that may incriminate the pilots?

P.S. Where is PJ?
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