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AF 447 Thread no. 4

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AF 447 Thread no. 4

Old 17th Jun 2011, 17:00
  #121 (permalink)  
 
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Originally Posted by PickyPerkins

Using the graphs in HeavyMetall's reference for the lowest aspect ratio (6), isn't it the case that stick forward, elevator down, would lead to a decrease in the lift on the THS, and hence a more nose-up attitude of the a/c?
This is maybe to some extent true if you change the angle of the THS.

But commanding ND Elevator does not change so much AoA but it changes the camber and thus the effective profile of the tail surface, thereby completely changing the lift curve.
You would need to compare the Cl of the cambered tailplane. I'm pretty sure the cambered tail will have more lift even at increased AoA.

Moreover as you already describe ND elevator would increase drag, thus increase ND forces. At these angles you can exopect the drag to be of similar magnitude if not higher that the Lift, especially for a heavily cambered profile.
what is also missing and what will also be influenced by ND elevator is the moment of the airfoil. Strong camber will add a ND moment.

Taken these aspects altogether, even if I have no specific data for the given case I would strongly tend to believe that ND elevator would have lead to Nose Down attitude change even at these AoA.
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Old 17th Jun 2011, 17:05
  #122 (permalink)  
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A33Zab;

Yours is a valuable contribution to this aspect of the discussion. My thanks to you for clarifying the THS system and correcting, for other readers, my mistaken impression of the THS operation. I had read the electric-motor component as "the slower" of the two driving systems, ostensibly due to the lower demands of the AFS and, in manual flight, the sidestick. I've gone back to what I have and re-read with your contribution in mind.

Chris Scott;
From the auto-trim point of view, the movement from 3NU to 13NU (after FL375) was very nearly continuous?
I've watched the A340s and A330s THS wheel on the pedestal move slowly in response to small stick movements and changes in airspeed. One can see the movement out of the corner of one's eye because of the paint but I always valued the aural confirmation of movement on those airplanes that had manual trim.

The THS response to sidestick orders is immediate, in other words. I believe then, that as the sidestick was held in the NU command position, (orders still gee-referenced but with rate/gain limits), the THS would follow through, likely at a varying rate in relation to slightly changing sidestick orders. I think once the AoA was below that which the autotrim ceases to function, it would respond to sidestick ND orders. As stated, it would certainly have responded to manual trim wheel input.

A good, (and needed) discussion on "NU" and "ND", thanks. That's exactly the way the "minus 4" symbol for the THS position on the FLT CTRL ECAM page was explained to us when learning the A320 in 1992. Minus meant "tail down, nose up", and I think made good sense once one understood.

On the "roll to the right" upon disconnection, I have to re-iterate that the fuel system for the A340 and A330 were next to flawless in maintaining cross-wing balance of fuel. And there is nothing in the BEA Note to point to cause but any fuel imbalance on this airplane is real cause for close observation. I can't see rudder trim being an issue because it is automatic as well. One doesn't "disconnect the airplane, trim, then reconnect" as one did with the DC8/B727, etc and at that point certainly, it isn't reasonable to consider that asymmetric thrust was a factor.

Last edited by PJ2; 18th Jun 2011 at 17:00.
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Old 17th Jun 2011, 17:52
  #123 (permalink)  
 
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Originally Posted by syseng68k
Hyperveloce, 105


Have been thinking about something similar, but in relation to wing loading
as an extra parameter input for the calculation of lift. For the control
surface case, this could be instrumented via strain gauge type transducers,
one would think, near the control surface root.
syseng68k
If I misnderstood what you wrote I apologize upfront, but the wing loading in a stable stall will be roughly the same as in level flight, i.e. 1g.

The main difference would be the exact point where the lift is applied. In a fully stalled wing this will move from the Quarter Chord to the middle of the chord, so the moment on the wing might change a bit but the vertical force won't change much.
In a really deep stall on the other hand it might indeed work as the force on the wing it will decrease a significant bit as the fuselage will take over some part of the lift. In the attitude of AF it might contribute ~30%, so it could work to some extent but only at a point where it might be too late.
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Old 17th Jun 2011, 18:14
  #124 (permalink)  
 
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henra:

With a little "back of the napkin" sketching, I arrive at this from your post number 120 of this thread.

With the THS not stalled, though at an angle of attack significantly displaced from the usual "nearer to horizontal" (horizontal reference being THS airfoil chord line), the effective lift force, and thus corrective pitching moment, (roughly perpendicular to longitudinal axis of the aircraft, and then acting along the fulcrum to tip the aircraft's nose back down) is reduced roughly as the sine of the angle (AoA). The larger the AoA, from a nominal airflow parallel to chord line, or AoA absolute value of zero, the greater the lost force (vector subtraction). I din't factor in camber, as I both don't know it, and expect that it would wash out. This presumes no stall. Once a THS is stalled, it's a different problem.

I sketched it out this way based on expected control forces, controllability, and control response from an input to an airfoil movement. Each knot of reduced airspeed robs the lifting force of magnitude proportional to the 1/2*k*v^2 relationships in the lift equation.

What does this do to the aircraft's response ...

even with the THS not being stalled at that unusual angle of attack, it would take longer for both the elevator to influence THS, and for the lift acting on the THS to move the tail up (and thus the nose down) since the lift force isn't pushing as hard for a given flight control pitch command.

At some point at very low airspeed, depending upon CG, the airspeed ( airflow? ) may be so low that there isn't sufficient pitching moment (due to the combination of low speed influencing lift reduction, low air density, and sine of the angle vector subtraction) to move the nose at all.

Why sine of the angle?

As I sketched out the FBD on the THS, my vector component NOT adding lift increased with increased AoA, lift vector component decreasing.

Does that make sense to you?

EDITED since it was really confusing to me, and I wrote it.

Second EDIT: the more I think of this, the more it seems to me that an AoA indicator might be handy at high altitude. The AoA margins are reduced up there, a point which strikes home as I review HazelNuts39's graph on Mach Number and AoA and stall warning. If you lose speed at high altitude, that thin air really puts you at a disadvantage in terms of what your flight controls can do for you.

Last edited by Lonewolf_50; 17th Jun 2011 at 18:28.
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Old 17th Jun 2011, 19:21
  #125 (permalink)  
 
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THS position-indication and "Vc Prot Law"

PJ2,
Thanks for reminding me that the THS display on the ECAM F/CTL page (the lower display unit, not normally displayed in the cruise), would have eventually shown either
"13° UP" or "-13° UP" for what I refer to below as "13NU". (Perhaps you can tell us which?)

As far as I know, the crew were not presented with any ECAM drills on that lower DU, despite the various failures timed at 0210 and 0211. Presumably, the CRUISE page would still have been displayed, which does not include THS? If so, the best way of reading the THS position would have been the scale on the manual pitch-trim hand-wheel, but that involves turning the head. As I think you said, there is no "whooler", or equivalent, to announce THS movement.

Quote:
The THS response to sidestick orders is immediate, in other words. I believe then, that as the sidestick was held in the NU command position, (orders still gee-referenced but with rate/gain limits), the THS would follow through, likely at a varying rate in relation to slightly changing sidestick orders. I think once the AoA was below that which the autotrim ceases to function, it would respond to sidestick ND orders.
[my highlighting]

I'm not sure you needed to include the clause highlighted, so will try to explain why.

From 02:10:51
Would the auto-trim look at AoA when and while the system considered the AoA data invalid? Was the "Vc Prot Law" or "limited-authority stability order" of Andy Tracey's paper disabled some of the time or constantly? When the stall warning was operating, the AoA was considered valid, so the THS would stop provided the stick was neutral.
When the stall warning was not operating, the THS could auto-trim if the AoA was below the stall-warning threshold. And if the AoA data was invalidated, my understanding is that it could auto-trim regardless of how high the AoA was.
But would the back-stick not have overridden the "Vc Prot Law" anyway? If that was not the case, would the THS have ended up at 13NU?

Hope this makes some sense.
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Old 17th Jun 2011, 20:31
  #126 (permalink)  
 
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Originally Posted by Lonewolf_50
What does this do to the aircraft's response ...

even with the THS not being stalled at that unusual angle of attack, it would take longer for both the elevator to influence THS, and for the lift acting on the THS to move the tail up (and thus the nose down) since the lift force isn't pushing as hard for a given flight control pitch command.

At some point at very low airspeed, depending upon CG, the airspeed ( airflow? ) may be so low that there isn't sufficient pitching moment (due to the combination of low speed influencing lift reduction, low air density, and sine of the angle vector subtraction) to move the nose at all.
I agree with the much slowed response at the low speeds and AoA. It is quite safe to assume that any recovery at these speeds and angles would have been relatively slow.
Where I'm not so sure if I understand it correctly is the thing with the Sine.
I will give it a try based on how I understood your approach:
As long as the flow does not separate it is not the speed along the chord but the speed along the Flight Path which is relevant for the lift. Therfore the lift component which is trying to tilt the aircraft Nose Down around an unknown center of rotation will be simply a function of the square of the speed along the trajectory.
The sine will apply with regard to the center of the Earth (i.e. the external coordinate system) but not for the coordinate system of the aircraft itself. Therefore it should be irrelevant for Nose Down with regard to the aircraft coordinate system. This assumes that the sum of external forces acts at one point on the aircraft and acts in the direction of the flight path.
This is a little oversimplification because the external forces consist of gravity, momentum, aerodynamic effects and thrust of the engines. It has to be noted that the latter two do not act exactly on the same point on the aircraft therefore making my assumption not 100% correct. Therefore the aircraft coordinate system is not completely independent.

Was a bit lengthy and probably even more difficult to understand what I mean but I hope I understood your description correctly if not you can ignore my comment.
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Old 17th Jun 2011, 21:01
  #127 (permalink)  
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Hi Chris - I'm struggling a little to understand what you mean. I'm not referencing anything too complicated, just the statement regarding the Abnormal Law, below.

Now I know the BEA has stated that the airplane never reverted to "Abnormal Law" as a result of the high AoA but they dont' explain why and that is really confusing. Another piece missing, which will show up sooner or later.

Here's my reference - not sure it answers your question:

Abnormal attitude law
This law from the FCPCs is engaged when certain aircraft parameters exceed predetermined values.

The laws in place would be:
- in roll: - yaw alternate law
- in pitch: - a modified Nz law, without autotrim.

After aircraft recovery, and until the aircraft is on the ground, the available laws are:
- in roll: - yaw alternate law, (in this case Alternate Law 2)
- in pitch: - Nz law, (with recovered autotrim).

"In haste", as you say Chris!...gotta do some work. Hor's de combat so to speak, over the next while.
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Old 17th Jun 2011, 21:10
  #128 (permalink)  
 
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henra, 122

If I misunderstood what you wrote I apologize upfront, but the
wing loading in a stable stall will be roughly the same as in level
flight, i.e. 1g.
Not at all and thanks for the patience. Shows just how much I (don't)
know about aerodynamics.

The main difference would be the exact point where the lift is applied.
In a fully stalled wing this will move from the Quarter Chord to the
middle of the chord, so the moment on the wing might change a bit but
the vertical force won't change much. In a really deep stall on the
other hand it might indeed work as the force on the wing it will
decrease a significant bit as the fuselage will take over some part of
the lift. In the attitude of AF it might contribute ~30%, so it could
work to some extent but only at a point where it might be too late.
Ok, if lift doesn't change significantly until beyond stall, but
the position does, other methods could be used, such as >1 pressure
transducers at critical points on the surface. If these were summed
differentially. the position of lift could be calculated, thus approach to
stall.

Going back to the strain gauge solution, assume that the lift position
produces a variable twisting moment to the wing ?. If so, this could
be measured to determine lift position.

Probably completely off the wall and i'm sure there's no shortage of
ideas, but it is Friday ...
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Old 17th Jun 2011, 23:08
  #129 (permalink)  
 
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Quote from Lonewolf_50:
The recorded cockpit briefing about temperature not developing as planned/forecasts may be of no moment ... or maybe a clue pointing to TAT readings sensing beginning to degrade as Airspeed sensing began to go wrong?

I'm inclined to think the former. Looking again at that Jetstar A330-200 incident FDR-trace that mm43 kindly provided us last night (GMT), the TAT down-spike when Mach and CAS collapse is quite small (~5C), whereas the coincident up-spike of SAT is large (nearly 25C), in order nearly to meet the TAT as you would expect at Mach 0.2.
When looking at the possibility of step-climbs, we look at the decreasing weight and the SAT (and its deviation from ISA), not TAT. Some large jets are limited mainly by buffet margins, others by thrust (the classic examples being respectively the VC10 and B707-320). With its high-bypass turbofans the A330-200 is thrust-limited, so the higher the temperature, the later the step-climb. The crew would have been comparing the SAT readings with the forecast OATs on their computer flight-plan: hence the remark.

Considering your second option, on the other hand: if the CAS/Mach readings had already started insidiously under-reading, causing an over-reading SAT, the A/C would have been actually speeding-up without their knowledge. Not sure if it's possible to prove or disprove that possibility. GS doesn't help because we don't know the wind-component.
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Old 17th Jun 2011, 23:16
  #130 (permalink)  
 
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Abnormal Law.

PJ2:
I didn't go into 'Abnormal Law" as stated by BEA and 'confirmed' by this data:
If CAS < 60 Kts AOAi & AOAc are set to 0° and SSM (System Status Matrix) is set to NCD (No Computed Data),
this is also valid for TAS 0 Kts if CAS < 60

If CAS < 30 Kts it declares itself invalid and outputs 0 Kts and NCD.
........
(Stall warning is generated by highest AOA and not the median value)
This means actual AOA could be indeed 40° while not triggering ABNORMAL ATTITUDE LAW due to a median *AOA* (ouch, made a mistake here, should be CAS) value below <60° (resulting in a AOAi of 0°) as already mentioned in the BEA ‘leak’.
http://www.pprune.org/tech-log/45283...ml#post6499464 for whole post.

@PJ2:

would this be a correct understanding of what you meant?

and

@Chris:
Please forgive me for offering readers a warning on terminology here.
Exact! Thx to you both for making this clear and with apologies, english is not the native language.
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Old 17th Jun 2011, 23:30
  #131 (permalink)  
 
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A33Zab;
(Stall warning is generated by highest AOA and not the median value)
This means actual AOA could be indeed 40° while not triggering ABNORMAL ATTITUDE LAW due to a median *AOA* (ouch, made a mistake here, should be CAS) value below <60° (resulting in a AOAi of 0°) as already mentioned in the BEA ‘leak’.
Did you actually mean:-

Actual AOA could be indeed 40° while not triggering ABNORMAL ATTITUDE LAW due to a median CAS value < 60 kts (resulting in a AOAi of 0°) as already mentioned in the BEA ‘leak’.
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Old 17th Jun 2011, 23:50
  #132 (permalink)  
 
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Abnormal Law.

@mm43:

Yes, indeed in this context from the BEA May, 27 update:


At around 2 h 11 min 40 , the Captain re-entered the cockpit. During the following seconds,
all of the recorded speeds became invalid and the stall warning stopped.
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.
and the 'leak':

After stalling, the A330's angle of attack stayed above 35°. But while this exceeded the threshold for the abnormal attitude law, the flight control computers had already rejected all three air data reference units and all air data parameters owing to discrepancy in the airspeed measurements.
Abnormal law could only have been triggered by an inertial upset, such as a 50° pitch-up or bank angle of more than 125°. "That never occurred," says French accident investigation agency Bureau d'Enquetes et d'Analyses.
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Old 18th Jun 2011, 00:22
  #133 (permalink)  
 
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Zoom climb

Nobody knows what is going to happen when an a/c enters a thunderstorm. No one.

If a strong enough updraft occurs it could push the plane up at 7000 fpm at the same time sending the nose down thus possibly causing nose up trim.

In addition no one knows for sure what the airplane will do when it is in a deep stall.

One thing i know, it is difficult to fly an out of control plane with trim alone.
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Old 18th Jun 2011, 00:28
  #134 (permalink)  
 
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uncommanded climb

GarageYears

"your are implying the climb was NOT of the pilots command? I can see nothing to support that assertion. "

Theres nothing to support it either.
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Old 18th Jun 2011, 00:45
  #135 (permalink)  
 
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Quote:
Hi Chris - I'm struggling a little to understand what you mean. I'm not referencing anything too complicated, just the statement regarding the Abnormal Law, below. Now I know the BEA has stated that the airplane never reverted to "Abnormal Law" as a result of the high AoA but they dont' explain why and that is really confusing.

Struggling? That makes two of us! Sorry it didn't make sense. By the way, I had not considered the possibility of Abnormal Attitude Law. Perhaps I should.

If the initial drop of IAS on ASI 1 and ASI 3 was sufficient to trigger Abnormal Attitude Law, what would have happened when the ASI 1 recovered to 215kt at FL375? The IAS/Mach, AoA, Pitch, and Roll no longer justified it. In these circumstances, you note:
After aircraft recovery, and until the aircraft is on the ground, the available laws are:
- in roll: - yaw alternate law, (in this case Alternate Law 2)
- in pitch: - Nz law, (with recovered autotrim).

So autotrim would have been recovered? In which case, it would have resumed trimming: partly, perhaps, to neutralise the increasing up-elevator that had been selected by the system to maintain the G demanded by the sidestick as the airspeed fell while the THS had been frozen. It would certainly have needed to trim nose-up for the continuing decay of actual CAS as the aircraft approached FL380.

It is unclear precisely when the AoA reached +30, but at that point the THS would presumably be disabled again. My assumption is that the 1 minute taken for the THS to move from 3NU to 13NU would have been completed before that point.

In the post you are discussing, I proposed that the PF's "nose-up inputs" may have overridden any stall protection in Pitch-Alternate Law. If Abnormal Attitude Law had been triggered, it seems from Andy Tracey's paper that stall protection is not provided.

Reading the above, it all sounds exceedingly speculative in the absence of data. I only post it to show that, after some thought, I share your present confusion.

PS
Thanks, A33Zab, I had no knowledge of the BEA's "Leak". Whoops...
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Old 18th Jun 2011, 01:53
  #136 (permalink)  
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A33Zab;

Thanks for your note re Abnormal Law...yes, I knew from the BEA 'leak' that the aircraft had not entered "Abnormal Law". In my note to Chris I was describing how the autotrim function would return once the aircraft was within the AoA limits for it's operation. Why it didn't go into Abnormal Law as a result of the high AoA does make sense to me, thanks.

To be sure, in all cases of all Laws, the THS is trimmable through the mechanical system unless jammed.

Please don't worry about your English...you're doing just fine.

Last edited by PJ2; 18th Jun 2011 at 17:01.
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Old 18th Jun 2011, 02:06
  #137 (permalink)  
 
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Turn rate during rapid descent

Forgive please, if this is already addressed...

During the descent - in a nearly level attitude - the aircraft turned either 90 to the right, or 270 to the left. Now then -

If the crew actually felt they were overspeed, and noted the fact they were changing direction, did they not feel the centrifugal force that would be created by that change in velocity vector?

With benefit of hindsight, this would seem to be a crude but effective means of differentiating between overspeed vs underspeed, when Pitot instruments are unreliable. Turning at anything approaching a standard rate, at or above cruise Mach, would glue one to his seat.

Last edited by barit1; 18th Jun 2011 at 02:17.
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Old 18th Jun 2011, 02:26
  #138 (permalink)  
 
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Hi,
A very humbly question: is the speed from Pitots corrected with AoA data?

If not, or if a 61° AoA is not take in compute (as "invalid"), the IAS from Pitots has to be: cos61 x square root (107˛ +107˛) = 0.48 x 151 = 72kts IMHO (61°, 107kts refering to post#100, first figure).

Last edited by Jetdriver; 18th Jun 2011 at 02:49.
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Old 18th Jun 2011, 02:43
  #139 (permalink)  
 
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Originally Posted by RR_NDB
Hi,
What kind of sensors you imagine in order to provide the data to the System? Some are yet available?
As syseng68k suggests, probably that simple strain gauge tranducers would do the job (on the mechanical link between the actuator and the control surface, and even on the control surface itself). All the strain signals (along with IMU signals, GPS signals, AoA signals, airspeed signals,...) could feed in real time an aerodymical model of the A/C (control surface+airframe) to derive these aerodynamical authorities and much more: in particular, it could also help to cross-check the integrity of all the incoming signals (navigation and attitude, AoA and airspeeds,...) in a more efficient/flexible way than it is currently done, and generate a reliable stall warning.
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Old 18th Jun 2011, 03:58
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Please forgive me for offering readers a warning on terminology here, from my experience of various jet transports with THS under one name or another. "Nose-up" is a term normally used to indicate the trim-effect on the aircraft, not the physical position of the THS. Because the THS is hinged at its aft spar, it's easy to think of "nose-up" as meaning that the leading-edge of the THS is up. Not so.

For maximum nose-up trim, a THS is at its fullest NEGATIVE incidence, with its leading-edge fully down. The A330's mechanical limit seems to be -14 (see PJ2/mm43 discussion). We call that "14 degrees nose-up" (14NU). For a typical take-off, the THS setting calculated on the trim-sheet by load-control might be 3NU. This requires the incidence of the THS to be -3 deg. However, load control and most pilots refer to it as "+3". Confusing, perhaps, but that's the way it is.
To further illustrate that good reminder from Chris Scott, here are two views from the left hand side of the THS :





Good eyes are needed, but a scale is visible just forward of the THS.
Both THS are at the GND SET position, which is also 4 deg UP, or minus 4. That’s the position where the THS is automatically reset after every landing.
The 0 (zero) position is also visible, with 2 units above and 14 below.

As the CG was pretty much forward for the AF447 takeoff, at 23%, the THS had probably been set initially at a position around 6 deg UP.


I ask again - why is it you're asking this of the BEA and no other accident investigation bureau/institution? Is one incident forever going to preclude acceptance of any finding of theirs that includes "pilot error" as a factor?
Also, you're a pilot, I'm an engineer - but I wouldn't trust myself to derive a correct conclusion from the raw data. What makes you so sure you could?
DozyWannabe, if tomorrow I don’t make it, I want my wife, children, best friends, to have a look TOO at MY data when my authorities will set their dirty move.

PS : Thanks for your friendly pm - I will answer or comment when time permits.


EDIT : Much better view on the scale on this image proposed by aguadalte
Punta Cana, Dominican Republic: Orbest Airbus A330-200 CS-TRA - detail of horizontal stabilizer - Punta Cana International Airport - PUJ / MDPC - photo by M.Torres - Travel-Images.com


Last edited by CONF iture; 20th Jun 2011 at 05:07.
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