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AF 447 Thread No. 5

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AF 447 Thread No. 5

Old 16th Jul 2011, 01:29
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Originally Posted by Bearfoil
THS is large, powerful, and slow moving
I don't think it is that "slow moving". I remember a figure from 1°/sec. up to 3°/sec in elecrical control, like for rudder trim, but I don't know where it is buried nor how much is the rate in manual control. Also, in this mode (back up), it could have a much higher rate than the one needed for precision trimming.

And again, I'm feeling now like the one having feed the (sleeping) Troll. You deliberatly use whatever is provided to make many assumptions based on nothing but your self ignorance. Your first assumption should be to give some credit to the engineers who designed those aircraft. You can be sure that there was a lot of thinking put in every design choice even if you don't quite understand the end result. Hence, study them in depth and, after that, come back with some constructive critisism about something you'll have really studied.

Last edited by takata; 16th Jul 2011 at 01:46. Reason: Added thinking
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Old 16th Jul 2011, 01:34
  #362 (permalink)  
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Hi takata. No problem. In 447's case, it was not THS authority that was desired, it was the desire (seemingly unaddressed, or ignored) to defeat it, to zero it. The Elevators were fine, yes? GY points out DIRECT LAW was not in. If TRIMMING can get the a/c into trouble, (fbw, or not ), it should have a Zero channel, no?

This is what I need to know. It is said the THS follows the Pilot's ss input.
This isn't emergency; on the way up, it was doing NORMAL LAW stuff? Given the dire nature of the a/c assiete at top of climb, How could the lack of Power to the THS be the reason it stayed at ~13 NU? It appears at this point, that the climb was a problem (!), independent of conjecture re: the accident, With a need for STALL recovery, and the THS limit of 2degrees ND, no matter the reason, how did she abandon the THS so NU?

This is a big question for Pilotless flight, notwithstanding whatever 447' PF did or did not do?

Thanks again for your pics, and wow. (words of wisdom)

edit re: upper post. Manual Trim was required, isn't that .65 degree per "hand sweep"? Also, in active dynamic flight , (UA recovery), power and rate are not desired coupled together? Q/N, (Quick and Nasty?) Cruise flight wants those tender touch, Yes?
 
Old 16th Jul 2011, 02:00
  #363 (permalink)  
 
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Originally Posted by Bearfoil
Hi takata. No problem. In 447's case, it was not THS authority that was desired, it was the desire (seemingly unaddressed, or ignored) to defeat it, to zero it. The Elevators were fine, yes?
Of course yes.
What we are talking about is that the aircraft seems to have done, so far, nothing wrong by itself. The THS was trimmed here and left here. The obvious reason is that there was no real attempt to pitch down with elevators. Hence THS remained where it has been ordered to be and helped to maintain this aircraft nose-up, certainly added to further NU elevators imputs by pilots...

Then, everything is pointing at the pilots not understanding the situation at all: that they were stalled, and that they will crash without pitching down the aircraft. Consequently, what happened to them? What caused them to believe something else happened and lead them to make those imputs?
Maybe the full transcript will tell us more about that. What was their strategy? We can't recognise any procedure applied from their acts alone, be it UAS, stall recovery... or whatever else.
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Old 16th Jul 2011, 02:23
  #364 (permalink)  
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I must agree. The only hesitancy I have is I am nonplussed the crew would apparently leave out the THS in the recovery attempt.

And also that I remain convinced the actual problem began perhaps many seconds prior to a/p loss, and became critical alomost immediately. Alas, further data from BEA will be most welcome.
 
Old 16th Jul 2011, 03:40
  #365 (permalink)  
 
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Puzzled by puzzlement

takata
Originally Posted by Bearfoil
…. In 447's case, it was not THS authority that was desired, it was the desire (seemingly unaddressed, or ignored) to defeat it, to zero it. ….
bearfoil
… The only hesitancy I have is I am nonplussed the crew would apparently leave out the THS in the recovery attempt. …
As a distant observer listening to all you experts being puzzled by the non-use of manual trim, I am in turn puzzled by your puzzlement.

We know that the training departments of the airlines had strenuously opposed any mention of the use of manual trim in training for upset recovery, even though the test pilots of both Boeing and Airbus had emphasized more than a decade ago that bringing the a/c into trim was in their opinion the first priority in a recovery.

So I assume (I don’t know) that if the crew had had upset recovery training (as distinct from training to recover from an approach to a stall) the use of manual trim would not have even been mentioned, and they may even had had warnings against its use (because of the danger of structural damage).

So why are you puzzled by the non-use of trim? They followed their training, which in part told them to forget what they were told in primary training, particularly WRT stalls. They were never trained to use manual trim. It wasn’t even mentioned.

QUESTION: Being a French crew for a French airline and knowing that any deviation from training and SOP might be investigated by a French Court, might they have been inhibited from deviating from training and/or SOP in case they might be blamed for any subsequent damage?

The crew were probably unaware of the characteristics of a deep stall and had never experienced one. One of Gums’ early posts mentioned a deep stall as being “just like cruise flight”. So while on this subject, how many of you experts are happy with the recent upset recovery screed which lists as one of the indications of a stall as “Buffeting”? While this is true, there is still no mention of the possibility of a buffetless stable stall. Are you all happy with this omission?

An unrelated QUESTION (my apologies if this was already been answered 2,000+ posts back):
From where did the BEA get their 107 kts ground speed on impact with the sea?
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Old 16th Jul 2011, 04:07
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Hi PickyPerkings,
Originally Posted by PickyPerkings
As a distant observer listening to all you experts being puzzled by the non-use of manual trim, I am in turn puzzled by your puzzlement.
Hopefully, you did not quoted me (it was Bearfoil's) as I'm neither an "expert", neither puzzled about the non-use of manual trim!!!
Nonetheless, you seem to be the one highly suffering from tunnel vision about a "manual" THS non-existant issue (related to AF447).
Quite simply, if you don't pitch down, autotrim won't certainly trim nose down, no? Then, why would you pitch up your nose and manually trim down your THS? Where would be the logic and what is the real issue?
Ask yourself!
Then maybe you'll understand why either "manual" nor "autotrim" are a NON issue from what is published so far from the investigation.

After this point, your comments seems a bit moot.
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Old 16th Jul 2011, 04:09
  #367 (permalink)  
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Not an expert. Also, far away. For me, TRIM is a verb. It implies through common usage the implementation of a small aerodynamic device to position a control surface in the airstream such that no continued force is necessary to keep it in place. It relieves the operator of a consistent and irritating need for pressure to be applied to the controls. As such it implies an adjustment in control pressure so that an attitude may be extended for a length of time. As used in large and flyable Horizontal surfaces, it takes on a somewhat different meaning. Rather than freezing a small device in place to relieve the pressure needed to maintain an attitude, THS essentially moves the whole kit and changes the "Angle of Incidence" of the Wings themselves.

So a THS is not strictly speaking a transitory attempt to relieve Pressure on the Controls, it changes the aspect of the airframe itself. It is in essence the 'Elevator'. It makes 'permanent' a flight attitude commanded by the FCS. It 'follows' elevator input, giving them a great deal more authority in Pitch.

So see, I am not an expert, as my clumsy explanation proves.
 
Old 16th Jul 2011, 04:48
  #368 (permalink)  
 
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Originally Posted by PickyPerkins
They were never trained to use manual trim. It wasn’t even mentioned.
(Sorry for the typos above about your name)
Did you find any detailed informations about this crew training?
Just have a look at the the two or three pages above and you'll see that manual trim use is a mandatory training as for being able to fly this aircraft in particular flight control modes. Did they ever had the real oportunity to experience it? Who know, but I really doubt it. Nonetheless, they were all certified pilots for flying this aircraft in all configurations.

As for the manual, there was a very short part about stall at cruise :
Originally Posted by FCOM 3.04.27
The aural stall warning may also sound at high altitude, where it warms that the aircraft is approaching the angle of attack for the onset of buffet. To recover, the pilot must relax the back pressure on the sidestick and reduce bank angle, if necessary. When the stall warning stops, the pilot can increase back pressure again, if necessary, tu return to the planned trajectory.
So, how would you interpret "relax back pressure on the sidestick"?
Would not one use the trim wheel?
Originally Posted by PickyPerkins
From where did the BEA get their 107 kts ground speed on impact with the sea?
Don't they have retrieved the DFDR? It should have recorded the acceleration at impact (assuming all those accelerometers). Hence, a basic function will give the ground speed at impact, if not provided directly by the GPS.
PS: onset of buffet... buffet may be expected close to stall speed, but gums was also certainly talking about a completely developped stall, where 1 g stall is fully achieved, that may be quiet in such a large and basically very stable aircraft.

Last edited by takata; 16th Jul 2011 at 05:47.
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Old 16th Jul 2011, 06:15
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So, how would you interpret "relax back pressure on the sidestick"?
Would not one use the trim wheel?
If you are getting a stall warning, it is because you are maneuvering (applying g).

Relax back pressure means do not pull so hard on the stick-period.

Using the trim wheel would have you trimming into a stall. Not a good idea.
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Old 16th Jul 2011, 08:17
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Hi Machinbird,
Originally Posted by Machinbird
If you are getting a stall warning, it is because you are maneuvering (applying g). Relax back pressure means do not pull so hard on the stick-period.
What you say makes perfect sense. I interpreted that for cruise level flight while being trimmed a bit too high. Stall warning could trigger in turbulence (basically the same effect as maneuvering). It's certainly due to tunnel AF447 thinking about its first stall warning that I'm trying to explain. I don't know also how the EFCS would render this trim feedback with neutral sidestick (hence it was stupid).
Originally Posted by Machinbird
Using the trim wheel would have you trimming into a stall. Not a good idea
I was certainly thinking about opting for the other way!

Last edited by takata; 16th Jul 2011 at 08:40.
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Old 16th Jul 2011, 11:15
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Originally Posted by gums #122
The modern commercial heavies have really great aero characteristics that can make a stall insidious. And the stall is not like the Airbus manuals depict on the lift versus AoA curve. There is no sharp break in the curve at "x" AoA. It's a very gentle curve and one can fly at fairly extreme AoA's without the sharp pitch excursion we all saw when checking out in a Chipmunk or Cessna or T-28 or......
Gums' point is illustrated in this graph: cLalphaM06.gif

It should be noted that only the blue data points are 'fact'. The extrapolation of the trend line beyond the last datapoint should be treated as a 'guess'.
Originally Posted by takata
PS: onset of buffet... buffet may be expected close to stall speed, but gums was also certainly talking about a completely developped stall, where 1 g stall is fully achieved, that may be quiet in such a large and basically very stable aircraft.
You will note in the graph that there is a substantial margin between the onset of buffet (alpha-max) and the 1 g stall. BEA says in its interim report no.2 that the stall is identified by 'vibrations' which is french for buffet, and I think they mean heavy buffet.
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Old 16th Jul 2011, 12:04
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Hi takata,
What we are talking about is that the aircraft seems to have done, so far, nothing wrong by itself.
I agree it seems to have performed as designed.

One has to wonder at the wisdom of starting with an aircraft concept which is naturally longitudinally speed stable, and design ALT LAW handling characteristics which allow the aircraft to be flown (with UAS), to stalling Alpha and beyond in a trimmed condition.

Last edited by rudderrudderrat; 16th Jul 2011 at 12:39. Reason: underline "nothing"
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Old 16th Jul 2011, 12:43
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Originally Posted by gums
- The stabilators, or horizontal stabilizers/elevators, moved +/- 21 degrees.
- They had a symmetrical airfoil, and...
- They were interchangeable from one side to the other
- They moved independently in order to help with roll, especially when supersonic
gums, many thanks for the data.
I am aware that the Viper has a delta wing and strakes that generate powerful vortices at high AoA. The stabilators also form a delta-shaped plan (but without the strakes). Such wings tend to “stall” rather smoothly AFAIK, meaning there is no sharp drop in lift as AoA increases past Clmax AoA, thanks in part to vortex lift.

I tried to understand why the stabilators efficiency (measured as the distance between the neutral and full deflections curves of pitch moment vs AoA) starts decreasing at a lower AoA for ND than for NU deflection. I surmised this had to do with the downwash of the wing and since the stab airfoils are symmetric it is possible to evaluate the downwash and stabilators local Clmax AoA. This gives 21 deg and 15 deg respectively. I was puzzled by a 21 deg downwash angle until I remembered that the FCS adapts the wing curvature at high AoA, drooping slats and flaperons.

Of course, we miss pitch moment curves for the A330 to understand what ND or NU inputs could do. Owain Glyndwr posted a very interesting explanation but I am not convinced the THS was not stalled at 60 deg AoA. We know that the elevator was still working from BEA’s note but perhaps this was due to the elevator presenting a larger frontal area to the airflow in ND position than in NU. The effect of a stalled THS would be a positive slope of the Cm curve in the stalled area, meaning a decreasing downward moment with increasing AoA.

Another point where I doubt Owain’s conclusions is the estimation that, at high AoA, the mean aerodynamic center will move longitudinally to the wing center of area, hence behind the CG. Looking at the Viper curves that move apparently occurs at about 60 deg AoA when the curves turn south. Owain considered the wing only whilst he should consider the wing/body assembly. Adding the fuselage will generally move the mean aerodynamic center forward so I think it is unclear whether the wing/body adds up to or opposes the ND moment of the tail at high AoA.

The stall was very probably recoverable but imho it would take much more time and obstinate ND input to recover than the crew expected.
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Old 16th Jul 2011, 13:55
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Yes DJ77, I wonder whether this 'lack of persistence' with ND inputs, was because no time in training (since ab initio) seems to have been allocated to 'stall recovery' - whereas 'stall avoidance' seems to have taken the allocated 'stall' training time... Without this recovery emphasis, few other than long time (old hands) would have any appreciation of what it takes to rotate such a beast back to a sensible AoA + time to accelerate back to a sensible speed.


Originally Posted by rudderrudderrat
One has to wonder at the wisdom of starting with an aircraft concept which is naturally longitudinally speed stable, and design ALT LAW handling characteristics which allow the aircraft to be flown (with UAS), to stalling Alpha and beyond in a trimmed condition.
That's I think what I was trying to get at a few posts back... I just wish I had the wisdom to chose those words - nicely put rrr

Last edited by Jetdriver; 16th Jul 2011 at 14:18.
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Old 16th Jul 2011, 14:19
  #375 (permalink)  
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DJ77

Have you addressed the THS' AoA as an independent entity from the a/c?

What was 447's trimmed state nearing the top of climb? I ask because at cruise, the Tail would be loaded and would want some (minor?) value of a/cND to create its lift (which is subtracted from its cruise duty AoA?)

Assuming an input of steady ND, (after the initial NURL), would there be any chance of the Tailplane Stalling and dropping the tail before the Mainplane Stalled? Given slow speed and gentle (?) entry, would lack of buffet and a rapid NU cause alarm in the cockpit? Would the a/c have assumed a very high NU communicating an alarming (seat of the pants) reaction from PF to input max ND? Did the a/c "recover" (actually not a recovery, but a predictable "fall" off). This would set the stage for the PF's return to NU inputs for the duration? For that matter, What was 447's trimmed state at Handoff? One way to enter a remarkable climb is to experience a lack of effectiveness from the tailPlane? The initial SW, the PF NU, the roc, things went South quickly.

If of no value, disregard.........

Last edited by bearfoil; 16th Jul 2011 at 14:36.
 
Old 16th Jul 2011, 15:37
  #376 (permalink)  
 
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Originally Posted by rudderrudderrat
One has to wonder at the wisdom of starting with an aircraft concept which is naturally longitudinally speed stable, and design ALT LAW handling characteristics which allow the aircraft to be flown (with UAS), to stalling Alpha and beyond in a trimmed condition.
To be fair, it would appear from the evidence we have to hand so far that it requires actions that would be pretty counter-intuitive from a trained pilot's perspective to do so though.
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Old 16th Jul 2011, 16:30
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Hi DozyWannabe,

I don’t think that’s a fair comment.

From PJ2’s post #282 "The Effect of High Altitude and Center of Gravity on Handling Characteristics of Swept-wing commercial airplanes" -
“For a statically stable airplane the required column force, as speed varies from the trimmed condition, is less at an aft CG than it is at a forward CG.The minimum average gradient allowed by U.S. Federal Aviation Administration FAR Part 25 is one pound for each six knots.”

If at FL 350 with UAS, the PF had pulled back with several pounds of force on the controls, and then manually selected several degrees of nose up stab trim – then I would agree with you and call that counter-intuitive.

I suspect that all he did was inadvertently pull back on the stick with a few ounces of force for a prolonged period of time (maybe the next report will clarify). The stall recovery action with the application of TOGA power was the start of the next chain of errors. (N.B, his action was i.a.w. the guidance in FCOM at the time – the new stall warning recovery procedure has removed that action because of the nose up couple it causes)

Last edited by rudderrudderrat; 16th Jul 2011 at 16:46. Reason: shorten text
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Old 16th Jul 2011, 16:40
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Bear,
Have you addressed the THS' AoA as an independent entity from the a/c?
AoA_tail = AoA_wing/body - downwash angle - THS trim angle.
What was 447's trimmed state nearing the top of climb?
No BEA info but close to 3 deg NU I think.
... would there be any chance of the Tailplane Stalling and dropping the tail before the Mainplane Stalled?
No.
As most people here, I cannot understand the PF insisting on NU inputs especially after the zoom climb. To me, it's akin to pilot incapacitation.
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Old 16th Jul 2011, 17:59
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Of course, we miss pitch moment curves for the A330 to understand what ND or NU inputs could do. Owain Glyndwr posted a very interesting explanation but I am not convinced the THS was not stalled at 60 deg AoA. We know that the elevator was still working from BEA’s note but perhaps this was due to the elevator presenting a larger frontal area to the airflow in ND position than in NU. The effect of a stalled THS would be a positive slope of the Cm curve in the stalled area, meaning a decreasing downward moment with increasing AoA.
My point was that to maintain 60 deg AoA you need a lot of UP elevator, which takes the THS away from stall. Gums curves and the curves in that NASA report on civil aircraft upset recovery (cited in thread #4 IIRC) show that the THS can stall if DOWN elevator is applied at that sort of AoA but there is no sign of stall with up elevator in either case. To hold 60 deg AoA you need a DOWNLOAD on the THS. What you describe is a stall with a POSITIVE THS load. What I meant was that ND moment can be obtained by reducing the up elevator just as well as applying down elevator - just a different starting point.

Another point where I doubt Owain’s conclusions is the estimation that, at high AoA, the mean aerodynamic center will move longitudinally to the wing center of area, hence behind the CG. Looking at the Viper curves that move apparently occurs at about 60 deg AoA when the curves turn south
As you note, the Viper is a delta wing, and the centre of pressure will be greatly affected by the strong LE vortices a la Concorde. Quite different flow patterns to a high aspect ratio swept wing.

Owain considered the wing only whilst he should consider the wing/body assembly. Adding the fuselage will generally move the mean aerodynamic center forward so I think it is unclear whether the wing/body adds up to or opposes the ND moment of the tail at high AoA.
Not that naive DJ - I did consider the wing body assembly - see my post of 12th July- and the net effect is the way I described it.

Last edited by Owain Glyndwr; 16th Jul 2011 at 18:23. Reason: change of original author
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Old 16th Jul 2011, 18:34
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THS Rate:

I’ve got these figures of THS rate.

The THS itself (the ballscrew) is driven by 2 hydraulic (B & Y) motors.

Max. Operating Load and Max. Speed (2 motors)

18940 daN (42578.81 lbf) ----- 0.4°/s
16950 daN (38105.11 lbf) ----- 1.0°/s
6770 daN (15219.56 lbf) ---- 1.2°/s

Limit load (both compression as tension) is 32500 daN (73062.9 lbf).
I assume this limit load is mentioned as being the limit for the Hydr.
motors to drive the THS.

Half speed for 1 motor (1 hydr. B or Y system failure) operation.

The rate for the BEA mentioned 1 minute to go from 3 to 13 ANU was 10°/60 = 0.16°/s.


For going back to the 3° ANU position:
The hinges are on the trailing edge and therefore the aerodynamic
load on the drive spindle is in the AND direction it would take 12 sec. to travel from 13° back to 3° ANU.

The manual trim wheel THS displacement is ~0,65° a stroke, if one need
1s to complete the stroke and 1s to re-grab the wheel, the rate will be 0,325°/s.

--

For the PFD messages "MAN PITCH TRIM ONLY":


ELEVATORS NORMAL OPERATION
Each elevator servocontrol is connected to two computers (one FCPC
and one FCSC).
In the normal configuration, the inboard servocontrol is in active mode
while the outboard is in damping mode.
FCPC 1 having the servo-loop control priority:
- sets its dedicated servocontrol in active mode and ensures the servoloop
control,
- commands the damping mode on the adjacent servocontrol (one solenoid
valve (S) energized).
For the elevator servolooping computation the computers need to acquire:
- the elevator surface position,
- the elevator servocontrol piston position.
This information is sent by servocontrol transducers (XDCRs) units and
the surface position transducer (RVDT).
In the event of large deflection demands, the two servo-controls can
become active to avoid the saturation of one servocontrol.

ELEVATORS ABNORMAL OPERATIONS

HYDRAULIC OR ELECTRICAL FAILURE
If a servocontrol being in active mode is either not hydraulically powered
or not electrically controlled anymore,
the faulty servocontrol falls in damping mode and the adjacent one
becomes active according the servoloop reconfiguration.
If both servocontrols of one elevator are depressurized (e.g. Hydraulic failure)
both servocontrols are in damping mode which prevents fluttering.
When P1, P2, S1 and S2 are no longer able to control their dedicated
servocontrol (ie: inputs missing, electrical failure, etc...), the servocontrols
fall in re-centering mode.
The elevator becomes a 'fixed' part of the THS. (=MAN PITCH TRIM ONLY)

When autotrim in not available (e.g. Direct Law, Flare Law in case RA is
unavailable or NAV IR DISAGREE the PFD message "USE MAN PITCH TRIM" is shown.
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