A33Zab, #230

Thanks for the excellent diagram. Not quite clear on the following:
So what you are saying is that:

The system has a total of (4 x 3) 12 pots per pitch and roll axis, total 24

FCPC1 and FCSC1 each get pitch and roll, triple redundant, 12 pots (minus 2 ?)

FCPC2 and FCSC2 each get pitch and toll, triple redundant, 12 pots

Question: Why are two pots unused ?.

Looks like a clever design though, in that the redundancy even extends
to the mechanical linkages. In the unlikely event that one linkage
disconnects or breaks, the second linkage and potentiometer set would continue to
function.

Question: What is the function of the solenoid and does that lock the
stick at any point (zero ?) in it's travel ?.

Indeed, the "a/c" trajectory is never really a "perfect" "straight line". It is rather a series of segments of the type I've mentioned - "up", "down", "left", "right", "level" - with the length of these segments depending on the degree of air turbulence, and how fine the control of the "a/c" is.

In non turbulent air, the 4 "non-level" segments are very short, while the "level" segments are long, or very long, and predominant.

That is different in turbulent air: the length of the 4 "non-level" type segments is a lot more significant, while the length of the "level" segments a lot shorter, possibly down to "zero", with a predominance of the "non-level" segments, versus "level" segments.

That implies more drastic or significant actions of the "a/c controls", that react to the change and transition from one type of segment, to the next, to keep the "a/c level".

My understanding is that that would be the case if there is no, or little turbulence.

However in turbulent air, at an "a/p and a/thr disconnect", which can be coincidental with a change of law, and loss of certain protections, the "inertia" and the "a/c control surfaces at normal position" would keep the "a/c" level, ONLY and ONLY if the segment is "level".

Otherwise, as I understand it, if the segment is "non-level", there is a good chance/probability that "inertia" and "control surfaces" as left after the disconnect, and lack of protections, can bring the "a/c" way out of being "level" - "up", or "down", or "left" or "right". The degree of how off from "level" depends also on the time interval between the "automation disconnect" and the taking of the controls by the pilot, as well as a correct control correction coming from the pilot.

The probability of 20% and 80% I've referred to implies an equal distribution and length of the 5 types of segments, which is a stretch, for the sake of an easier explaining/understanding .

But it is a stretch in both directions!!!

Which means that for a very active turbulence, for the duration of that turbulence, the segments might be only "non-level", in which case in that time interval the "probability" goes from 80% to 100% - which makes it "a sure thing".

syseng68k, the SS probably did not fail. The PF was able to achieve ND type inputs on several occasions. And if the PF pulled the stick and got NU he'd no doubt announce it and turn control over to the PNF whose stick would presumably work.

And I'd be tempted to look into "springs" and strain gauges. (The sticks gums cites very probably were based on strain gauges since they had so little movement.) Strain gauges have nothing to get dirty, noisy, or erratic. So they're a little more reliable.

takata - post #235 - you accuse Bear of posting "such a load of cr*ap about a subject" and then do it yourself. I repeat - there is NOTHING in the BEA report to tell you that the pilots caused the climb. A 'nose up' input of unspecified size or duration does NOT prove that and may well have been of short duration - you do not know. As for your bit about "There is another way to change pitch attitude than pulling up on the sidestick as simply applying thrust could do the job if the amount is large enough." - you don't say? Now show me where the report tells you they increased thrust. You are making this up!

Now to TC-JDN - have another look at the trace. The a/c began pitching while the engines were throttled back, the side stick did not move but the THS did.

I note you come 'from Toulouse'.

Thanks for your clarifications...

Hm.... Perhaps I've missed this.... are the variables/parameters determining the values of the Stall Warning Threshold function and thus the shape of the curve valid, for the entire time (X) axle in the graph?

If NOT, then, "invalid", or "non-defined values" for the variables determining the values of the function/curve, means "invalid" or "non-defined function values", which means segments of curve which would be "invalid", or "non-defined". This means that the "non-defined" portions of the curve can be represented accurately only as a discontinuity - gap - on the non-contiguous curve.

Ultimately, it does not matter much, as these curves are just an illustration to help the understanding and communications of info on this thread.

Would the Throttle setting be cruise detent? Could the actual thrust of the engines be somewhat higher than "normal" due a/p increases to maintain straight and level? If so, at handoff, the Throttles would lock? An assumption could be that with high N1, a change in Pitch (up) might be aided by thrustline? (These are rephrasing of my earlier question, so if they seem repetitive, they pretty much are just that).

:p (no offense)

g load
 

hi everyone. could some one please explain the g loading on a aircraft in laymans terms. many thanks

Short explanation of g loading:

G is an expression of acceleration.

If you or I simply sit in our chair, we feel the normal 1 g load on our body. That is due to earth's gravity acting down toward the center of the earth.

If we are in a roller coaster car, and go racing down an incline, when we reach the bottom, and then race up the next incline, we feel more than 1 g acceleration as we go through that arc on the bottom of the track. (we feel pushed down into our seat a bit). This is like a pitch rate change in an aircraft, pitching the nose up.)

If we are in a plane flying along straight and level, we should feel 1 g.

If the plane goes into a 60 degree angle of bank turn, straight and level, the vector forces sum up to 2 g's. You'll feel that in your seat. (You'll feel a bit heavy).

If you are flying with the Red Arrows, or Blue Angels, or Snow Geese flight demonstration teams, you may do a high G turn over the field to show how maneuverable your jet is. You can induce g's up to 5, 6, 7 ... depends on which aircraft you fly. While doing such a maneuver, if you try to raise your arm, it will feel heavier to lift than when under 1 G. Under 5 g turn, your arm feels about 4 times more heavy than just sitting in your seat. (You can usually still lift it, but it feels strange to do so, much effort).

If you are in level flight, and you push the stick forward, you will tend to feel light in your seat: you are feeling < 1 g. If you fly the plane in a particular manner, beginning nose up, you can push over into a parabola shaped flight path that will induce a zero G condition, temporarily: if not strapped in, you can float a bit in the aircraft cabin. (Astronaut training used to include such events. Not sure if it still does).

If you choose to roll the aircraft inverted, and fly level, you will feel 1 G acting in the opposite direction to your sitting down: you will feel pushed OUT of your seat, not held into it. (Keep those harness straps on nice and tight). That G is typically referred to as negative G. It makes the blood rush to your head. The earth pulls on you the same, but your orientation made you experience that force differently.

A vigorous nose down push on the stick from level flight can induce negative G, and you will fall "up" toward the aircraft's cabin ceiling if you are not strapped in.

For much smaller changes, you can induce a 1.2 g or 1.1 g, or slightly less than 1.0 G (0.9, 0.8) via small pitch changes, as a result of control inputs that you choose.

G onset tends to be rate sensitive. If I make a very slow input, nose up or nose down, I induce a small acceleration, so the "g" of that maneuver, or the "change in G force" is small.

If I make a rapid input, the G tends to increase, and it is readily felt.

Does that help?

If you like a fuller article, this one is OK, more detail.
g-force - Wikipedia, the free encyclopedia

lonewolf 50
 

thanks very much for your time sir:ok:

As I said, in a given configuration SWT depends only on Mach. I have explained in a recent post how Mach etc. are calculated. The calculated speeds are the speeds that the airplane physically has in the simulation, and do not reflect any erroneous or 'invalid' speed that the systems may have 'thought' the airplane had. You have to consult the BEA Note for those. The SWT shown on the graph is valid for the 'simulation' Mach.

It is not entirely clear how the system calculates SWT in the event of erroneous or invalid airspeed. BEA#2 says that the system then sets it at the low speed value of 10 degrees. But several UAS incidents show that that does not occur. Maybe we just need to have better understanding of 'erroneous' and 'invalid'. For the NCD condition it doesn't matter where SWT is, or does it?

@A33Zab
 

Thank you for your posts #202 & 230.

I just love the way theories seem to gain momentum in the ether, only for them to be dashed by the cold FACTS.

pitch moments, gee and tranducers
 

I welcome the "fresh blood" to the fray. Attaboy.

- Although the pitch moment chart I posted is for another aircraft, it illustrates two points ( one positive, one negative) that result in little or no pitch control authority for the existing HS/elevators/stabilators in either positive or negative direction.

My main point presenting that chart was to counter all the folks that think a deep stall is only a concern for the T-tail designs. I also wanted to show the c.g. of the Viper, and point out that under some instances the 'bus has a very aft c.g. The reduced trim drag of the main wing is reduced if we can fly with a more neutral longitudinal stability. Simple, really, as the HS doesn't have to induce a nose up moment, and can basically "float". The big savings are the reduced AoA of the main wing and associated reduced induced drag.

In my case, we had a great cruise drag reduction, but we also had the HS and the wing contributing to lift when in a turning fight. Yeah, I can just see the 'bus in a turning fight with a Sopwith Camel, heh heh. Except for the inital HS movement, our HS (stabilators) usually was "limiting" the nose up tendency. So when we got to the deep stall scenario, out HS were commanded full nose down. Turned out there was a small AoA/c.g. range that allowed the jet to settle into a classic deep stall.

- The gee and roll rate are two physical phenomena that pilots can sense instantly ( as opposed to the drone pilots at Creech AFB and other places). We tried pitch rate for the longitudinal axis, but seemed to most pilots that the gee was easier to sense and control. Rates are still involved to prevent overshoots of pitch commands and to "smooth things out" for the SLF's. No big deal.

'bus takes into account actual pitch attitude, so at a 30 deg climb angle you get a 0.87 gee command, not one gee. With a constant one gee command you would continue to pitch up once a few degrees greater than level. Because we weren't worried about the SLF's, we normally let the basic one gee trim setting alone and just manhandled the sucker. But many pilots would trim for zero gee and simply hold back pressure. Would be willing to bet that the Thunderbird slot pilot trims for slightly less than one gee, and prolly the wingies.

The Viper concept was that we flew around in something close to the 'bus "direct law", but we could never command actual physical movement of the control surfaces. Only exceptions were rolling down the runway and if we wound up in a deep stall with AoA above 30 degrees. Even then, we had no rudder control, as HAL cut us off, heh heh.

All of our "sub-laws" were due to things like gear down while airborne, special setting for carrying external ordnance and "standby gains" in case we lost air data. Interesting, that last feature, ya think?

- Our stick employed piezo-electric transducers - 4 of them for pitch and roll, Rudder pedals moved one half an inch and were linear voltage tranducers. After a few jets, we had 1/8 inch of movement and some springs or whatever could be felt. Initial jets had zero stick movement - all pressures.

As with every fighter built from the mid-fifties, there was no feedback from the control surface pressures. Some jets had bellows to provide a "stiff" stick at high speeds. Some had bobweights to help pulling too many gees too quickly. But zero actual physical feedback. The feedback was from your butt and inner ear.

Hi Dozywannabe,

You keep telling me that and I've tried to explain it to you before.
Would it help if I called it "sense of longitudinal speed stability"? The aircraft has it naturally by design concept.

Normal Law removes it, but adds Alpha Prot & Alpha Max.
Alternate Law removes it and has no such "protection" or "Limit".
Direct Law would restore it. Ask any conventional aircraft pilot - they would understand it.

Well, this is quite authoritatively expressed... but also quite illogical as for addressing the fact that any pilot nose-up imput (whatever duration and amplitude) could not have caused this aircraft to climb (at whatever rate, as this is likely proportional).
Considering this high speed profile, manual flight and elevators sensibility, what would be the illogical part about me mentioning that?

The BEA is providing this information about a pilot imput in the same direction as the trajectory followed by the aircraft, what would be the logic to affirm : the climb wasn't related to this imput?
Obviously, you are taking the possibility (that I'm also sharing) that this full climb rate could not have been fully achieved by pilot imput alone by translating it into "nothing in the BEA is telling us that the pilot caused the climb".

Hence, you are rhetorically jumping into the breach of us lacking the full detailed picture of this imput as making an illogical point by saying that there is absolutely no causality between a pilot imput nose up and a following climb.

For my part, I'm saying that whatever else could have been added to the climb rate (turbulences, initial pitch rate at AP off, whatever else you'll like more...), this single imput "nose-up" was certainly part of it because, of course, the system would have taken it as a commanded +x g maneuver order, translated it into pitch attitude increase and, once achieved, would maintain the trajectory in climb (1 g).
What we'll need to know is the value of "x", the resulting maneuver g-load, adding/substracting possible turbulence impact, as to verify if something else was not a factor of the climb rate achieved... which is certainly what the BEA would have to acertain before mentioning, as a fact, that climb was entirely due to pilot imput.
Saying otherwise (it was unrelated) seems fallacious.

I'm just looking at the joined DFDR tracks of N1 % thrust and read that, when autothrust kicked off, N1 was already auto-throttled back at ~70%, then moved to 100% during the climb sequence.
You should know that thrust would freeze where it was (~70%) when autothrust disconnected; Hence, this increase was manually applied, even if not reported in the narative, which include an incoherence about "alpha-floor" kicking which is not available at such speed (inactive above Mach 0.53). This is based on AIB interpreted pilot confusion about "Alpha-lock" corrected by "Alpha-floor", while it certainly was "Thrust-lock" alarm (when ATHR disconnected).

Look by yourself if I'm making up this stuff:
http://takata1940.free.fr/Airprox340.jpg

The system will use every surface control and thrust to maintain 1g flight. Changes in THS trim were very limited during the whole sequence (much less than 0.5 deg), including a "zoom climb", "apogee", "descent"... while elevators moved in both sense with a much larger amplitude. Nonetheless, the system trimming THS seems to have not been "pitch up", but the other way as there is no such 5ND in the range of THS.

Right, I was there some time ago. I also note that you come from "Per Ardua ad Astraeu", but who cares?

syseng68k

No, it's just an insurance if something goes wrong......

@syseng68k:
 

No more computers to connect. (5 in total)


The solenoid is activated by the A/P and add's a force threshold in neutral stick to prevent any unwanted switching to manual control, while keeping
the possibility to override the A/P if required.

simple schematic of xdcrs:

http://i474.photobucket.com/albums/r...pitchInput.jpg

I've been asking a few people about it behind the scenes, but the answers I got back were too technical for me to comprehend easily (my brain has an annoying habit of shutting down when presented with algebra)... there's a pile of emails/PMs looking suspiciously at me as I type.

Let me see if I understand what you're getting at, and if I'm wrong please correct me. Prior to this I got the impression from your post that it related to the presence of autotrim - that is, when manually trimmed (with the THS at a fixed angle), the aircraft will tend to stabilise at a certain pitch angle and speed will remain more-or-less in the required ballpark. The presence of autotrim means that the THS is correcting itself based on sidestick input, and as such the trim angle is changing. In Normal Law, Alpha Prot and Alpha Max keep the trim settings within certain limits, but in Alt 2 those are not there, so there's nothing to stop the trim getting the aircraft into difficulty if the sidestick input is on the aggressive side.

Am I following you OK?

If I am, I think it's important to recognise that it takes a significant amount of input to cause the THS to get itself to such an extreme angle, and it's also important to recognise that autotrim has other benefits as well. It's pretty clear that Airbus FBW training at a basic level instructs pilots to be more careful with their sidestick inputs when law degradation has occurred. It's also worth bearing in mind that if you're in that situation and don't like what the trim is doing, all you have to do is set the manual trim as desired and avoid making large and consistent pitch inputs on the sidestick - this should cause the autotrim, when it kicks back in, to maintain a pitch angle at or near where you've just manually set it.

If I've read the "note" correctly, this isn't a case of a slight back-stick input in combination with (presumably corrective) roll input causing the THS to move to the 13deg nose-up angle in a matter of seconds, this is a case of the elevators causing the zoom climb in response to input, followed by the THS moving over the course of around a minute as repeated full or near-full backstick was held on the way down. From what I've read, THS movement from the autotrim is not particularly sensitive, nor is it likely to come into play with inadvertent sidestick deflection - the pilot *really* has to command it to be at that kind of angle for it to have got there, which is why the initial nose-down commands weren't enough to get the THS moving.

Hi DozyWannabe,

You are getting closer.

In Alternate Law, even with the stab trim wheel held stationary, longitudinal speed stability is not restored. The elevators will be held at a displaced position in order to satisfy the pitch stable law. (as in Loss of G & Y Hyds on A320, (or G & B I think on A330)). With UAS, the pilot would still not "feel" the aircraft getting slower and heavier in pitch.

Hi Rudderrudderrat,
This is not entirely true.
In Normal Law (pitch), some feedback is artificialy maintained.
In Alternate Law (pitch), same, but there is less gain and feedback.
In Alternate Law (general), Valpha_prot is changed by Vstall_warning (g sensitive) There are still speed limits and speed protections available... but, of course, only if the system still have some valid airspeed sources.
In Direct Law (pitch), feedback is direct as you said (but artificial, as electric).
In Back up Law (no power for computers), pitch is changed using the mechanical THS trim.

- er - my bit refers to 447, not TC-JDN..............................:confused:

ergo. Indeed, mine (quoted) refers to TC-JDN, not to 447...:O
(looked clear to me when I wrote it... if not, sorry about my poor syntaxe!)

Bearfoil: The THS will move to satisfy long term trim solutions PROVIDED the `g` demand is not greater than -.5g ( namely -.6 to -1g ) or High Speed Protection is active. Once away from these constraints then it should resume movement if necessary.
HOWEVER manual movement of the THS is always available to the pilot and its authority is very large.

Given that the sticks are not backdriven, they wouldn't feel it anyway, unless you're talking about in their bodies through the motion of the aircraft - in which case I'd say even if it could be felt, is it not preferable to use the ADI and thrust settings to make sure you're at a safe AoA?

With regards to inertia, I agree.
Regarding Control surfaces I tend to disagree.
As I already stated the AP disconneting will not so much be the factor here.
The AP makes requests to the FCPC's like a human pilot would do.
In normal law the AP will ask for a roll or pitch demand and the FCPC will provide the 'implementation'.
Therefore Control Surface deflection will depend on FCPC and not directly on AP.
If they had still Normal Law after AP disconnect this would have assured wings level.
It is the reversion to Roll Direct Law that will dictate what the Control Surfaces will do.
As in Direct Law the position of the control surfaces is directly proportional to stick position it is reasonable to conclude that in the given scenario the ailerons would have moved to neutral after change to Alt2.

Hi Henra,
Very well explained.
To illustrate this point, the schematic of the control system, (note that AP box channel is treated like a Pilot imput channel):
http://takata1940.free.fr/flightcontrols.jpg

And the AIB annexe to the report quoted above on A330 behavior in full auto flight facing turbulences. (There is no THS track, but the FCPC would not have time to adjust it anyway, as such pitch corrections are mostly left to elevators and autothrust).
http://takata1940.free.fr/Airprox330.jpg

Dozy,

Without judging what is better I would like to try and point out the difference:
With the THS at a given position there is a speed to which the aircraft will return by itself if you let go the yoke, even no matter what thrust you set.
If we assume that during cruise the trimmed speed is 270kts, and you command a Pitch Up, if you let go the yoke, the aircraft will lower the nose and finally settle again at 270kts, albeit at a slightly lower FL and after a couple of nice and accentuated phugoids.

You can only overcome this by constantly pulling the yoke or by deliberately changing the trim.

With any kind of Autotrim (be it AP or FBW) you do not have this constant point to which the AC will revert by itself. The Aircraft will follow a demanded flight path.
It needs clever logic (elaborated C*) in the FCS to deal with the fact that you could demand a flight path that could not be sustained by the AC.
This is done by blending over from 'g' demand to pitch demand. This makes sure the AC doesn't try to chase 1g at all costs.

Edit:
Caveat for any one who wants to bash Airbus Autotrim now: An AP mistrimming slowly and quietly and then disconnecting and handing back to the pilots can be equally bad or even worse as you cannot fix it just by applying ND on the Yoke, which would work on an AB as long as you have Autotrim.
It just has to be noted that trim is a very important aspect of flight in any aircraft and not clearly poinitng that out and training it is a clear mistake.

Hi DozyWannabe,

I'm still not quite there am I?
Er .... No.
- They don't have to be.

In Direct Law, the aircraft is stick free speed stable (just like a conventional aircraft). Provided I don't move the thrust or stab trim, it will continue on a trajectory at the same speed.

If I experienced UAS, all I have to do is leave it alone. If whilst attempting to hold the wings level, I inadvertently pulled back on the stick a little for a prolonged time, it would climb and slow down a little. If I relaxed on the stick, it would descend and regain speed again.

In order to get it to fly much slower, I'd have to pull back really hard or move the stab trim. That effect is what we call "elevator feel".

It is absent in ALt Law, and makes the pilot completely reliant on power and pitch. A human pilot, deprived of this sensory feed back, is reduced to an attitude clues processor - much like the autopilot that just gave up.

PS Please see JT's post #282, which explains it better.

If the AC would try to chase 1g at any cost, disregarding pilots maneuver demand, it would make the aircraft unable to change its trajectory in any circumstance, as each maneuver is an excursion out of 1g flight. (this is the part I'm clearly unable to understand about THS Fantasy Law theories).

I'll bet that any test pilot would have noticed this Airbus design flaw... at first attempt to fly it!

Thx Henra, I wasn't aware of that. In the case of AF447, when the pilot 'maintained nose-up inputs' after 2:10:51, and continued that '15 seconds later', reaching a pitch attitude of 16 degrees, and supposing he released the stick at that point, would the FCS then maintain 16 degrees pitch?

The last sentence regarding the rudder is a prime example of the FCOM portraying something in the BASIC PRINCIPLE that turns out not to be true.

Just a few pages back I noted someone suggesting that "plenty of rudder" could have helped the situation, and it is worthwhile repeating that the rudder has two forms of protection, i.e.

• Rudder Travel Limiter Unit (RTLU) which limits rudder travel between 35° at 150 KCAS and decreasing to 4° at 350 KCAS respectively,
• Pedal Travel Limiter Unit (PTLU) which provides input dampening.

On latching of ALT2 LAW the rudder was limited for the remainder of the flight to +/- 7.9°.

Not necessarily. I don't know the exact implementation of the C* law by Airbus. If it were a straightforward pitch rate law below a certain speed this would be the case but I'm sure it is not exactly implemented this way all the way to Stall speed.
Unfortunately I don't know how exactly they factor in AoA in the very low speed regime but it would not be too difficult to overlay AoA thresholds over a pitch rate law and I cannot see them not factoring this in.

Edit:
If someone has got access to a good sim the exact behaviour of the 'bus should be rather easy to find out. Put it in Alt2 in a constant climb at 5-10° NU pitch and wait what happens.
Take notes of Speed, AoA, Pitch, Trim while speed reduces close to the apogee.

Hi mm43,
It should be added to your sentence, "until the slats are extended"... but now, without valid airspeed, this is another question rising as there is also speed limitations to their extension.
Concerning the FCOM, this part is quite old, it could have been modified later. But you are certainly right that its litteral "meaning" is not exact (and curious in such a manual). They certainly meant something else like no rudder "active" protection is part of the flight envelope as for roll and pitch axis.

mm43

I think you are being a bit harsh.

It is quite clear that they are referring to the type of the protection mentioned. The FCOM statement is absolutely accurate. The rudder has no protection that will prevent excessive maneuvres or envelope exceedance. The travel limiting system only limits the travel and so depending on conditions, excessive manuevres/envelope exceedance are indeed entirely possible.

PJ2, #232
A lack of completeness is to be expected in training, as it can only
ever be an overview in any complex professional discipline. However, it
is also a given that further study is expected and that this and
experience fill in the gaps over the years. Continuous education and the
learning of new techniques is what makes some types of work interesting
in it's own right. I doubt if training is being skimped in any way that
could affect safety, but perhaps the omission of certain aspects tends
to instill a false sense of security in terms of system capabilities,
especially at the edges. If the automation handles everything perfectly
99.999% of the time, is it really necessary to get into more than the
sops and regs demand, I guess is the received wisdom.
Passion is the right word, but passion is not always appreciated,
perhaps even denigrated in this modern age and beancounters don't
understand it at all. I've seen this in industry for years: The
difference between those who love their work and those for whom it's
just a gig. Anything that smells of instinct, intuition or creativity
might be seen as "unconventional", yet those are just the sort of
qualities that intelligent individuals need outlet for and that are also
needed to get out of unexpected situations.
Safety has been improving over the years in any case, because of
advanced engineering techniques, more sophisticated design and
verification tools and improved infrastructure. The fact that fbw
techniques are used may not be a significant factor and imho, fbw can
only be for the good longterm. The problem is perhaps indirect, in the
culture that arises from it and it's promises.
Thanks for that. It just goes to show how complex the system is. I was
more interested in a state transition diagram / logic flow chart that
shows the input conditions, discreet and variable, required for
transition between the various laws. Gums posted a drawing for one of
his machines some time ago, but haven't been able to find it via forum
search. I don't see how anyone can analyse af447 without it, though I
bet the BEA have all this info and more...

Hi Takata,

Prior to the BEA Note, I had used the "until slats extension", but as you have suggested there were other issues in this descent.

busTRE,

I agree that I may be "pushing the boundaries" on this one, but Airbus had determined in their wisdom that the vertical stabilizer needed protection from rudder inputs, and the computers are programmed to provide that as per the graph. I accept that it is not directly "g" related but in all other respects is designed to protect the aircraft.

"Way off"... I'm not that sure (could be very few degrees on each axis)... Then a bit "off", certainly with light to moderately turbulent air (severe turbulence should have been notified by BEA). And that seems also the correct explanation (rather than fuel or CG issue) of this initial roll to the right at autopilot disconnection : rudder returned to neutral? (note = damping mode, no resistance to airflow) position, then possibly needed some correction and eventually retrimming.

As for AP/FD and ATHR corrective imputs preference during autoflight phase, it will also depend on Flight Director mode selected, that we don't know yet.
Diagram (not up to date) of the Flight Management Guidance and Envelope System (FMGS) :
http://takata1940.free.fr/FMGC.jpg

Hi gums,

It is nice to read that you are still with us.
I've got a question for you about pitch.
We know from BEA that AF447 had a MAC set at 29%, but also an optimum target that would be closer to 39% MAC, both being inside the safe certification limits. Now, people here tend to agree that, for an attempt to recover from a developed stall, having an aft CG would be a real issue.

Unless I'm mistaken, would not a further aft CG provide a better elevator authority (at least sensitivity)?
I seem to remember that moving the CG forward is a trade off for elevator sensitivity. Hence, in case of stall recovery, may be it is better to have more elevator control with a CG further aft, at least until the point that it is still inside the safe envelope.
What would you think preferable, in this case, 29% or 39% MAC?

takata,

Good question for Gums, it is what I was trying to understand when I posted this:

I just wasn't as clear as your post is.

Stepping in for gums (temporarily and with apologies):

Aft c.g.

Easier to stall
Easier to unstall (in theory) *

* Unfortunately though, a/c may well have departed - so you could be too late :eek:

Worth bearing in mind that the aft c.g. limit is not a completely black and white line, rather a grey area, especially with FBW and FCS messing with things. I guess that in the shorter term, everything is equiv to the old 'stick-fixed' stability, due to powered controls

: Would you call that stalling though? ;)