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AF 447 Thread No. 5

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AF 447 Thread No. 5

Old 13th Jul 2011, 20:18
  #261 (permalink)  
 
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Originally Posted by BOAC
- er - my bit refers to 447, not TC-JDN..............................
ergo. Indeed, mine (quoted) refers to TC-JDN, not to 447...
(looked clear to me when I wrote it... if not, sorry about my poor syntaxe!)
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Old 13th Jul 2011, 20:21
  #262 (permalink)  
 
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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.
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Old 13th Jul 2011, 20:33
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Originally Posted by rudderrudderrat
With UAS, the pilot would still not "feel" the aircraft getting slower and heavier in pitch.
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?
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Old 13th Jul 2011, 20:38
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Originally Posted by airtren
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".
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.
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Old 13th Jul 2011, 20:52
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Hi Henra,
Originally Posted by Henra
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 on AP.
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):


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).
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Old 13th Jul 2011, 20:56
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Originally Posted by DozyWannabe
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?

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.

Last edited by henra; 13th Jul 2011 at 21:07.
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Old 13th Jul 2011, 21:05
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Hi DozyWannabe,

I'm still not quite there am I?
unless you're talking about in their bodies through the motion of the aircraft
Er .... No.
Given that the sticks are not backdriven
- 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.

Last edited by rudderrudderrat; 14th Jul 2011 at 10:56.
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Old 13th Jul 2011, 21:21
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Originally Posted by Henra
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.
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!
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Old 13th Jul 2011, 21:35
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Originally Posted by henra
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.
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?
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Old 13th Jul 2011, 22:03
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However, when in normal law, regardless of the pilot's input, the computers will prevent excessive maneuvers and exceedance of the safe envelope in pitch and roll axis.
However, as on conventional aircraft, the rudder has no such protection.
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°.

Last edited by mm43; 13th Jul 2011 at 22:57. Reason: corrected highspeed KCAS
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Old 13th Jul 2011, 22:12
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Originally Posted by HazelNuts39
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?
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.

Last edited by henra; 13th Jul 2011 at 22:22.
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Old 13th Jul 2011, 22:14
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Hi mm43,
Originally Posted by mm43
On latching of ALT2 LAW the rudder was limited for the remainder of the flight to +/- 7.9°.
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.
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Old 13th Jul 2011, 22:27
  #273 (permalink)  
 
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mm43

I think you are being a bit harsh.

However, when in normal law, regardless of the pilot's input, the computers will prevent excessive maneuvers and exceedance of the safe envelope in pitch and roll axis.
However, as on conventional aircraft, the rudder has no such protection.
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.
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Old 13th Jul 2011, 22:42
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PJ2, #232
These are not taught very thoroughly in initial ground schools at least
in my experience, and if one wasn't in the air force one's knowledge was
increased largely through one's own efforts.
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.
The frustrations of not knowing and not being able to find things out
easily have been endemic - while easy to fly and a joy to hand-fly,
"automation" has become as much a marketing tool as it has a way of
solving the problems of flight. The resistance to knowing more than the
"NTK". need-to-know, ground-school curriculum requires, comes first from
how expensive it is to train well, and next from a lack of knowledge in
those who must do the teaching, always of course, with wonderful
exceptions from those memorable instructors who's passion takes them,
and their students beyond NTK.
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.
This isn't "THE" problem, because clearly the aircraft and the design's
record is no worse in terms of fatal accident rates than conventional
types - in short, the airplane and the design work extremely well but
one should never be in a position to not understand and not anticipate
what his airplane is doing.
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.
Below is a nuts-and-bolts schematic of the pitch-basic loop. I hope it
is of some service in understanding the pitch control of the A330
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...
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Old 13th Jul 2011, 22:46
  #275 (permalink)  
 
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Henra:
Thanks for your post, and to Takata's. The latter provides a good illustration of some of the elements I was referring to...

Glad to read you're in agreement on "inertia".

Two things regarding the disagreement on control surfaces, that may help bringing you in agreement:

1. You’ve quoted selectively from my paragraph, omitting the last sentence (marked in blue):
Originally Posted by airtren
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.
That last sentence is important, as it has put an emphasis on the time window/interval between the AP & A/THR disconnect, and the time the Pilot is taking control and providing an input control/correction through the Stick - which is the time in which there is NO input (gap in input) to the F/CTL. The two are in the Takata's post's drawing, the two Input sources into the F/CTL Computer.

2. Your reference to Normal Law behavior is different from the case I refer to, and may indicate your missing of my reference to “lack of protections” (marked in blue) referenced as in pertinent protections, as a case in which the system is NOT in NORMAL LAW.

Note: Regarding the Takata's post text/excerpt of "...when in normal law, regardless of the pilot's (or AP) input, the computers will prevent excessive maneuvers and exceedance of the safe envelope in pitch and roll axis." I seem to be more conservative on the interpretation of the "prevent... exceedance of the safe envelope....", in regards to the extent of the creation of orders by the F/CTL Computers if no input is present from the AP or Stick..

That being said, during that time window mentioned above, and other than Normal Law, the control surfaces are left in a neutral/static position, and thus (a) they may be neutral, (b)they may reduce, or (c) they MAY AMPLIFY the effect of air flow change due to strong/heavy turbulence during, before, and after that window.

That effect may be NULL (a), may counter (b), or may compound (add to) (c) the “inertia” in getting the “a/c” way off the “level” flight.
Originally Posted by henra
Originally Posted by airtren
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".

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.
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Old 13th Jul 2011, 22:46
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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.
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Old 13th Jul 2011, 23:43
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Originally Posted by airtren
That effect may be NULL (a), may counter (b), or may compound (add to) (c) the “inertia” in getting the “a/c” way off the “level” flight.
"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) :

Last edited by takata; 13th Jul 2011 at 23:54.
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Old 14th Jul 2011, 00:19
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Hi gums,

It is nice to read that you are still with us.
Originally Posted by gums
- 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.
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?

Last edited by takata; 14th Jul 2011 at 00:34.
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Old 14th Jul 2011, 01:21
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takata,

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

However, in the BEA Update, dated May 27, 2011, The weight of the aircraft was again reported at around 205 t, but the balance was changed to 29%, or in other words moved forward 8% or so. I thought the aft balance (37.3% - 37.8%) reduced drag and improved overall efficiency.
So my questions are: How did this happen? Why did this change happen? Does this have any effect on maintaining pitch either by the automatics or in a manual fly mode?
I just wasn't as clear as your post is.
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Old 14th Jul 2011, 01:31
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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

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

:
Originally Posted by Anecdote
Aft c.g. (at or behind the neutral point) was encountered on heavily fuelled early mark PR Spits Usually just during the early initial climbout while rear fuel burnt off.. I think one or two were lost due to pitch osciallation and departures, fighters being pretty neutral anyway..

Occasionally NA P51s with certain types of arming & fuel loading got pretty close to neutral I believe, certainly overcontrol was responsible for pulling the wings off a few... albeit like as not in combat.
Would you call that stalling though?

Last edited by HarryMann; 14th Jul 2011 at 01:53.
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