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AF447 Thread No. 3

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AF447 Thread No. 3

Old 6th Jun 2011, 19:32
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The computers seemed to be experiencing wild fluctuations in speeds. When the speeds became very low, the computers marked them as unreliable. But they were consistent. Surely icing has the opposite effect - readings would be very slow moving, and inconsistent (- why would two sensors fail simultaneously) ?
I believe it depends on the mode of icing - if the drain-holes did NOT become blocked and the forward opening DID, with the static ports unblocked, then the pressure within the pitot drops toward static, leading to a low speed (ultimately zero) reading. Given that pitot heating was ON, then icing of the leading port ONLY is the most likely scenario. Particularly given the fact that the speeds appeared to recover and read correctly a little later in the sequence (assumption being that heating led to the ice melting...).

Given that you note you have NOT read the entire thread (which is a task, but really the only way to avoid rehashing half-baked ideas), I'm loath to state anything more than you are quite a long way from the current line of thinking. There is little contention that the pitots iced, leading to AP and AT disconnect (per design) with the crew taking manual control in ALT law... it was uphill from there out.

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Old 6th Jun 2011, 19:35
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PJ2
Quote:
Originally Posted by MurphyWasRight #1476
If I understand correctly the Airbus sidesticks do not provide any feedback, they are "input only".
The following schematic illustrates why this understanding (and many understandings here) is not true:
Thanks for the chart, it shows that there is feedback between sidestick position and force that is "local" but that does not provide tactile feedback to the pilot regarding the requested inputs affect on the aircraft.

In the "old school" derived from original direct cable controls (later emulated via artificial feel) the same chart would have opposing forces generated by "artificial feel", harder to push if speed higher or whatever.

I don't have an informed opinion on whether this lack of artificial feedback is a "good thing" or not, just wanting to be clear, in my own mind at least, on how it works.

Despite the lack of "informed opinion" I will guess that feedback might not be that helpfull since the relatively limited range of motion makes this as much as "force" sensor as position. That combined with different meaning of inputs across the laws would create yet another layer of possible confusion.
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Old 6th Jun 2011, 19:44
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I cannot find the post of PJ2 from above in which he mentions a schematic and neither can the search facility.
I believe that with fbw we have created two classes of pilots.We have the old school who are probably better polers and have been brought up on tactile feedback through feel,and sound of their aircraft and those who have only flown an electric jet where the aircraft has never let them down.The latter have flown in alternate and direct laws only in the simulator and trust the automation implicitly because normally it always works.They are used to a low workload and probably the odd single failure not the multiple that would have been introduced by unreliable airspeed..
The former do not like the fbw as it does not fly like a proper aircraft,the tactile clues such as engine noise,pitch changes with large power changes ,the nose dropping in turns unless you put in a little nose up pitch are all missing.The thing they hate most is that you do not have a direct stick to control surface relationship and you cannot feel your colleagues inputs through the sidestick..who is the better pilot and who is better at dealing with unusual situations?
I believe that Airbus decided that the weak link is the pilot and they do not want you flying the aircraft.They also want to make it easy to use low experience pilots in their aircraft but this is fine until some real stick and rudder skills are required.What happens then if you never learned these skills?Perhaps the lessons of AF 447 will show us.

Last edited by tubby linton; 6th Jun 2011 at 19:58.
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Old 6th Jun 2011, 19:45
  #1484 (permalink)  
 
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Dozy, I appreciate your stubborn defense of this design decision, but do not accept from that side of the rooom, as it were, that the decision necessarily allowed for the "right" balance between man and machine in the man-machine interface.

An assumption in most machine design choices is that (operators) pilots retain piloting (operating) skills as a step zero in the process. (You put an untrained machinist on a lathe, and watch the money bleed from your production via waste and lost). Without feedback, without tactile sense, your brain to muscle memory goes to hell and your piloting skills are less than 100 %, unless you train to a new skill set.

In three separate, and long threads, if appears that a non trivial number of the airline pilot community have deep reservations about the matched triplet of training, currency and proficiency.

Try this.

Teach someone who has never flown to fly VFR, to include stalls, landings, and spins. Then, for the first time, put him or her under the bag, zero visual reference, vertigo rampant, and have him or her, even with some sim training, try to fly instruments in mild or light turbulence, through climbs, turns, descents, power changes, and some unusual attitudes. The odds of overcontrol or scan breakdown, even on conventional controlled aircraft, are pretty high. (For your own amusement, I'd like to be able to transmit a video of my own first ever on instruments flight. A bit of a mess, that one ... as expected in most training scenarios). How do you overcome that? Training and proficiency.

Time and again people have pointed this out, in this thread and others ... how pilots fly is informed by all of their flying experience, and all of their training.

If one takes the tactile feel, which at this point in aircraft design has been "manufactured" for a generation, (to include the helicopters I've flown) out of the loop, you have robbed the pilot of a standard tool to use (working around a trimmed position, or a null position, using vestibular and tactile sense) in making sense of his situation -- hence the term situational awareness -- and making the plane fly as he or she wishes it to.

That is what flying is. You make the plane (within its physical design capability and limit) do what you want it to do.

That said, the Captain from the AvWeb article knows how to fly the 330, and has some advice on those who wish to do it well. He's pointing out that the design has in some senses (hardly all) vacated the above assumption. What is apparent is that the new assumption has not necessarily been trained and embedded into the pilots who fly it. (Again, 99+% of the ones that take off come back down, standard day at the office, the pilots and their machine interface just fine).

Go back to our novice on his, or her, first instrument hop.

The novice pilot has to have the vertigo resistance trained into him, or her, so that when that distraction arises, an appropriate response is made, and the machine is obedient to the pilot's will.

Mayhap the captain's point is that retraining, and proficiency, in the "play it like a video game" approach needs to be incorporated into the training and education scheme better than it has been to date.

If, like vertigo resistance, you have to be trained and taught no to rely on stick feel, on vestibular and tactile sense, then train to proficiency, educate to understanding, then rinse and repeat until it's as embedded as "pull stick pack, cows smaller, push stick forward, cows bigger."

Last edited by Lonewolf_50; 6th Jun 2011 at 19:57.
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Old 6th Jun 2011, 19:52
  #1485 (permalink)  
 
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Control inputs and feedback

Salute!

Hmmmmmm, PJ's post was there, but I can't find it now. Good charts.

So I shall post two graphs of another FBW stick inputs from my personal Photobucket albuyms that I made myself from data and an Excel spreadsheet. Will they disappear as well?

All inputs were force and not stick movement. As with the Airbus, there was no feedback as to how much you were commanding, just body rates and gees that you felt in the seat of your pants.

Roll command:


Pitch command:


The tactile feedback most pilots here talk about is actual or synthetically-generated pressures on the stick from the control surfaces ( not spoilers or flaps). So a high speed aileron flutter is easily felt, as the stick is vibrating left-right. At high AoA, the stick gets "mushy" ( real technical term we used as instructors, heh heh). i.e. you could move it rapidly but nothing happened and you kinda knew that the elevator or ailerons had no meaningful effect. Then, at high speed, the stick became "stiff", and required more force to get the control surfaces to move than when in a stall or approaching a stall.

So I agree with the folks that talk about a giant video game.
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Old 6th Jun 2011, 20:10
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Dozy, I appreciate your stubborn defense of this design decision, but do not accept from that side of the rooom, as it were, that the decision made for the proper balance between man and machine in the interface, given that an assumption in most design choices is that pilots retain piloting skills as a step zero in the process. Without feedback, tactile sense, your brain to muscle memory goes to hell.

<snip>

If one takes the tactile feel, which at this point in aircraft design has been "manufactured" out of the loop, you has robbed the pilot of a standard tool to use (working around a trimmed position, or a null position, using vestibular and tactile sense) in making sense of his situation -- hence the term situational awareness -- and making the plane fly as he or she wishes it to. That is what flying is. You make the plane (within its physical design capability and limit) do what you want it to do.
Well, I'd like to hear from "gums" on this one - the Viper has no feedback per se for the sidestick - it is a force sensor (for want of a better description).

Much of this discussion lately seems to operate on the assumption that the control 'stick' of the 50's is the 'right stuff' and any other way of doing it is somehow bound to fail.

However the current generation have grown-up using joysticks and control inputs such the Wii controller, etc, and have 'learned' very differently from days of yore... Having been involved in a number of UAV programs, you only have to see the control stations for these aircraft to understand that the "pilots" (go with the flow...) are a different breed. Are they so different from the 'bus pilots?

If the slate were entirely blank and we were tasked with developing a control system for a new cockpit what would it look like? My guess is such a design would be rather different if you happened to be age 24 versus age 54.
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Old 6th Jun 2011, 20:11
  #1487 (permalink)  
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'different' is not 'wrong', necessarily. Attack, defend, ad absurdum.

Different has challenges that fairly speaking, need addressing. Or, redefine the operator syllabus, and accept problems that appear in transition. Too much PE lately, of all description. Where it can be identified, it needs to be corrected.

The pilots first wanted a window, then a Stick, then some Press (Mercury).

This debate has become tiresome, and actually forecloses progress toward that which is all important
 
Old 6th Jun 2011, 20:13
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Airbus feedback?

Where does this idea that there is no feedback on the Airbus control system come from? the side stick itself resists input via a spring so that a certain force is needed to move it through a certain deflection, and then that deflection commands (initially at least) a normal acceleration which is constant over most of the envelope in all laws except direct, as I (mere Boeing pilot) understand it. I got the following from a 1992 paper which I assume is still roughly right:

Limits:

Pitch
Max. load 10 daN
Threshold 0.5 daN
Deflection +-16 deg.
Orientation 20 deg fwd.

In non engineer units doesn't that mean you need roughly 10kg or 23 lb to demand the maximum allowed normal g of 2.5. The feedback is firstly the spring rate of the side stick, and secondly the response of the aircraft, what more should one want?

I would still like to understand why the first two stall warnings were generated before everything went pear shaped, doubtless the full BEA description will clarify that, but it would seem that when the PF took over he put in a mighty demand for no very obvious reason

Last edited by gonebutnotforgotten; 6th Jun 2011 at 20:15. Reason: table formatting
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Old 6th Jun 2011, 20:27
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Originally Posted by Lonewolf_50
Dozy, I appreciate your stubborn defense of this design decision, but do not accept from that side of the rooom, as it were, that the decision necessarily allowed for the "right" balance between man and machine in the man-machine interface.
I'm painfully aware of that, and if you think I'm being blase or somehow dismissive of tactile feedback in it's place I promise you I am not. However, when I talk about these things I am talking with my "engineer's hat" on. As I've said countless times before, there are failure modes of a backdriven system that are distinctly non-optimal, as well as the consideration of extra complexity.

The "feel" that a pilot gets back through the yoke from the Comet onwards has been mechanically interpreted. You are not "feeling" what the aircraft is doing, you are merely feeling what the artificial feel system thinks you should be feeling based on the input parameters it is getting. In the Comet up through the 767, what you were feeling was mechanically driven. The A320 and her descendants dispensed with it entirely and the 777 actually had that feedback response programmed into the computers. That was a design choice, and while a non-trivial number of pilots have reservations about the fact that Airbus dropped it, it is also true that a non-trivial number barely missed it at all. It is therefore a matter of personal preference. Dealing with the visual-only feedback channel is a matter of training.

We don't know whether the PNF spoke up about the strange attitude or not at this point, so any argument over backdriven sticks is basically irrelevant to the subject at hand. We *do* know that when whoever was in the LHS tried to take control, the PF spotted it and handed over without a fuss. In the stick-shaker regime, connected control columns did neither Birgenair nor Aeroperu's crews any good, as the cacophony of lights, sounds and warnings made problem-solving practically impossible. This was a nightmare situation that would test even the best pilots no matter what aircraft they were flying.

Originally Posted by gonebutnotforgotten
I would still like to understand why the first two stall warnings were generated before everything went pear shaped, doubtless the full BEA description will clarify that, but it would seem that when the PF took over he put in a mighty demand for no very obvious reason
According to an earlier BEA report on a different incident (I'm still trying to track down the original research - any help much appreciated), tests indicated that in a panic situation 80% of pilots would instinctively pull back on the stick. I'm not saying that's what happened here, but it's definitely one of the possibilities.
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Old 6th Jun 2011, 20:38
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A very bad Airbus design feature is thrust levers that do not move while in autothrust. They are instead set in a detent which would equal climb trust in manual mode. If the pilots did not reset the thrust levers to equal the last cruise power setting, they likely eventually ended up in climb power, making it difficult to reset the proper cruise power setting and adding to what was likely already a great deal of confusion.


If my English reading skills are ok, capt is saying that, if Autothrust trips (due to UAS, or any other cause), engines go to CLIMB power without crew intervention!!!

And, at cruise setting, the inference is, AT trip will likely induce pitch up in A330 as consequence of thrust increase?

So AF447 likely to had pitch up produced both by thrust increase PLUS induced by PF sidestick
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Old 6th Jun 2011, 20:38
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Is it possible they suspected the tailplane (only) was stalled? Perhaps due to ice?

On other aircraft I understand the standard proceedure for recovering from a tailplane stall is stick back and reduced power. I believe the thinking is that increasing the camber lowers the stalling speed.
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Old 6th Jun 2011, 20:38
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Bovine scaling

.... then rinse and repeat until it's as embedded as "pull stick pack, cows smaller, push stick forward, cows bigger."

Perhaps better expressed in this instance as "push stick forward, cows bigger - pull stick back, cows smaller - keep pulling stick back, cows get bigger again......
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Old 6th Jun 2011, 20:54
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Originally Posted by sirgawain123
So AF447 likely to had pitch up produced both by thrust increase PLUS induced by PF sidestick
No, he's wrong. When autoflight trips out, the trim and power settings remain at their last assigned setting. Thrust will not change from that setting no matter what detent it was in until one of the pilots manually moves those levers.

This makes me highly suspicious of his credentials - if he's flown those thousands of hours in an A330 and doesn't know that then I'd be a little leery of getting on his aircraft.
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Old 6th Jun 2011, 21:09
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THS trim law

Hey, DOZE!!!!!!

Can you show us the law for the THS trim position? Would help a lot of us to understand the first minute or so of the accident.

For example, if I hold a slight back stick to command a slightly higher gee ( a gee command plane, right?), then does the THS move to "help" me achieve the gee?

Secondly, is the THS law related to airspeed/mach?

In short, how does that sucker work?
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Old 6th Jun 2011, 21:10
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Dozy, thanks. It does not matter one whit that "since 1950" the force gradients and feedback mechanisms have been added ... they were necessary due to the magnitude of control forces as planes flew faster, and in part due to the undesirability of direct feedback heading back down the pipe into the cockpit.

What evolved was the concept of the "trimmed position" which is a null point about which correction is made. Many of the advances since just after Wilbur and Orville hung up their cleats amount to is ways to aid and abet the standard task loop of flying, and to do so from around a known "neutral" position ... not of the airfoil, but of that which controls it.

Set condition (what you want the airplane to do) via control inputs (which
includes power, we'll leave gliders out of it, yes?) and then check to see if it does what you want it to, or not. If yes, hold, and then trim, that condition until you want to change. If no, change and retrim until you get to your yes ... and you are in a steady state climb, level flight, turn, descent, or combination thereof.

Again, there was good reason, both behavioral and psychological, to retain the use of tactile sense in the control loop as an error detecting input.

I disagree with your attempt to dimiss it in this little throwaway.

The "feel" that a pilot gets back through the yoke from the Comet onwards has been mechanically interpreted. You are not "feeling" what the aircraft is doing, you are merely feeling what the artificial feel system thinks you should be feeling based on the input parameters it is getting
I disagree with your merely. The point is to scale it so that what you feel is a tactile representation of what the aircraft is doing. (Good example is nose trim: "I gotta push to keep the stick outta my lap, nose is in, and trimmed in, the wrong place for this flight condition.") That feedback system is useful for pilot decision making. The art and craft of feel and feedback developed over some years. It allows you to add the tactile sense to your suite of tools, rather than limit it or even eliminate it.

Consider the training that gums and his comrades underwent on a continuing basis. (The USAF is nothing if not zealous about training).

Your operator is a necessary element of the control system.

I was hoping you'd see the link, and that I was pointing to the underlying issue: you have to train to proficiency. Since most people don't begin flying in FBW, and do begin with an ergonomically sound, and proven, method that applies the tactile senses to the system (man and machine in the gestalt ... ) you have to pay attention to how you do your transition training, and your proficiency and habit building. If you design your system with erroneous assumptions on that score, you open up a nice can of risk whoopass.

I am done with that, thanks for your insights.

Note: thanks to poster who explained better the feel system in the Airbus design.
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Old 6th Jun 2011, 21:24
  #1496 (permalink)  
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tubby, others, I deleted the post temporarily as I was unhappy with it. Sorry for the loss of signal. This stuff has to be done so carefully so as not to cause wrong understandings and lead others down blind rabbit trails! - pj

Originally Posted by MurphyWasRight #1476
If I understand correctly the Airbus sidesticks do not provide any feedback, they are "input only".
The following schematic may help:




Machinbird;

Re "one of the most experienced A330 pilots"...Perhaps so, perhaps not.

We have to approach this article with the same caution as any which "arrive" in the publice venue.

We have absolutely no way of verifying that this is from an A330 pilot.

As with the comments from a Professor Hüttig last week, (who I do not believe was "an Airbus pilot" because he doesn't tell us what exactly his Airbus/airline experience is), we have no information on this author who claims to have 4500hrs on the A330. Probably does but the stakes are far too high, the desire for accurate information too great, to expect others will take an internet posting as the truth. Establishing credibility takes time, discussion and a good smeller.

Whoever he is, I think he's wrong on comparing the airplane to a video game and he's wrong on the artificial feel matter; the non-moving thrust lever item has been around for decades and won't be resolved by his or any others' comments; they are, for the many who are experienced on the airplane, a non-issue.

The A330 is what it is in these specific respects and I think it is reasonable to state as an experienced airline pilot that after one is trained and finishes an informal "apprenticeship" in real airline operations one gets accustomed to one's airplane, especially after 4500 hrs on the machine. One gets used to "what is" and, as a professional, maintains a regime of continuous learning by staying in the books and practising one's craft by hand-flying including disconnecting the autothrust.

In reference to the article, it is worth asking oneself the question, Why are people saying the things they are saying? Motivations are numerous and very complex and no question, no comment is "innocent". Do we follow the money, the ethics, the lofty goals of learning? All are choices which we make, for the most part, subconciously, when something either feels right or smells a bit. Why would someone "go public"? Attention?, setting the agenda?, enframing the discussion? We are not told who wrote the article. We are free to judge that for ourselves. Is that intentional or done naively unaware that others would ask such impolite questions?

How many who continue to pronounce on "the dangers of Airbus" and the "confusion of laws" have ever flown any airplane at all let alone the A330? How many here have had the A330 Course and flown it in-service in real life?

I submit that the act of summarizing the details of the A330's AFS laws and trying to make academic sense of them either to oneself or others is the same kind of cognitive act as trying to describe to someone exactly which specific muscles to move, and when, to walk up a flight of stairs. One googles "smart-cockpit" and like so many who use the internet for personal qualifications, consider themselves sufficiently knowledgeable to actually say something about the airplane without ever having flown it. If it were that easy and that simple, we wouldn't need the training and checking regimes that regulators require and airlines routinely provide.

That the design and execution of the design most definitely deserves comment and critiquing. There are some serious questions to be asked.

Dozy...re, "No, he's wrong. When autoflight trips out, the trim and power settings remain at their last assigned setting. Thrust will not change from that setting no matter what detent it was in until one of the pilots manually moves those levers.

This makes me highly suspicious of his credentials - if he's flown those thousands of hours in an A330 and doesn't know that then I'd be a little leery of getting on his aircraft. "

Yes, exactly. He may be who he says he is; doesn't matter. THR LK has been around a long time, but not forever. I flew the A320 without thrust lock. One remembers very quickly how to disconnect the autothrust. But when the engines went to the donuts, it was a non-event...an embarrassing cause for questions from the back but nothing beyond that.

For those who seem to believe that CLB or even MCT at FL350 on the CFs or RRs is cause for a huge pitch-up followed by an immediate overspeed or an uncontrolled climb for the A330, it needs to be understood that there is not much residual thrust remaining and the aircraft response from what "boost" there is, is emminently controllable. On the A340 one could likely leave the CFM-56's in TOGA and you'd take all day to overspeed the airplane...they didn't call them hair-dryers for nothing.

Last edited by Jetdriver; 6th Jun 2011 at 22:26.
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Old 6th Jun 2011, 21:26
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THS trim law

@ Gums:

There isn't a separate THS law.......there are pitch laws!

In manual mode (Side stick control) short orders are handeld by elevator deflection.
Long term orders by THS. (autotrim)

Just like any other design.

The gains depends on Vc, CG and Flap/Slat position.
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Old 6th Jun 2011, 21:29
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Originally Posted by boofhead
The old-style control column provides feedback, but that is dependent on a separate set of pitot tubes.
I presume you are just trying to be facetious ?
Otherwise your post makes no sense.

Last edited by Jetdriver; 6th Jun 2011 at 22:27.
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Old 6th Jun 2011, 21:37
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@gums

There's a useful document here:

A330 Flight Controls

But as I understand it, it boils down to "if the pilot is still holding the stick back at the limits of elevator authority, trim nose-up to assist until the pilot stops issuing the command". This should hardly ever be necessary, as it is intended to escape from extreme attitudes only, but it is there.

@Lonewolf_50

I *know*, and I know that many pilots agree that the loss of tactile feedback was a mistake. I do not "dismiss" anything and I do not reduce pilots to "operators". I think we're poking at cross tortoises here. Airbus aircraft do retain a degree of artificial feel to allow the pilot to know what the *aircraft* is doing, but what CONF and the others are banging on about is the fact that on the Airbus you can't directly feel what the other *pilot* is doing. That's the issue, and that's what I'm talking about when it comes to back-drive.
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Old 6th Jun 2011, 21:42
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Dozy, thanks, sorry I beat that poor horse into glue.
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