roulishollandais

@Dozy
I don't think it is a Winnerhofer original.
Use of "URA" is from somebody who didn't read the BEA report
I don't agree with that complicated "philosophy". Pilots don't fly philosophy, not enough time to "do nothing"

gums

Salute!

O.K., 'bird. I thot the jet was in Alt 1. So Alt 2 is a different beast WRT roll commands and aircraft reactions.

It still bugs me that I have to figure out exactly what law I am operating with.

Seems to me that "stby gains" ( gear up or gear down values) and basic gee commands/protections and AoA inputs should have saved the day. Then there are all the inertial/rate sensors that don't need a single atmospheric input.

When I talk about the various modes, I am talking about limits on max bank angle, max pitch, max gee, ,max AoA, etc. Then there are the control surface rates that depend upon "Q" - the "gains".

Oh well, the AF447 tragedy shall remain the "poster child" concerning airmanship and aircraft software implementaion for at least a few more years.

DozyWannabe

This surprises me, because you've certainly been active enough on the thread of doom to know otherwise!

The difference is fairly academic really. I think it boils down mainly to the aircraft being slightly more sensitive in roll - other than that it's fairly transparent.

We covered this in the mega-thread though. The "soft" protections would have stabilised the aircraft until well into the sequence *had the PF just let go of the sidestick*.

The assumption in the EFCS design is that if there has been a failure sufficient to degrade from Normal Law, then the human pilots are given priority over the systems on the grounds that they are likely to know more than the systems do about the status of the aircraft. Such priority is exercised by being able to override the soft protections via control inputs.

@Winnerhofer - OK, we know AF has had a few issues in recent times, but I'm struggling to understand what you're trying to say by providing us that link.

roulishollandais

@Winnerhofer
I was seeing that you replaced the unknown "URA"s by the "UAS"s (i.e. BEA and AF SOP). Much better !

But that text is not usable by pilots in flight and if you teach pilots on ground with that text about UAS they will start to write a book waiting 30seconds (Eternity) until the pitots ice is melted (perhaps never in case of failure) and systems fit again. You have to pilot your bird with pitch and power, that is "doing nothing" but as Machinbird said it "without keeping your hands in your pockets". Gums suggested the good delay for observation, appropriate decision, and starting correction without precipitation : 1 or 2 seconds...

That delay of 30 seconds appeared with A320 in some office. I was amazed since February 1988 that Pierre Baud accepted it.

SOP must be simple, but understanding the SOP during the time you do it must be simple too, as often a succession of factors and results happen in an emergency.

After some months gums pointed alt1 instead alt2b and came back to the C* definition, with limitation and rate limitations from fondamental pilot controlled parameters - pitch, bank, gee, power and their variations with help of reliable information like intertial data. More ordinary pilots cannot do more complicated than gums or Machinbird. Accurate pedagogy is a condition of survival in flight.

DozyWannabe

I don't think it was intended for that, I think it was an attempt at a general summary.

Right, I don't think anyone's suggesting anything different. I think what does need to be pointed out, though, is that outside of a significant mechanical failure or sudden change in the weather conditions being transited, the pitch and power at the point of AP disconnect will usually be more-or-less correct for the conditions, and one should only change it immediately if a significant problem is developing.

Sorry, need some more detail on this - what are you referring to?

As a rule, you don't really need to worry about systems functions at that level. The aircraft will still go where you point it.

roulishollandais

Perhaps you can help me to find again that document in a ICAO magazine from january or february 1988, where Pierre Beaud was describing the strange innovations of the A320 : Push the red button 30s after the systems decide you have to learn piloting again, addition of both inputs, A320 doesn't stall. Etc. I no more remember the tittle and exact reference, and since ICAO does no more sell their documentation in Neuily/Seine I found it too difficult to find anything.where you point your HUD ! Free falling I learned "Body goes where eyes are looking"

Machinbird

Dozy, you continue to astound me with your blithe statements on subjects you have little practical knowledge of. That "academic" difference just might have been the difference between AF447 making in into the barn that day and taking that big swim. That sensitivity difference permitted initiation of a roll PIO from what I can see.

gums

I am gonna go with 'bird and gang up on the Doze! Heh heh.

There is one helluva difference between the "direct" roll commands in Alt 2 and the roll implementation in Alt 1 ( thanks, 'bird..... I forgot the mode -- exxxxcuuuuseee me!)). My documents show something that looks like "stby gains" for Alt 2 according to configuration, but also states the control surface deflection is directly related to stick delection - full stick gives you full deflection and the "gains" move the surfaces at an appropriate rate ( just like the Viper did over ten years before, except our roll was still a body rate command and not an actual control surface deflection).

So @ Doze: Easier to get into a PIO in Alt 2 than Alt 1 or "normal". Doesn't excuse the "startle" factor we have discussed, if that's what happened, but going from a rate command with all the protections and such to a "direct" mode is not trivial. Feedback to the pilot is basically "seat of the pants" and your inner ear gyros. Not sure what the pounds per degree of stick deflection are, but the Viper was basically 16 - 17 pounds full pressure/rate command for roll ( my leading edge flap failure had me holding about 15 - 16 pounds for the 15 minutes I was airborne just to get zero roll rate in actuality).

Still appreciate your inputs, Doze, and wish I coulda been there withya in the sim and trying to recover the jet. I thot someone else tried a "recovery" from the "top" of the climb and entry to a deeply salled condition.

Owain Glyndwr

Just got back home after driving through WW1 battle areas, so I am going to stick my head above the parapet and duck back quickly!

I'm not at all convinced that it is fair to describe the lateral oscillations immediately following the AP drop out as being a PIO, although I can see why people should describe them as such. PIOs, technically speaking, have control inputs out of phase with the necessary corrective actions and the bank angle excursions should be increasing, not falling.

I think the point being missed in the discussions of 'sensitivity' and 'startle' is that there was a fundamental change in the piloting dynamics when the FCS reverted to direct law. IIRC in normal and Alt laws a sidestick movement commands a roll rate, but in direct law sidestick commands roll acceleration as in most aircraft.

Now I am not a pilot, but what I learned in studies of several aircraft is that to get stability the necessary control techniques are very different in the two cases.

What I see in AF447 is a pilot struggling to come to terms with an unfamiliar control dynamic; OK, it is one he had seen before throughout his early career and training, but not one which he had used recently. The (damped) oscillations result from his re-learning process and personally, I think he made a pretty good fist of it. I can't see them as a fault on his part, which describing the motion as a PIO somehow implies.

gums

Sounds like a trip thru memory lane, OG

I don't think the "direct" roll implementaion is an "acceleration" command of the control surfaces, but actual control surface position. The "rate" of movement seems to be dependent upon the "gains" that the FCS uses according to aircraft configuration. This was same for the primitive FBW law we had in the Viper - "x" dynamic pressure according to gear up or gear down.

The "direct" command of aileron position is vastly different than commanding a roll rate or whatever. So at stick being neutral, the ailerons should move to whatever position the "neutral" position of the stick is using according to last trim condition.

Our force on the Viper stick was pretty simple, and just relaxing on the stick with zero force resulted in the jet rolling according to the last trimmed roll rate ( usually zero). You can see this on the Thunderbird solo pilots that do rolls to inverted passing in front of the crowd. The sucker flat stops rolling and no overshoot.

Just my experience and observations, but the 'bus FCS seems to implement the Viper laws except for the max bank angle.

Owain Glyndwr

Hi gums,

Guess I should have been a little more precise. Direct law does produce aileron movement proportional to stick movement as you say; but aileron movement produces a roll acceleration, which is why I wrote that stick movement commands a roll acceleration. That develops eventually into a roll rate which depends on aileron deflection, airspeed and the natural roll damping derivative. That means the characteristics are speed dependent whereas in the other laws you get the same roll rate for a given stick movement at any airspeed.

Putting the stick back to neutral in direct law will kill the roll rate not leave it at a commanded value as it does in normal/alternate. That is same as non-FBW aircraft and differs from your description of the Viper operations.

DozyWannabe

You remember right, however in this case the aircraft did not degrade to Direct Law, but to Alternate 2 (the equivalent of Alternate [no speed stability] on the A320 IIRC). In Alternate 1, roll is still rate-commanded, but in Alternate 2 the roll commands become direct. In Alt 2 the pitch commands are still rate-commanded, unlike Direct Law.

While I agree the dynamics definitely change in Alt 2 versus Normal Law, there are two caveats - namely that there was no immediate need to make significant roll inputs upon AP disconnect (and thus risk potential PIO), and that as the PF had no training in high-altitude manual handling at all it is conceivable that he may still have initially overcontrolled even if the roll aspect was still rate-driven.

EDIT : Though, as pointed out, I am not an aviation professional, I do however submit that this should at least be considered a matter of common sense - namely that if a malfunction (which will initially be of an unknown nature) has occurred, then it would be advisable to treat the flight controls very gingerly at first, because you don't know what works and what doesn't. I believe the phrase used in the mega-thread to describe the inputs made by the PF was "stirring the mayonnaise" - and this is something to be avoided absolutely unless there's an obvious and life-threatening reason to do so.

Indeed, most of the incidents and accidents I can think of in which the outcome was at least relatively successful have involved the crews concerned doing just that - "feeling out" the aircraft to see how it responds, what works and what doesn't. Examples include UA232, QF72, BA009 and NWA85 - not to mention the other 30-odd incidents of Thales AA-related UAS problems!

Owain Glyndwr

Yeah I knew that Dozy, but talking of a lateral oscillation the pitch mode is irrelevant and for all practical purposes my statement that the roll degraded to a direct law is correct even if you label it Alt2

DozyWannabe

Of course - I was just being irritatingly pedantic (as usual) for the benefit of thread-watchers who were new to the discussion.

Generally speaking, I think it's important not to get too hung up on the technical aspects of control laws, as some people's eyes tend to glaze over at the mention of them - while the specifics are definitely of interest to a technical audience, I think the best way of explaining them is in terms of more traditional setups.

Specifically, in real terms, all the control laws and their underlying mechanisms actually do is provide maximum controllability in the event of systems or mechanical failures - the technology means that this re-configuration is automatic, but ultimately it's no more than the equivalent of what a flight engineer would do in the event of a e.g. a dicky hydraulic pump or a stuck control cable on an older type.

The common thread remains, however, that the presence of a malfunction requires a gentle touch with the aircraft until the consequences of the malfunction on the aircraft's handling are understood and compensated for by the crew.

Machinbird

Welcome back from the fronts Owain.
The definitions of PIO have evolved over time and encompass a broader spectrum of oscillations.
The modern definition of PIO is Pilot Involved Oscillation and gets away from blame and fault concepts. Without the pilot in the control loop, the oscillation does not occur.

You can bet your money that Bonin was not rocking the wings for the fun of it. From his control inputs, I can conclude that he desperately wanted it to stop. Unfortunately, he chose a bad strategy to stop the oscillation and thus exacerbated his problem. If you look at the oscillation cycles, you will be able to see that there were a number of low points in the oscillation amplitudes followed by an increase in amplitude in the opposite phase to the immediately prior oscillation. To me, this is an indication that he attempted to get ahead of the cycle of oscillation.

To him, it must have seemed that he was a little too late with his control input, so he accelerated his control inputs, i.e. went faster. I think that you can see this pattern in aircraft behavior and in control behavior if you look at the timing on a cycle to cycle basis.

Too bad he didn't have prior experience in flying in Alt2 Law. He would have learned that all he had to do was stop making control inputs.

The development and implementation of Bonin's control strategy must have heavily dominated his thinking and almost certainly explains a lot of the otherwise inexplicably bad flying he demonstrated.

As pilots we learn to put our aircraft under accurate control. When we have an aircraft that is not behaving as we command, your whole attention is drawn to the problem, and it is difficult to leave sufficient attention for other important requirements of flying. The one very short roll PIO I experienced in a simulator was traumatic enough to leave a lasting impression. It compared to the feeling you get when you trip and begin to fall, and you are looking desperately for a hand hold. In my case, I was desperately looking around for a visual attitude reference that did not have a time lag in its display. I am relaying this so that the reader can get some idea of how compelling a PIO problem can become.

Owain Glyndwr

Originally posted by Machinbird

Which confirms my fear that I am thoroughly out of date on some things!

That is my point really. Control of the oscillations is all about getting the phasing of the control inputs right. Roll rate is 90 deg out of phase with roll acceleration, so if he was accustomed to a roll rate response to stick he could have been very confused by an aircraft that took a little longer to develop the response he was anticipating. That would be entirely consistent with your remarks

Machinbird

Sometimes getting control of the oscillations just requires the pilot to get out of the control loop, but this is not a one size fits all solution.

I can think of at least one PIO event where this course of action had immediate fatal results.(Sageburner).

roulishollandais

Henri Poincaré feared to say that the solar system was not exactly periodic and could be unstable (in the traditional use of "PIO" divergence or non convergent oscillation).We know that periodic dynamic is governed both by phase and frequency, but traditionally lack of math tools focused on the only phase control which does not involve energy use at the contrary of frequence modification.
Another well known oscillation in airliners is passengers' hyperventilation. Frequense of breath is increased by different factors disturbing the CO2 and O2 regulation. Trying to correct it with phase has no chance of success.

Sometimes periodic movements are not more or less sinusoidal oscillations, I.e. chalk squeaking on the blackboard when pushed and doing rapid successions of jumps.

Progressively theory of chaos taught us that difficult art of controlling oscillations.
Multi variable problems are more difficult to describe in short words.

"Pilot" in PIO may be Autopilot too, as we see it in the KC-135R dutch roll report when the captain tried to stop it with A/P. The notion of "fault" is of no use in science. We are concerned by cause.

Machinbird

The human is a variable gain device when it comes to aircraft control systems.
For some type of input, you can get either a large or a small output and that output can have further modification as to phase angle or even anticipation of a signal.

The autopilot is a machine that is constrained by its construction and design to create a certain type of output for a particular input. Normally an oscillation involving an autopilot is not considered to be a PIO. As I understand the KC-135R accident however, the presence of human feet on the rudder pedals caused additional inputs to the system that resulted in an increasing Dutch Roll oscillation. If my understanding is correct, then that would be a PIO under the newer PIO definitions.

xcitation

@Mac / Chris

IMHO the most perplexing of all the holes in the swiss cheese is the total absence of discussion given to the stall warnings by all 3 pilots. Was it a Titanic belief in the un-stallable nature of the airbus that the warning must be a glitch not even worthy of discussion?
Mac - if the patients heart stops during surgery and you hear an alarm I would hope that it gets someones attention.