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  • #16
    Originally posted by Evan View Post
    This is sort of along the lines of what I was thinking. The roll occillations just so abruptly stop, as if something in the flight control authority has suddenly changed, like a mode change, for instance. Or perhaps that is the nature of rotor turbulence caused by a large Spanish rock. I wouldn't know.
    Again, the rotor may have caused the initial upset, but it cannot explain the control inputs that we can see in the video. But the control input we see in the video CAN explain the roll motion we see in the video.

    Click image for larger version

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    1) First frame of the video: The plane is in a left bank but rolling right quite fast. The left bank angle was obviously steeper before the start of the video.
    2) Bank pass approx wings level, still rolling right fast, indication that we have right-roll command input.
    3) Roll spoiler starts to deflect (almost unnoticeable in the picture). However the roll rate had already slowed down a bit by then, indication that the right-roll control input had diminished or been neutralized before this picture.
    4) The wings reach the max right-bank, roll rate is hence zero, the spoilers have not reach the max deflection.
    5) Wing is coming down, spoilers reach maximum deflection.
    6) Bank pass wing level, spoilers still in maximum deflection for left roll.
    7) In a left bank and rolling left, spoilers start to retract.
    8) Spoilers complete retraction, plane is still rolling left. Cannot be seen in the video but most likely the command has to be for right roll after this frame, however the plane keeps rolling left due to inertia.
    9) Pane reaches maximum left bank, roll rate is zero, plane starts to roll right after this frame.

    2nd row is the second cycle, which is basically a copy-paste of the previous one, except with a little bit less of amplitude (but then these were the last 2 cycles).

    I was thinking that, since the initial roll excursion was uncommanded (an upset rather than an intention), would the AP logic—up to a point— simply rely on the aircraft's lateral stability to restore wings level (no flight control inputs)? And if that point is around 33° bank, would something in the logic THEN command a counter-rolling flight control movement?
    No, there is no such a thing as natural lateral stability, and the roll control inputs by the AP are done on any little deviation not only from the bank target but aslo from the roll rate target, which is NOT what we see in the video (not if the target is constant heading or wings level, but may be yes if the target is to follow a certain track, like a localizer.

    Because that is what appears to be happening in the video showing the delay in the spoiler deployment. I wouldn't expect such a delay from the autopilot unless it was intentionally delayed.
    It is worse than that. What we see in the video is the plane banking THROUGH wings levels with control inputs in the same direction than the roll rate.

    The A320 normal law in roll requires a pilot in manual flight to maintain full stick deflection to exceed 33° of bank. It also introduces THS limits at this point. So perhaps there is a similar functional threshold in autoflight at this bank angle.
    What will happen in normal law if the plane banks 20 degrees and the pilot makes no corrective input? The plane will stay at 20 degrees. The same will happen in AP if the AP doesn't make an input.

    Is it possible that there is a bit of an oversight in some flight control algorithm that only reveals itself in this rare combination of circumstances? Something that can lead to A/P-induced occillation... until another mode is selected or the A/P is disconnected (something I might expect the pilots to do if this was happening under autoflight)... ?
    I don't know. Everything that is not impossible is possible.

    --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
    --- Defend what you say with arguments, not by imposing your credentials ---

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    • #17
      Originally Posted by Evan
      This is sort of along the lines of what I was thinking. The roll occillations just so abruptly stop, as if something in the flight control authority has suddenly changed, like a mode change, for instance. Or perhaps that is the nature of rotor turbulence caused by a large Spanish rock. I wouldn't know.

      The scale of wave systems is determined by the size of the obstacle creating them. In the case of, for example, the Andes, massive. In this case the rotor,created by the Rock, would be of a relatively small scale and the time required to transit short. At the same time, the transition from the violent turbulence of rotor to the smooth laminar flow of the wave itself is always sudden and remarkable.

      It may be interesting to have a look at www.mountain-wave-project.com/ which has done a lot of work on the subject much of which is relevant to commercial flight operations. Most remarkably,Klaus Ohlman, a German glider pilot flew a distance of 3008km at an average speed of over 200kph in the Andes wave.

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      • #18
        Originally posted by pegasus View Post
        Originally Posted by Evan
        This is sort of along the lines of what I was thinking. The roll occillations just so abruptly stop, as if something in the flight control authority has suddenly changed, like a mode change, for instance. Or perhaps that is the nature of rotor turbulence caused by a large Spanish rock. I wouldn't know.

        The scale of wave systems is determined by the size of the obstacle creating them. In the case of, for example, the Andes, massive. In this case the rotor,created by the Rock, would be of a relatively small scale and the time required to transit short. At the same time, the transition from the violent turbulence of rotor to the smooth laminar flow of the wave itself is always sudden and remarkable.
        Again, it is quite clear (to me) that while the initial upset may have been caused by the rotor and perhaps it contributed to some shaking afterwards, but it is NOT what is generating the roll oscillations, which are clearly (to me) commanded.

        It may be interesting to have a look at www.mountain-wave-project.com/ which has done a lot of work on the subject much of which is relevant to commercial flight operations. Most remarkably,Klaus Ohlman, a German glider pilot flew a distance of 3008km at an average speed of over 200kph in the Andes wave.
        Ha! Remarkable that of the Andes wave? (yes it is but...) Then what about soaring beyond 76,000 ft?
        https://www.flyingmag.com/perlan-2-g...et?enews090418

        --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
        --- Defend what you say with arguments, not by imposing your credentials ---

        Comment


        • #19
          Originally posted by Gabriel
          No, there is no such a thing as natural lateral stability...
          Ok, you're doing it to me again. Aerodynamically, the A320 has positive spiral stability, correct? And this is due to a combination of positive directional stability and positive lateral stability, is it not?

          What will happen in normal law if the plane banks 20 degrees and the pilot makes no corrective input? The plane will stay at 20 degrees. The same will happen in AP if the AP doesn't make an input.
          I am distinguishing commanded roll from uncommanded roll. I'm speculating that the logic might respond to uncommanded roll by letting the aircraft's static stability resolve the upset—up to a point, and then...

          But if you are telling me the A320 lacks this, then that blows my theory.

          Comment


          • #20
            Originally posted by Evan View Post
            Ok, you're doing it to me again. Aerodynamically, the A320 has positive spiral stability, correct?
            I don't know. Spiral-mode stability is not required and is typically very very small (either positive or negative) to the point that, in many cases, small variations in the construction from one specimen plane to the next can make it change from positive to negative or the other way around and any minor asymmetry in the weight or construction will generate a larger roll moment than the restoring roll moment (if any) of the spiral dive. For one given plane, it is typically not consistent in all the envelope. Depending of speed, load factor, CG, bank angle, etc... it may become a little bit more positive or negative (or change from one to the other, but always a little bit). But in general, the time to double or time to halve the amplitude of the initial perturbation is always many seconds, when not minutes. We can say that planes are roughly neutral in the spiral mode. That's why, as a rule of thumb, if you want to turn a plane you make an aileron input to get to the desired bank and then neutralize the ailerons to keep the bank, making only small inputs as needed to keep the bank angle.

            That said, because transport category jets with swept-back wings tend to have a quite big dihedral effect (that is one of the competing factors in the spiral mode), they tend to have positive spiral stability over most of the envelope, but again, it is very small and it takes many seconds to half the amplitude, so you cannot count on that to level the wings after an upset.

            And this is due to a combination of positive directional stability and positive lateral stability, is it not?
            No, it is because the diherdal effect (rolling moment induced by sideslip), which is the factor that tends to make the spiral mode stable (and worsen the dutch roll mode).
            The other competing factor (there is no name of this one as far as I know), which tends to do the spiral mode unstable, is the fact that that in a turn the outer wing goes faster than the inner wing, thus generating more lift and tending to steepen the turn. This effect is created by the existence of the positive directional stability (that is what makes the plane start turning when you just bank it), so actually the positive directional stability plays against the stability of the roll mode.

            There is no such a thing as lateral stability. Take a plane flying straight and level. Suddenly, hit "pause", bank it 20 degrees, and press "play"again. Do you see any restoring moment being created by the bank itself? There is none. Both the weight and the lift still pass through the CG, thus they have no arm to make a rolling moment.

            Compare for example with the directional (or longitudinal) stability. Have the plane flying straight and level, hit "pause"and change its heading 20 degrees to the left, and press "play" again. Do you see any restoring moment? Of course! The air is now hitting the fin from the right (it has an AoA from the right) and pushing the fin to the left, which in turn makes the plane yaw right. And similar if you are flying straight and level and increase the pitch, the air pushes the tail up and hence the nose down. There is no such restoring effect with the bank. Not until it starts to slip, and that will take time whether it will be end up being stabilizing or unstabilizing (we are back in the directional stability and the dihedral effect fight in the spiral mode).


            I'm speculating that the logic might respond to uncommanded roll by letting the aircraft's static stability resolve the upset—up to a point, and then...

            But if you are telling me the A320 lacks this, then that blows my theory.
            No, no AP does that. I mean, I don't know the specific design details of any specific AP, but that would make no sense for the reasons explained above.

            --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
            --- Defend what you say with arguments, not by imposing your credentials ---

            Comment


            • #21
              Originally posted by Evan View Post
              https://www.youtube.com/watch?v=IkGTg_UOBNY

              Something's not cricket here. PIO?
              Did I mention that I hate forum entries where the a/c type is not mentioned? So, let me guess, British Air to Gibraltar, the only rwy is
              09/27 with a length of 1670 meter.

              The video shows a jet with 2 engines. Only a rough guess, not a British B744. Now for the experts, which is the biggest jet you would
              like to land on a 1600 m (5,200 ft) strip . I'd say, that definitely depends on the alt AMSL. LXGB is 15 AMSL.

              Since a few weeks I operate an experiment with Brazilian airports, just to understand what happened, or better, what did not happen on flight Varig #254,
              September 3rd, 1989, on board a VG-B732... Spatial disorientation, if you ask me, and they even received 2 US-Dollars for what they did, per month, but as I hope not after 09/89!

              There is one difference. To Guarulhos, I used what we always use, the B747 (simulator by Randazzo). For the infamous Uberaba - Uberlândia - Goiânia - Brasília - Imperatriz - Marabá route,
              with a final arr at Belem, I use a Beech King Air 350. Brazilian strips are so short. Imho, SNAG Araguari is too high and too short for a Beech B350.
              SNAG is 1490 meters short, @ 3107 AMSL . That's why even x and y (pilot names are not mentioned) did not try Araguari in their 737-200.

              So, let's say, I'd try Gibraltar in a Beech King Air 350 simulator. But what happens if I asked Flight Captain Spohr. 1600 m is short, not to say really f* short for the t/o of a fully loaded LH-A320, isn't it..

              PS: For all people who don't yet have understood who has stolen the summer temperatures in February 2019..
              Rio Galeao weather, +27°C OAT at this very moment? That's why we all love the LH-B744 thermometer, it's good for good news. I'd say, for the de Janeiro summer (southern hemisphere..), we definitely don't need an extra fuel temp thermometer..
              Last edited by LH-B744; 2019-03-03, 03:18. Reason: Rio Galeao weather? Well what do you assume in Brazil.
              That's what airlines are good for, amongst others,
              The Gold Member in the 747 club, 50 years since the first LH 747.
              And constantly advanced, 744 and 748 /w upper and lower EICAS.
              Aviation enthusiast, since more than 35 years with home airport EDDL.

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              • #22
                Evan, have you ever been able to visit the Southern Hemisphere? It's quite confusing..

                The European winter is summer on SBGL Rio Galeao, and beginning with April 1st, they start into autumn (?!). But even with a night temperature in summer of +27°C, we would be very happy if not overwhelmed..

                SBGL Rio Galeao time zone is EDDL minus 4. That sounds so very near. But nevertheless we need my avatar to reach the Copacabana.

                Sorry for that off topic entry.
                Gibraltar never was a Boeing 747 topic, as far as I can remember. +13°C ? Sorry, but that's not a temperature which makes me board an a/c.
                That's what airlines are good for, amongst others,
                The Gold Member in the 747 club, 50 years since the first LH 747.
                And constantly advanced, 744 and 748 /w upper and lower EICAS.
                Aviation enthusiast, since more than 35 years with home airport EDDL.

                Comment


                • #23
                  Originally posted by Gabriel View Post
                  It looks intentional, based on the spoiler inputs (and the lack of screams inside the plane).
                  And little or no turbulence- camera appears steady and don’t see wing flex. Screams fly-by/air show/something.

                  I sure hope they briefed and consulted manuals, checklists and placards.
                  Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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