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Thread: British Airways fly-by salute, or...

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    Senior Member Evan's Avatar
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    Default British Airways fly-by salute, or...

    https://www.youtube.com/watch?v=IkGTg_UOBNY

    Something's not cricket here. PIO?

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    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    https://www.youtube.com/watch?v=IkGTg_UOBNY

    Something's not cricket here. PIO?
    It looks intentional, based on the spoiler inputs (and the lack of screams inside the plane).

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    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    It looks intentional, based on the spoiler inputs (and the lack of screams inside the plane).
    The only other thing I can imagine is a flight control issue, perhaps an issue with the flight stick or perhaps something delaying pilot commands to the control surfaces, perhaps resolved by a priority take-over. But I don't have any precedent for that. So it's looking to me like PIO.

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    Quote Originally Posted by Gabriel View Post
    It looks intentional, based on the spoiler inputs (and the lack of screams inside the plane).
    Diverted to Malaga due to strong winds

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    It was a go-around from 300 ft in the middle of a tight turn.
    https://flightaware.com/live/flight/...835Z/EGLL/LEMG

    I would still not call it PIO in the traditional way since I see no overshooting due to inertia and lack of dampening, at least in the last couple of oscillations that were captured from inside the plane. It looks more like the pilot (or autopilot) overcorrecting the TRACK while trying to keep the plane in the very narrow space between the Spanish airspace and the Rock, both of which they are not allowed to overfly.

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    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    It was a go-around from 300 ft in the middle of a tight turn.
    https://flightaware.com/live/flight/...835Z/EGLL/LEMG
    121kts

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    Quote Originally Posted by Evan View Post
    121kts
    Groundspeed. They had like 25 knots of headwind on final.

    Metars:
    LXGB 251050Z 09023G33KT 9999 FEW025 16/10 Q1032 NOSIG=
    LXGB 250950Z 08026G36KT 9999 FEW022 16/10 Q1032 TEMPO SCT022=
    LXGB 250930Z 09025G35KT 9999 FEW020 16/10 Q1031 BLU TEMPO SCT020 WHT=

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    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    Groundspeed.
    Aha.

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    Lee wave rotor?

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    Senior Member Gabriel's Avatar
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    Quote Originally Posted by pegasus View Post
    Lee wave rotor?
    Looks like you are the winner!!!

    https://www.youtube.com/watch?v=tjv3xBB-HmY

    https://www.youtube.com/watch?v=3BkOgZPjZX4

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    Quote Originally Posted by Gabriel View Post
    Us glider pilots know a thing or two about wave, A fellow pilot has flown 1000km and returning to home base using the phenomenon.Incidentally, I think there is an error in the Mentour presentation, the lee wave rotor was created by a Northerly wind not one from the South.

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    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    Looks like you are the winner!!!

    https://www.youtube.com/watch?v=3BkOgZPjZX4
    Great video. I'd love to see a DFDR plot of this event, to see how A/P handled the roll excursions. My guess is that there would be very little, if any, rudder involved. I also wonder if the autopilot inputs were excessive or poorly timed. Has there ever been a case of auto-pilot induced occillation?

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    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    My guess is that there would be very little, if any, rudder involved.
    Probably, especially if it was on AP.

    I also wonder if the autopilot inputs were excessive or poorly timed. Has there ever been a case of auto-pilot induced oscillation?
    It shouldn't. APs (even old analog ones) have at least what is called a PID control system (Proportional-Derivative-Integrator). Modern digital APs have even more robust control logic, but basically this kind control system has a target bank angle, in function of the difference between the target and actual bank angles it establishes a target roll rate, and in function of the difference between the target roll rate and the actual roll rate it establishes a control input. In other words, even if the AP wants the plane to roll to the right towards a specific bank angle (say it is in a left turn and wants to return to wings level), the control input may be to the left if the plane is already rolling to the right towards the target at a rate higher than the target rate. And the target roll rate diminishes as you approach the target bank (and becomes zero at the target). With the parameters correctly set, an AP should be at least critically dampened and should not have PIO. Now, being this a digital AP, there can be a bug or some specific circumstances or combination of variables that was not foreseen when developing the code. But it would be strange that it had not surfaced until this week. The A320 has too many years in service for something like this to remain hidden.

    What is strange here is that in the video you can clearly see that the control inputs to "level the wings" persist even after the plane has already overshot the wings-level condition, as if it intentionally wanted to bank to the other side now. That is not PIO.

    That's why I said that it looks more like the pilot (or autopilot) over-correcting the TRACK while trying to keep the plane in the very narrow space between the Spanish airspace and the Rock, both of which they are not allowed to overfly. Like a PIO but not in roll but in track (which is colloquially called "sewing").

    I am totally speculating now, but perhaps the AP was set in a lateral mode that it would track a VOR radial or the ILS LOC, which become more sensitive as you approach the station (because you cover more angular deviation from the same lateral deviation). Old APs were known to have problems with that, causing "sewing" (and human pilots too), so the practice was to change the AP from NAV to HDG when you were close to the station. But new APs should be immune to that. They know the distance to the station so they know what is the actual lateral deviation for a given angular deviation. They also have INS information to integrate in the logic and even without all that, given the TAS (which they know) they could easily "reverse-calculate" the actual lateral deviation from the speed that the angular deviation changes when the plane changes the heading during the maneuver.

    Now say that. for whatever reason, this was not the case. Say that the AP had a strike of "nostalgia" for the old APs, or that something went wrong with the tricks to prevent "sewing" or that the human pilot was blindly following the commands of the FD (which should also be immune to sewing, but say that it was not). This would very nicely explain why so many roll "excursions" (that were at the same time well controlled so the plane never exceeded the normal bank angle limits) and suddenly everything stopped.... the instant the pilot selected HDG mode.

    I'd also love to see the FDR (and CVR would be interesting too, if not funny). But since this apparently didn't even qualify as an incident, chances are that we will never know.

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    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    This would very nicely explain why so many roll "excursions" (that were at the same time well controlled so the plane never exceeded the normal bank angle limits) and suddenly everything stopped.... the instant the pilot selected HDG mode.
    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.

    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? 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.

    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.

    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)... ?

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    Quote 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.

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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.

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    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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    Senior Member Gabriel's Avatar
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    Quote 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

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    Senior Member Evan's Avatar
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    Quote 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.

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    Quote 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.

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