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Thread: A380 Rudder Reversals

  1. #41
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    Quote Originally Posted by Gabriel View Post
    BBobby, just out of curiosity, how well (if at all) would the autoland manage a situation like the one faced by this A380? (you can limit your answer to a 747-4 and 747-800 autoland if you want)
    Interesting that you ask this question...You know I'm not a details person, but I thought autoland was a written procedural no-no when the wind became 'significant' and that it is designed more for super-fine adjustments while you land in a reasonably calm fog bank.

    Is this a trick question?
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    Quote Originally Posted by Gabriel View Post
    Do you realize how close that was from an engine strike?
    AND- Evan's video is NOT a gusty crosswind landing as much as an 'unstabilized-not-so-slick' turn to final at Kai Tek.

    The guy is not fighting the wind per se, as much as he is forcing a huge airplane to get lined up from a very cramped 'base-to-final' turn that he didn't set up as well as he should have...the lack of a gusty cross wind means that we do have steady rudder inputs in this video.

    As is clear from all of these discussions- there are a wide variety of landings (and an endless variety of wind gusts) and a variety of suggested crosswind techniques...Review additional YouTubes...you will see rudder reversals on some...it's not rare, it's not the end of the world, it's not bad training.

    Someone also made a very interesting comment...that they weren't overly concerned with a perfectly-straight touchdown...That comment is insightful in numerous ways- not the least of which is that (ironingly) we see plenty of sideways touchdowns that come from over-correction (See Footnote).

    Footnote: We also see plenty of touchdowns that come from a very pure crab into the wind with virtually no 'kick-out' nor 'wing dip'...and I do believe that is an FCOM-blessed procedure as well as being better than many of the over-corrected touchdowns.
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  3. #43
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    Quote Originally Posted by Evan View Post
    ***Watch as many of thee as you want, watch the rudder and watch the bank angle.***
    I have.

    As I state above- Pilot inputs vary- but rudder reversals are not rare, not the end of the world, not bad technique and not bad training...[Your black and white thinking blinds you to the ones where rudder reversals occur].

    Also see above- please look at gusty CROSSWIND videos and not botched-last-minute-alignment-at-Kai-Tek videos...While SOME of the techniques may be related, the 'gusty' part requires a different pace of control inputs. [Your black and white thinking blinds you from distinguishing stabilized left and right track adjustments from crosswind gusts- these evolve into a gray area.]

    ...Actually after the A-380 dudes over corrected to the left, I don't fault them for going "oh-crap-right". That was probably a reasonable move for the tires, landing lights, paperwork department, PR department AND their careers. [Your black and white thinking blinds you from seeing why it might be reasonable to switch to right rudder (at~140 knots) when you are making a beeline for the left edge of the runway...no...one must never ever switch from right to left, without pausing at neutral].
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    Quote Originally Posted by Evan View Post
    The thing is, I'm not a pilot and I never pretended to be one so I'm not going to get into an argument over the AMOUNT of control input that is needed. I am concerned here with the NATURE of the control input.
    Not sure I follow your logic, namely why not being a pilot enables you to only argue about one of the above. I would think it would be both or neither.

  5. #45
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    Quote Originally Posted by ATLcrew View Post
    Not sure I follow your logic, namely why not being a pilot enables you to only argue about one of the above. I would think it would be both or neither.
    The actual AMOUNT of control input needed is of course what you learn by doing, by flying in specific situations on specific types. However, the PHILOSOPHY of how you should use control surfaces can be determined by people without without even a single hour of flight time, people known as engineers and incident investigators, through research alone. I read what they publish, all those manuals, reports and periodicals, and I try to learn from that knowledge. But when it comes time to decrab an A380 in a gusting crosswind, I can't tell you specifically how much bank that requires or how much rudder. What I can tell you is that it should never involve large rudder reversals on an aircraft like this because that is going to get you into an overcontrol PIO situation really fast. I also know that a pilot with a tendency towards unnecessarily aggressive rudder inputs on large transport aircraft (which might not be the case here) is a potential threat to the aircraft and everyone on it. As I said at the beginning, this may simply be a case of aircraft-pilot coupling and may not be indicative of poor airmanship. I just hope that, when they get the bill for the tires and the gear inspection/overhaul they will be inspired to find out.

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    Quote Originally Posted by Evan View Post
    Now open your mind. Take a look at this one (or any of the hundreds of others). Notice the use of rudder. Notice the use of bank. You see rudder INPUT (not occillation) in one direction, then neutral rudder, then a modest amount in the opposite direction needed to decrab ALONG WITH BANK, DOWN LOW and a touchdown IN A BANK in what is obviously a very significant crosswind. There are no reversals involved. And it all ends with the aircraft stable on the centerline, not zigzagging down the runway. Watch as many of thee as you want, watch the rudder and watch the bank angle.

    https://www.youtube.com/watch?v=nehZ9v4YB6o
    The a380 has a much wider wingspan (80m) than the 747 in the video which means the pilot is going to have less room to bank. Adding to Gabriel's comments about engine strike, I'm assuming that landing on Youtube was in conditions with a strong but constant crosswind. I imagine in gusty winds near the ground, the last thing the pilot wants to do is risk putting the wingtips toward the ground through banking -- that german wing strike comes to mind.

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    Quote Originally Posted by Evan View Post
    Now open your mind. Take a look at this one (or any of the hundreds of others). Notice the use of rudder. Notice the use of bank. You see rudder INPUT (not occillation) in one direction, then neutral rudder, then a modest amount in the opposite direction needed to decrab ALONG WITH BANK, DOWN LOW and a touchdown IN A BANK in what is obviously a very significant crosswind. There are no reversals involved. And it all ends with the aircraft stable on the centerline, not zigzagging down the runway. Watch as many of thee as you want, watch the rudder and watch the bank angle.

    https://www.youtube.com/watch?v=nehZ9v4YB6o
    Also, it sure looks like there is significant left rudder applied at 9 seconds.

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    By the way...

    1. Exactly how many rudder reversal crashes have occurred in the history of aviation?
    2. Is there any evidence that this landing overstressed the tail?
    3. Is there any evidence that this landing overstressed the landing gear?
    4. How many run off the side crashes have occurred in the history of aviation?
    5. Look at 1 through 4 as a group and please explain what the problem is here? (aside from a 'remarkably' ugly-LOOKING landing.)
    6. I'm going to stick to why well-trained pilots (albeit rarely) sometimes execute the universal deliberate stall procedure to a high degree of accuracy.
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  9. #49
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    Quote Originally Posted by Schwartz View Post
    Also, it sure looks like there is significant left rudder applied at 9 seconds.
    Yes, IN ONE DIRECTION. That is what the rudder is there for, to compensate for things like crosswind and engine failure. The issue here is active rudder REVERSALS which can lead to pilot-induced occillations, overcontrol, upset and even loss of control. At higher airspeeds it can also lead to structural failure, as we have witnessed. The ONLY defense is proper training on rudder and vigilance by other crewmembers.

    If you in a situation where line-up without the risk of engine strike requires a large, sustained rudder reversal, you need to abandon that approach and try again. The reason for this is that if you continue to land you will be touching down with a high unarrested yaw rate (as well as bank) and then, well, just watch the video...

  10. #50
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    Quote Originally Posted by 3WE View Post
    By the way...
    1. Exactly how many rudder reversal crashes have occurred in the history of aviation? I'm not sure off the top of my head, but isn't one enough for you?
    2. Is there any evidence that this landing overstressed the tail? I never even implied that. That issue here is overcontrol and upset. The risk is high-speed runway departure, gear collapse, engine strike, etc...
    3. Is there any evidence that this landing overstressed the landing gear? There is a large vertical force and at the same time a large lateral force applied. An inspection should be mandatory here. I wouldn't be surprised if things needed to be repaired or replaced.
    4. How many run off the side crashes have occurred in the history of aviation? How many mid-air center fuel tank explosions have there been? I'm not going to play "how many" with you. One is too many. And two represents a failure to learn from it.
    5. Look at 1 through 4 as a group and please explain what the problem is here? (aside from a 'remarkably' ugly-LOOKING landing.) See my above post to Schwartz.
    6. I'm going to stick to why well-trained pilots (albeit rarely) sometimes execute the universal deliberate stall procedure to a high degree of accuracy. I'm all for well-trained pilots.

  11. #51
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    Quote Originally Posted by Evan View Post
    ***There is a large vertical force and at the same time a large lateral force applied. An inspection should be mandatory here. I wouldn't be surprised is things needed to be repaired or replaced.***
    You're sure about that?

    ...and you have seen other crosswind techniques- in particular the one where you simply touch down in a 'full' crab?
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  12. #52
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    Quote Originally Posted by 3WE View Post
    You're sure about that?

    ...and you have seen other crosswind techniques- in particular the one where you simply touch down in a 'full' crab?
    Am I sure that this merits a mandatory close gear inspection? Yes. You have vertical forces large enough to result in a bounce combined with repeated large lateral forces in opposite directions. You don't wan't a gear collapse on the next hard landing do you? Do you understand WHY flying is so safe these days?

  13. #53
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    Quote Originally Posted by Evan View Post
    Am I sure that this merits a mandatory close gear inspection? Yes. You have vertical forces large enough to result in a bounce combined with repeated large lateral forces in opposite directions. You don't wan't a gear collapse on the next hard landing do you? Do you understand WHY flying is so safe these days?
    If you believe that bounces is the result of hard landings, you didn't understand how the shock absorbers work.
    This landing doesn't take place at too high vertical speeds.The reason for the bounce is that the wing is giving more than 1G lift at the moment of touchdown (as result of flare and ground effect), that greater-than-1G is what is reducing the vertical speed (which is of course still negative at the moment of touchdown, otherwise you would not touchdown), So when the plane touches down the shock absorbers absorb the vertical speed but, since the wings are still making more that 1G-lift, the plane is still accelerating up, and up we go again. So initially there is no great vertical forces (the shock absorbers do not need to bare weight, just to cancel the vertical speed, and they are assisted by the wing while doing that). The vertical force in the shock absorbers is greater when the spoilers extended and the nose touched down than during the bounce. That means that all the lateral skidding that takes place during the "low vertical force" period also does'n not impose a too hard lateral load, since the friction has an upper cap at mu * vertical. I need to go back and check the certification standards, but I believe that basically you cannot damage the landing gear with lateral forces while the weight-on-wheel (or on the shock abosrber) is not more than its share of the airplane weight.

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  14. #54
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    If you believe that bounces is the result of hard landings, you didn't understand how the shock absorbers work.
    This landing doesn't take place at too high vertical speeds.The reason for the bounce is that the wing is giving more than 1G lift at the moment of touchdown (as result of flare and ground effect), that greater-than-1G is what is reducing the vertical speed (which is of course still negative at the moment of touchdown, otherwise you would not touchdown), So when the plane touches down the shock absorbers absorb the vertical speed but, since the wings are still making more that 1G-lift, the plane is still accelerating up, and up we go again. So initially there is no great vertical forces (the shock absorbers do not need to bare weight, just to cancel the vertical speed, and they are assisted by the wing while doing that). The vertical force in the shock absorbers is greater when the spoilers extended and the nose touched down than during the bounce. That means that all the lateral skidding that takes place during the "low vertical force" period also does'n not impose a too hard lateral load, since the friction has an upper cap at mu * vertical. I need to go back and check the certification standards, but I believe that basically you cannot damage the landing gear with lateral forces while the weight-on-wheel (or on the shock abosrber) is not more than its share of the airplane weight.
    Sorry, I forgot to painfully carefully about what I say around you. No I don't imagine the plane is merely bouncing off the ground like a rubber ball. I realize it is still flying and since the pilot has flown it down hard rather than letting it settle, the flare AoA and airspeed are lifting it off again, etc, etc. Now, maybe I'm wrong, but it certainly looks to me like the descent is arrested by a) the ground and then b) the compressiblity of the tires and then c) the shock absorber travel limit followed by d) structural things. In other words, the possibility of some impact on the gear strut attachment structures due to the shock absorber bottoming out. Not a million pounds of impact, but an impact significant enough to warrant some concern. However, if it were simply that, it probably would not warrant a detailed inspection. But in this case, the gear is also touching down with lateral and torquing forces, and beyond that, it is experiencing repeated lateral and torquing forces in both directions. Considerable A380 forces, and yes, the wing spoilers are deployed at that point.
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    Quote Originally Posted by Gabriel View Post
    If you believe that bounces is the result of hard landings, you didn't understand how the shock absorbers work.
    This involves gray areas, and thus is largely incomprehensible to Evan.

    -You have softer landings.
    -Harder-totally-normal landings.
    -Harder landings where pilots give each other grief.
    -Harder landings that really should not happen.
    -And finally the rare stuff that has genuine structural implications.

    And floating up and down this scale is the very subjective rating of looking ugly.

    Visual subjectivity is another area Evan struggles with. He see's that those guys really might have torqued the gear and there is no gray area..."The gear was likely over stressed (no gray) and must be subjected to special formal inspections"...

    Contrasting with this is the unbelievable comment from one Airbus pilot who said it's in the FCOM that it's OK (if not sort of almost recommended?) to land in a 'full' crab...(which isn't that different than the pilot inspired sideways touchdown.) (Who in the hell would want to listen to an Airbus pilot anyway?)

    Also- In one sentence Evan says it's bad to run off the side. In another he objects to hard right rudder as the plane starts heading strongly to the left...

    He still sees that the extra second of hard left ruder is a training problem. I continue to recommend that Evan take a little more time driving his vehicle pictured below, so that he can see for himself that sometimes, humans make minor errors in control inputs that are not preventable, nor are they the end of the world.

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    Quote Originally Posted by Evan View Post
    ***STILL PHOTOGRAPHS***
    Please note the following, highly technical concept: F = M * A

    I will speculate from your still photographs that M is very impressive.

    I will make no speculation at all whatsoever (very black and white of me), regarding the magnitude of A from a still photograph. (Oh, maybe the gear is compressed but hey, maybe I've seen that before on youtubes of even normal landings.)

    Furthermore, speculation of the magnitidue of A from a telephoto youtube is problematic at best. I have yet to see Gabby, Bobby, Schwartzy, ATLie nor 3BSie stating that it is clear to them that this was a structurally significant landing.
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    Quote Originally Posted by 3WE View Post
    Please note the following, highly technical concept: F = M * A

    I will speculate from your still photographs that M is very impressive.
    Well, I'm not Gabriel but I suspect it is A that is making them impressive. And those oleo struts are taking more F because of it. For them the trick is to reduce the A and thus dampen the F because the M is the constant here. Anyway, what we are taking about here are simultaneous F(s), vertical, lateral and torsional. Even more highly technical.

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    Quote Originally Posted by Evan View Post
    ***Anyway, what we are taking about here are simultaneous F(s), vertical, lateral and torsional. Even more highly technical.***
    Ok then. Given that higher level of understanding, you should have no problem with this concept:

    Either those multiple F(s) sum into something structurally significant.

    OR

    They don't.
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    Quote Originally Posted by 3WE View Post
    Ok then. Given that higher level of understanding, you should have no problem with this concept:

    Either those multiple F(s) sum into something structurally significant.

    OR

    They don't.
    That they sum into something POTENTIALLY structurally significant is my only concern here. The A380 gear is way overbuilt, but that video seems to be one way to test the limits.

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    Quote Originally Posted by Evan View Post
    1. Exactly how many rudder reversal crashes have occurred in the history of aviation? I'm not sure off the top of my head, but isn't one enough for you?
    2. Is there any evidence that this landing overstressed the tail? I never even implied that. That issue here is overcontrol and upset. The risk is high-speed runway departure, gear collapse, engine strike, etc...
    3. Is there any evidence that this landing overstressed the landing gear? There is a large vertical force and at the same time a large lateral force applied. An inspection should be mandatory here. I wouldn't be surprised if things needed to be repaired or replaced.
    4. How many run off the side crashes have occurred in the history of aviation? How many mid-air center fuel tank explosions have there been? I'm not going to play "how many" with you. One is too many. And two represents a failure to learn from it.
    5. Look at 1 through 4 as a group and please explain what the problem is here? (aside from a 'remarkably' ugly-LOOKING landing.) See my above post to Schwartz.
    6. I'm going to stick to why well-trained pilots (albeit rarely) sometimes execute the universal deliberate stall procedure to a high degree of accuracy. I'm all for well-trained pilots.
    Hi Evan,

    I may be remembering incorrectly, but I thought in the one case of the New York crash there were several contributing factors:
    1) The pilot made several alternating full deflection rudder control inputs unnecessarily to deal with wake turbulence
    2) The A300 did NOT have fly by wire and thus the rudder inputs at high speed were extremely sensitive to control inputs and it was easy for the pilot to call for full deflection. My understanding is that in most fly by wire jets, the controls are dampened at higher speed meaning the pilot has to provide a lot more pressure at the limit of the pedals to get a full deflection
    3) The pilots were trained by AA to aggressively deal with wake turbulence which was unnecessary in large airliners

    In this case:
    1) The A380 is fly by wire and undoubtedly the rudder controls are quite different from the A300 that crashed
    2) This plane was landing at low speed and not at full maneuvering speed
    3) I don't see anywhere near full rudder deflection. The A380 looks like it has two separate control surfaces on the rudder and the first few larger deflections before touchdown only involve one of the surfaces. Once the plane is down, the two segments seem to operate together, but again no where near full deflection.

    So, it doesn't seem like any the conditions that caused the A300 NY crash were present here.

    EDIT: Just to add here, this landing looked ugly because the pilot overcompensated on the first couple oscillations. It was similar to a car at high speed who over swerves at first to avoid something that fell out of the car ahead. You can't even do a full steering deflection, but it can cause loss of control or an ugly looking swerve oscillations for a few seconds while the driver recovers. This is what it looks like to me.

    One question I have for the big pilots, is the nose wheel steering and rudder coordinated by the computer when landing one of these big jets?

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