Originally posted by Evan
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A380 Rudder Reversals
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Originally posted by Evan View PostNow 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
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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.Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.
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Originally posted by Schwartz View PostAlso, it sure looks like there is significant left rudder applied at 9 seconds.
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...
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Originally posted by 3WE View PostBy the way...
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.
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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.***
...and you have seen other crosswind techniques- in particular the one where you simply touch down in a 'full' crab?Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.
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Originally posted by 3WE View PostYou'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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Originally posted by Evan View PostAm 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?
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.
--- 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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Originally posted by Gabriel View PostIf 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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Originally posted by Gabriel View PostIf you believe that bounces is the result of hard landings, you didn't understand how the shock absorbers work.
-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.
Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.
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Originally posted by Evan View Post***STILL PHOTOGRAPHS***
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.Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.
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Originally posted by 3WE View PostPlease note the following, highly technical concept: F = M * A
I will speculate from your still photographs that M is very impressive.
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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.***
Either those multiple F(s) sum into something structurally significant.
OR
They don't.Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.
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Originally posted by 3WE View PostOk 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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Originally posted by Evan View Post1. 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.
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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