A380 Rudder Reversals

[BoeingBobby]

Changing the subject- I am amazed at all of the swerving and sliding that SEEMS to be depicted on these crosswind videos...

...did you ever do crosswind landings that resulted in 'impressive sideways movement'.

Sometimes it seems like there's two variations and no middle ground.

Variation 1: A well-timed crab kickout with a nice SLIGHT upwind wing dip and relatively straight touchdown.
Variation 2: Rather nasty-looking sideways touchdown. (Although I believe more and more that it only LOOKS nasty- sure the passengers would feel some swerves, but not a near-death expereince).

-Heck, a little back and forth swerving on a typical takeoff is pretty common from my experience in 23A.

Me? Never! All of mine were like butter! cough...cough

[Evan]

1. So what is the difference between 'a bit of overswing' and 'a reversal'?

A reversal, which we are witnessing here, is a successive cyclic rudder deflection from one direction to the opposite direction without first centering it in the neutral position. It doesn't appear to be full rudder deflection reversals but who knows. The structural danger in doing this probably isn't present at landing speeds, but overcontrol can result and this can lead to further occillations, as I believe we seeing here.

An overswing results when a rapid rudder input is applied. An overswing is a sideslip angle that is initially greater than the steady-state sideslip angle resulting from the same rudder input and occurs because of the slightly underdamped nature of the airplane’s motion in the yaw axis.

[Evan]

No.

Yes. You should read the report some time instead of living in denial of the fact that a large transport aircraft rudder is not designed to be used like that of a Cessna.

If you don't trust the NTSB, you can read the same thing from Boeing, Airbus and the FAA.

[Schwartz]

I would love to hear from Gabriel, but I'm guessing that once the nose gear is down, the nose friction is going to provide the overriding force controlling direction in these cases. In the video above, it only looks like the main gear skidded sideways just a small bit on the outside of two of the swerves (there was a small puff of rubber smoke on 2 of the main gears wheel assemblies). The rest of the "sliding" is actually the plane zig zagging on the runway with the main wheels following the track of the nose, just like a car would.

[Gabriel]

Well...blue font really.

The sarcasm was so obvious that I didn't even realize about the blue font.

[Gabriel]

I have things to say here, but I don't have time now.
But I will say this. The video is long with several differently edited takes of the same approach and landig, you have close ups, slomo, longer takes..
Watch it ALL and, above all, appreciate the vegetation. Seriously, that's a cue.

[Roshni]

I liked this crosswind video. Being a newbie I am knowing and getting familiar with different things.

[Black Ram]

A reversal, which we are witnessing here, is a successive cyclic rudder deflection from one direction to the opposite direction without first centering it in the neutral position. It doesn't appear to be full rudder deflection reversals but who knows. The structural danger in doing this probably isn't present at landing speeds, but overcontrol can result and this can lead to further occillations, as I believe we seeing here.

It's a FBW Airbus, so the pilots can do whatever they want and it's still safe.

[Gabriel]

Cue 2 and 3...

Why does it require to have a sing down and the ball off-center to keep coordinated flight with an engine our?
May the force be with you.

(cue 1 was "appreciate the vegetation")

[Gabriel]

Ok, here is my analysis, for whatever is worth that I know some think is nothing if not less than that.

Disclaimer, I have not seen any METARs or weather info, and my analysis is bases solely in what's seen in the video + laws of physics.

The plane was approaching with a 1) fair crosswind from the right (as seen from the pilot's seat, which is from the left of the video frame).
The wind was obviously not steady, there was some 2) turbulence, 3) some windshare (significant change in wind intensity and/or direction that takes place in a small vertical or horizontal distance), and 4) wind gradient (gradual change in wind speed with altitude).

The 1 is obvious from the crab to the right (ok, I mean a crab with the nose to the right compared to the flight path).

The first 2 are obvious for the video take that captures a larger part of the approach. In that video it looks like the turbulence is very strong and sudden, but keep in mind that this take is played at higher than normal speed.

The last is obvious from the vegetation. The leaves of the trees barely move, indicating a low surface wind.

The pilot had to fight with all that, and they didn't do it great.

Shortly (but not immediately) before landing, the plane starts to deviate to the right even when the nose was still pointing in the same general direction. This is probably due to a reduction in the crosswind speed.

Then, immediately before and during touchdown, the pilot applies what seems to be a full travel and somehow long left rudder input. This has 2 effects: of course the nose goes to the right. But why does the nose that? Because the left rudder generates a left yawing moment. Why? Because the left rudder generates a right lift on the fin which, combined with the long arm of the fin behind the CG, generates a left yawing moment.

But what happens with the right force to begin with? Yes, moment, yaw, yadayada. But what about F=ma?
Exactly. Big left rudder deflection = big lift on the fin towards the right = big acceleration towards the right.

The plane, which was already starting to move the the right due to the reduction of the crosswind, suddenly has a significant force to the right. So right we go.

The pilot keeps the large left rudder input through the touchdown, and not only as needed to align the nose with the runway, the pilot keeps it through the nose passing from aiming left to aiming right. The pilot was trying to steer the plane with yaw, something that is not effective until the wheels are on the ground, the spoilers fully extended, and the noseweel down on the ground. That's why most of the acceleration to the right happens with the main wheels already on the ground, despite the nose of the plane moving left and against the main wheels skidding to the right.

The spoilers extend, the nose go down, and now we have a significant weight on the main wheels and hence a significant traction and, with the nose pointing left, left we go.

At that point we enter a PIC or PAC (pilot-induced oscillation or pilot-airplane coupling). The pilot wants to steer for yaw, instead of steering for yaw rate (which is what you need to avoid or kill an oscillation). So, for example, the pilot reverse from rudder significantly left to rudder significantly right when the yaw goes through what the pilot feels is the right heading to go back to the centerline, but then the plane keeps yawing left a little more even with right rudder to inertia. Then the left yawing stops and, still with right rudder applied, it starts yawing right and passes through a point where it is aligned with the runway heading and more or less on the centerline. At that point the pilot reverses again to significant left rudder, but the plane keeps yawing right a bit more due to inertia.

The oscillation is however damped and after a few come and go the plane ends up on a solid and stable ground track.

The oscillation could have been killed much earlier had the pilot applied reverse rudder before reaching the target yaw, so as the yaw rate would slow down and stop by the target (or close), then center the rudder at zero yaw rate as to not induce any new yaw rate, and then fine tune it as needed.

Overall, it was a good landing by definition, but not a nice one, the pilot did a fair job in challenging conditions, but could have done better.

[BoeingBobby]

Ok, here is my analysis, for whatever is worth that I know some think is nothing if not less than that.

Disclaimer, I have not seen any METARs or weather info, and my analysis is bases solely in what's seen in the video + laws of physics.

The plane was approaching with a 1) fair crosswind from the right (as seen from the pilot's seat, which is from the left of the video frame).
The wind was obviously not steady, there was some 2) turbulence, 3) some windshare (significant change in wind intensity and/or direction that takes place in a small vertical or horizontal distance), and 4) wind gradient (gradual change in wind speed with altitude).

The 1 is obvious from the crab to the right (ok, I mean a crab with the nose to the right compared to the flight path).

The first 2 are obvious for the video take that captures a larger part of the approach. In that video it looks like the turbulence is very strong and sudden, but keep in mind that this take is played at higher than normal speed.

The last is obvious from the vegetation. The leaves of the trees barely move, indicating a low surface wind.

The pilot had to fight with all that, and they didn't do it great.

Shortly (but not immediately) before landing, the plane starts to deviate to the right even when the nose was still pointing in the same general direction. This is probably due to a reduction in the crosswind speed.

Then, immediately before and during touchdown, the pilot applies what seems to be a full travel and somehow long left rudder input. This has 2 effects: of course the nose goes to the right. But why does the nose that? Because the left rudder generates a left yawing moment. Why? Because the left rudder generates a right lift on the fin which, combined with the long arm of the fin behind the CG, generates a left yawing moment.

But what happens with the right force to begin with? Yes, moment, yaw, yadayada. But what about F=ma?
Exactly. Big left rudder deflection = big lift on the fin towards the right = big acceleration towards the right.

The plane, which was already starting to move the the right due to the reduction of the crosswind, suddenly has a significant force to the right. So right we go.

The pilot keeps the large left rudder input through the touchdown, and not only as needed to align the nose with the runway, the pilot keeps it through the nose passing from aiming left to aiming right. The pilot was trying to steer the plane with yaw, something that is not effective until the wheels are on the ground, the spoilers fully extended, and the noseweel down on the ground. That's why most of the acceleration to the right happens with the main wheels already on the ground, despite the nose of the plane moving left and against the main wheels skidding to the right.

The spoilers extend, the nose go down, and now we have a significant weight on the main wheels and hence a significant traction and, with the nose pointing left, left we go.

At that point we enter a PIC or PAC (pilot-induced oscillation or pilot-airplane coupling). The pilot wants to steer for yaw, instead of steering for yaw rate (which is what you need to avoid or kill an oscillation). So, for example, the pilot reverse from rudder significantly left to rudder significantly right when the yaw goes through what the pilot feels is the right heading to go back to the centerline, but then the plane keeps yawing left a little more even with right rudder to inertia. Then the left yawing stops and, still with right rudder applied, it starts yawing right and passes through a point where it is aligned with the runway heading and more or less on the centerline. At that point the pilot reverses again to significant left rudder, but the plane keeps yawing right a bit more due to inertia.

The oscillation is however damped and after a few come and go the plane ends up on a solid and stable ground track.

The oscillation could have been killed much more earlier had the pilot applied reverse rudder before reaching the target yaw, so as the yaw rate would slow down and stop by the target (or close), then center the rudder at zero yaw rate as to not induce any new yaw rate, and then fine tune it as needed.

Overall, it was a good landing by definition, but not a nice one, the pilot did a fair job in challenging conditions, but could have done better.

Once again, ^^^ 100% ^^^

[Evan]

Ok, here is my analysis, for whatever is worth that I know some think is nothing if not less than that.

Disclaimer, I have not seen any METARs or weather info, and my analysis is bases solely in what's seen in the video + laws of physics.

The plane was approaching with a 1) fair crosswind from the right (as seen from the pilot's seat, which is from the left of the video frame).
The wind was obviously not steady, there was some 2) turbulence, 3) some windshare (significant change in wind intensity and/or direction that takes place in a small vertical or horizontal distance), and 4) wind gradient (gradual change in wind speed with altitude).

The 1 is obvious from the crab to the right (ok, I mean a crab with the nose to the right compared to the flight path).

The first 2 are obvious for the video take that captures a larger part of the approach. In that video it looks like the turbulence is very strong and sudden, but keep in mind that this take is played at higher than normal speed.

The last is obvious from the vegetation. The leaves of the trees barely move, indicating a low surface wind.

The pilot had to fight with all that, and they didn't do it great.

Shortly (but not immediately) before landing, the plane starts to deviate to the right even when the nose was still pointing in the same general direction. This is probably due to a reduction in the crosswind speed.

Then, immediately before and during touchdown, the pilot applies what seems to be a full travel and somehow long left rudder input. This has 2 effects: of course the nose goes to the right. But why does the nose that? Because the left rudder generates a left yawing moment. Why? Because the left rudder generates a right lift on the fin which, combined with the long arm of the fin behind the CG, generates a left yawing moment.

But what happens with the right force to begin with? Yes, moment, yaw, yadayada. But what about F=ma?
Exactly. Big left rudder deflection = big lift on the fin towards the right = big acceleration towards the right.

The plane, which was already starting to move the the right due to the reduction of the crosswind, suddenly has a significant force to the right. So right we go.

The pilot keeps the large left rudder input through the touchdown, and not only as needed to align the nose with the runway, the pilot keeps it through the nose passing from aiming left to aiming right. The pilot was trying to steer the plane with yaw, something that is not effective until the wheels are on the ground, the spoilers fully extended, and the noseweel down on the ground. That's why most of the acceleration to the right happens with the main wheels already on the ground, despite the nose of the plane moving left and against the main wheels skidding to the right.

The spoilers extend, the nose go down, and now we have a significant weight on the main wheels and hence a significant traction and, with the nose pointing left, left we go.

At that point we enter a PIC or PAC (pilot-induced oscillation or pilot-airplane coupling). The pilot wants to steer for yaw, instead of steering for yaw rate (which is what you need to avoid or kill an oscillation). So, for example, the pilot reverse from rudder significantly left to rudder significantly right when the yaw goes through what the pilot feels is the right heading to go back to the centerline, but then the plane keeps yawing left a little more even with right rudder to inertia. Then the left yawing stops and, still with right rudder applied, it starts yawing right and passes through a point where it is aligned with the runway heading and more or less on the centerline. At that point the pilot reverses again to significant left rudder, but the plane keeps yawing right a bit more due to inertia.

The oscillation is however damped and after a few come and go the plane ends up on a solid and stable ground track.

The oscillation could have been killed much earlier had the pilot applied reverse rudder before reaching the target yaw, so as the yaw rate would slow down and stop by the target (or close), then center the rudder at zero yaw rate as to not induce any new yaw rate, and then fine tune it as needed.

Overall, it was a good landing by definition, but not a nice one, the pilot did a fair job in challenging conditions, but could have done better.

WIth a fin of that magnitude, do you really need reversals to get things centered? I would expect the plane to recover from the gust without much, if any, opposite rudder correction, and all that would be necessary is what is needed to decrab before touching down. These were very extreme, gusting conditions, so a steady-state crosswind input is going to experience some upset but is it really better to use rudder to recover or to simply let the plane recover on its own? I've read lit from Boeing, Airbus and the FAA emphasizing that rudder controls on a large transport aircraft are only intended for steady-sideslip and engine failure. The Boeing 'maxim' was "If you are in a stall, don't use rudder. If you are not in a stall, you don't need rudder" (obviously that doesn't include very high AoA upsets where rudder is all you have left). But the operative word there is "need", as the force of the fin itself will facilitate recovery from yaw excursions.

The reason I posted this is to question whether some pilots are STILL not being trained properly for rudder on these big jets and are STILL using them with a small plane mentality.

[Gabriel]

WIth a fin of that magnitude...,

The fin of that magnitude, the yaw inertia is of that magnitude.

do you really need reversals to get things centered?

No, as I clearly explained in my previous post. Neither with a fin of that magnitude or with a C-152.
The problem is when you hold the input too much, then you may need some reversal. Actually, rudder reversal is the normal input done by the yaw damper.
input = -k1*yaw - k2*(yaw rate) -k3*(yaw integral in the last t seconds).
What is called a PID control.

I would expect the plane to recover from the gust ....

What gust? I don't think that the yaw upset was caused by a gust. Rather, by... ok, read my previous post.

By the way, there was not only a yaw upset, but also a lateral lineal motion upset. To a point, the pilot had to do something to keep the plane from going out to the right of the runway. Hadn't the pilot apply rudder, we would be reading an AvHerlad article of the type "temporary departure from the runway while landing". I don't know of many other options other than allpying left rudder ehrn he did immediately before the landing (a bit of left bank maybe, but I don't think it would have been enough). Now, he held it too long, and that required the reversal and started the PIO.

[Evan]

The fin of that magnitude, the yaw inertia is of that magnitude.


No, as I clearly explained in my previous post. Neither with a fin of that magnitude or with a C-152.
The problem is when you hold the input too much, then you may need some reversal. Actually, rudder reversal is the normal input done by the yaw damper.
input = -k1*yaw - k2*(yaw rate) -k3*(yaw integral in the last t seconds).
What is called a PID control.


What gust? I don't think that the yaw upset was caused by a gust. Rather, by... ok, read my previous post.

There was a very powerful storm system moving through northern Germany and large, sudden gusting winds around that time. I assumed that was the culprit.

I don't know of many other options other than allpying left rudder ehrn he did immediately before the landing (a bit of left bank maybe, but I don't think it would have been enough).

I would expect left bank and removal of the left rudder input to suffice. In all the A380/747 crosswind landings I've viewed, I've never seen a significant rudder reversal involved.

[Gabriel]

At what point? How much would you bank 1 or 2 seconds before touchdown?