Exciting landing and a lot of Rudder Deflection

[kent olsen]

I'm an old tailwheel pilot so I'm not adverse to using the rudders but not at high speeds. You remember the DC-8 70 series with the big CFM engines. Well the inboard engines were about 18" off the ground with the struts compressed. So crosswind landings where a skill. I would teach to line up just on the upwind side of the center line. As you would start your flare push the nose out of the crab with the rudders and start to lower the upwind wing, keeping in mind that at 8 deg you are close to hitting the engine. One good thing about the -8 was the ground spoilers where not automatic. You had to grab the handle and deploy them once on the ground, which was after both main gear where on the ground.

When Douglas folded and became a part of Boeing, Boeing had a great big meeting for all the Boeing/Douglas operators in the world. One of the Boeing engineers was talking about crosswind landings and mentioned that it was ok to land in a crab. Well after his talk the next guy up was from the old Douglas. The first words out of his mouth where "At Douglas we never taught any pilots to land their airplanes in a crab" !!! Well he got a standing ovation!!! Keep in mind that nearly all those in attendance where Chief Pilots with years of experience probably starting with the DC-3.

[Evan]

That also. But it is also perfectly ok to use somehow large but quite short rudder inputs to correct sudden yaw disturbances caused by turbulence and wind gusts.
What the AA guy did was as large as they come, and very long, rudder deflections reversing them at the point of each overawing.

I believe he was trying to control roll disturbances with rudder, which is my point. The reason this is not such a hot idea is, as you know, the lag between when the rudder command is input and when the sideslip induces roll, so things get out of phase quickly. The sudden delayed roll can confuse the pilot into believing it is due to external upset and cause him to command opposite rudder, until the out of phase maneuvering builds up to full reversal from overswing and the fin snaps off like a Twix. Even Boeing jets are not certified to withstand a single deflection from overswing to full opposite rudder. It's one of those traps you must train pilots never to get in. How? Maybe by telling them this:

Transport pilots should be made aware that certain prior experience or training in military, GA, or other non-transport air
craft types emphasizing the acceptability of unrestricted dynamic control application typically does not apply to transport
aircraft or operations. Excessive structural loads can be achieved if the aircraft is manoeuvred significantly differently
than what is recommended by the manufacturer or the operator’s training program.

In simple pilot terms, if you are in a stall, don’t use the rudder; if you are not in a stall, you don’t need the rudder. The rudder in a
large transport aircraft is typically used for trim, engine failure, and crosswind takeoff and landing.

[Gabriel]

I believe he was trying to control roll disturbances with rudder, which is my point. The reason this is not such a hot idea is, as you know, the lag between when the rudder command is input and when the sideslip induces roll, so things get out of phase quickly. The sudden delayed roll can confuse the pilot into believing it is due to external upset and cause him to command opposite rudder, until the out of phase maneuvering builds up to full reversal from overswing and the fin snaps off like a Twix.

Yes, it seems that that was what the pilot was thinking.
I never proposed to use rudder to control roll. I was clearly talking about using it to control yaw/sideslip disturbances caused by turbulence and gusts at the onset to minimize yaw excursions (not to create them). Using rudder to induce roll is the wrong thing to do almost always, the only exception being in very specific instances of very-high-AoA upset recovery where and if ailerons become ineffective.

Even Boeing jets are not certified to withstand a single deflection from overswing to full opposite rudder.

I have no proof but I've heard (i.e. read somewhere) that, unlike Airbus, Boeing has an internal requirement for the fin to withstand a full-deflection anti-yaw rudder application from the max overswing obtained by starting from all neutral and then suddenly applying and holding full rudder in one direction until achieving max overswing, and at that point applying full opposite rudder, only that instead that using a 1.5 margin coefficient between the limit and ultimate loads, they use 1.2 for this (they can do it because it is an internal requirement that goes beyond the FAR 25 structural requirements)

[Evan]

I was clearly talking about using it to control yaw/sideslip disturbances caused by turbulence and gusts at the onset to minimize yaw excursions (not to create them).

And I, of course, see no problem with that if it doesn't involve the sort of cyclic reversals that lead to PIO (such as the ones we saw on that A380 last year). Regardless of the picture 3WE like to paint.

I have no proof but I've heard (i.e. read somewhere) that, unlike Airbus, Boeing has an internal requirement for the fin to withstand a full-deflection anti-yaw rudder application from the max overswing obtained by starting from all neutral and then suddenly applying and holding full rudder in one direction until achieving max overswing, and at that point applying full opposite rudder, only that instead that using a 1.5 margin coefficient between the limit and ultimate loads, they use 1.2 for this (they can do it because it is an internal requirement that goes beyond the FAR 25 structural requirements)

From what I've read, Boeing certifies only from max overswing to opposite steady state sideslip. Under no circumstances should a pilot be getting into cyclic overswings, but that is what can happen when using rudder for roll upsets.

[ATLcrew]

And I, of course, see no problem with that if it doesn't involve the sort of cyclic reversals that lead to PIO (such as the ones we saw on that A380 last year). Regardless of the picture 3WE like to paint.

Noted.

[Schwartz]

And I, of course, see no problem with that if it doesn't involve the sort of cyclic reversals that lead to PIO (such as the ones we saw on that A380 last year). Regardless of the picture 3WE like to paint.



From what I've read, Boeing certifies only from max overswing to opposite steady state sideslip. Under no circumstances should a pilot be getting into cyclic overswings, but that is what can happen when using rudder for roll upsets.

I think you see similar swings (maybe not full deflection, but certainly large fast swings in opposing directions) in the video posted here of that 380 landing, it's just the angle is different, so it looks a bit different.

[Gabriel]

I think you see similar swings (maybe not full deflection, but certainly large fast swings in opposing directions) in the video posted here of that 380 landing, it's just the angle is different, so it looks a bit different.

The real difference is that the inputs in that video, while large, are short enough that you don't see large sideslip angles being induced (or large yaw movements altogether).
The problem comes when you hold the rudder at a large deflection long enough to induce a large sideslip angle and then slam opposite rudder. That creates a condition that is beyond the certification criteria and which, at not-that-slow speeds, can induce loads beyond ultimate load and cause a catastrophic failure of the fin structure with the consequent loss of the airplane and the lives.

[Evan]

Noted.

As a passenger, you understand. I just dislike being yawed into the poor chump next to me and then counter yawed until my skull cracks against the window and then into the seatback when we end up in the grass. Nor do I like being dashed headlong into the ground for no good reason.

[3WE]

As a passenger, you understand. I just dislike being yawed into the poor chump next to me and then counter yawed until my skull cracks against the window and then into the seatback when we end up in the grass. Nor do I like being dashed headlong into the ground for no good reason.

Yes, ATL, I always hate it when you are yawning me back and forth like that, and wonder what it will take to make you stop...as if we aren’t being clear on how to address the issue.