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Use Of Rudder In Large Transport Category Airplanes

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Use Of Rudder In Large Transport Category Airplanes

Old 24th Mar 2021, 08:59
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Use Of Rudder In Large Transport Category Airplanes

I just read a interesting IFALPA briefing leaflet titled "Use of rudder on Boeing aircraft". One conclusion towards the end of that article was that (quote) "The rudder in a large transport aircraft is typically used for trim, engine failure, and crosswind takeoff and landing." In other words, do not step on the rudder pedals during normal operations except when required in crosswinds during takeoff or landing.

At this point I would like to clarify that I am familiar with the AA587 accident, its reasons and conclusions. I agree that the incorrect use of the rudder was a causal factor for the loss of control there, and I want to stress that I do not want to start another discussion about the inappropriate and/or excessive use of the rudder in large jets.

What I would like to know is your opinions and experiences regarding the subtle use of the rudder in situations where it would be perfectly warranted in smaller aircraft types, such as after departure or in a traffic pattern. What I could so far gather from my fellow colleagues out on the line is that the vast majority of pilots shies away from using the rudder like the devil shies away from holy water, and understandably so when reading that even the manufacturer recommends to do so (as mentioned above). In most situations at normal speeds during climb, cruise and descent that makes perfect sense to me as well, but there are some situations where I would like to question this mindset. For example, when flying a SID which requires a large turn at low speed after departure the slip/skid indicator in a B744 shows a considerable deflection (with or without the autopilot engaged). The airplane is actually in a sideslip since the aerodynamic forces at low speeds are not strong enough to prevent the tail from "hanging" down into the bank. This is not an issue at higher, "normal" speeds, but at low speed and low altitude it kind of goes against everything I've learned back in the days on the single engine trainer. The more uncoordinated the airplane flies, the larger the turn radius will become, and to some small extent there will also be a little bit of performance loss (even though I will admit that this should have a negligible effect in a all engine operating condition).

So, in airplanes where the autopilot does not control the rudder directly, would it really be so terribly wrong to GENTLY and SLOWLY add a little bit of rudder in such situations until the slip/skid indication indicates coordinated flight again?

I often hear the argument " Don't you know what happened to that Airbus in New York?", but like I said above, a gentle and appropriately slow rudder application has nothing to do with what happened with AA587. It would be the same like saying that we should not use the yoke to control pitch during approach because we might pull back too hard, stalling the airplane or overstressing it at higher speeds. As highly trained pilots we should all be able to judge the amount of control input required to maintain our flight path, and if we aren't, what does that say about our abilities to safely fly the airplanes we are rated on?
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Old 24th Mar 2021, 09:59
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Not taking into account any manufacturer recommendations or SOPs, I tend to agree with your point - especially considering the massive rudder deflections needed with an engine out, there shouldn't theoretically be any problem in using normal rudder deflections on takeoff / approach. The airplane should be designed to be able to handle that, as long as you're not doing left/right full deflections with 300+kias.

Nonetheless, I never noticed any significant slip on the NG yet so definitely no need for rudder...will have a look next time I fly (so expect that to be in 5 years or so )

Last edited by flyfan; 24th Mar 2021 at 12:37.
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Old 24th Mar 2021, 10:13
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Spoilers on the down going wing are sized, so Boeing tells us, to eliminate any need for rudder. And it really works as advertised.
Rudder is not used in normal flight manoeuvres.
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Old 24th Mar 2021, 10:57
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In the aircraft I fly (747) the yaw damper provides turn co-ordination without causing any movement of the rudder pedals. I think that's fairly common across the Boeing range. The turn and slip indication on the PFD is (so I'm told) not especially accurate. Quite honestly I don't see any need to overcomplicate matters. I only touch the rudder pedals during taxy, crosswind takeoff and landing and in the sim during an engine failure. If Boeing says don't touch, I don't touch.
When I have deviated from SOPs in the past, I try to ask myself whats in it for me. The answer is often nothing.
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Old 24th Mar 2021, 11:34
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On my current plane (A320), my feet are on the floor once airborne. The yaw damper does a pretty good job. Ive only ever needed to use the rudder on one airframe.
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Old 24th Mar 2021, 12:01
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The slip & skid indicator is the most unreliable instrument (together with the compass) on larger transport aircraft. The YD keeps it coordinated, there is no need for rudder at all. On top, slipping an aircraft of this size (jet engines, swept wings) creates extra hazards you don't want to learn in reality (disturbing the intake airflow for the jet engines, different stall characteristics in low speed turns).

Rule n1: don't fly a jet like a Cessna, it will get you killed. Especially in low speed. It's the start of all line training for pilots flying swept wing jets for the first time. Never ever "tighten the turn" with rudder in low speed on a swept wing.
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Old 24th Mar 2021, 12:16
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Most large aircraft have an integral turn co-ordination function within the yaw damper to minimise the lateral acceleration by applying a small amount of rudder in turning flight
The co-ordination system is generally independent of the AP - a stand alone system, or if part of an integrated flight control system, it would not depend on AP engagement.
In an aircraft with well optimised turn co-ordination and yaw damping there should be no need to use rudder in normal flight. Any manual rudder input could confuse the system, temporarily, or be out of phase, resulting in even more acceleration.
The quality of turn co-ordination relates to the aircraft characteristics, control system (e.g. spoiler for roll), flight conditions, and system fidelity; an ‘optimised compromise’, there may be some transient deviations when rolling, which reduce, to become balanced in a steady turn.
Note; check which parameter the ‘slip/skid’ indicator uses and its scaling, - lateral acceleration?
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Old 24th Mar 2021, 13:56
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Pure gold 🤣
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Old 24th Mar 2021, 14:22
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I think we should listen to Airbus, Boeing and company SOPs over a stranger on PPRuNe.
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Old 24th Mar 2021, 15:17
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I don't understand?? I thought this was a discussion and not rule based no matter who posts
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Old 24th Mar 2021, 15:34
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On the DHC Dash 7, full rudder authority was not available until some flap was selected; are there any similar systems on the Boeing fleet?
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Old 24th Mar 2021, 15:41
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Originally Posted by NGjockey View Post
What I would like to know is your opinions and experiences regarding the subtle use of the rudder in situations where it would be perfectly warranted in smaller aircraft types, such as after departure or in a traffic pattern.
Depends on how you define "smaller aircraft types."

It is definitely the case that propeller aircraft suffer from asymmetric engine and prop yaw-control effects that are neglible in turbojet/fan aircraft.

P-factor, engine torque, gyroscopic precession, and (in singles) spiral propwash impacting the entire tail-fin assymetrically, all combine to make prop aircraft yaw, especially during climb-out (high nose angle). And the smaller the aircraft (relative to engine power and prop mass) the greater the effect.

Unless, of course, the aircraft is a multi-engine and the manufacturer goes to the trouble and expense of installing engine(s) on one side that are "backwards" (counterrotating) and balance the yaw forces (Piper Seminole twin-trainer, for example).

See also: "Critical engine" - a problem very important to prop fliers if they lose an engine, but generally non-existent in flying jets.
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Old 24th Mar 2021, 16:09
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Originally Posted by Archive mole View Post
I have flown several Cessna jets, and I was not killed.
Lol. Good for you.

"As highly trained pilots we should all be able to judge the amount of control input required to maintain our flight path, and if we aren't, what does that say about our abilities to safely fly the airplanes we are rated on?"

The answer is easy: highly trained pilots learn to fly the aircraft they are flying as explained by the constructor, it's part of being "rated" on the aircraft.

Better?
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Old 24th Mar 2021, 17:02
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@ BraceBrace:

I beg to disagree. On the type I fly (B744), the slip/skid indicator derives its information from the IRU, which is also the source for heading, wind, track angle, drift angle and ground speed information. That is information we usually put quite some significance on, so why would only the slip/skid information be inaccurate enough to be allowed to be dismissed? Boeing (and all other manufacturers) would get into hot water if they deliberately installed an indicator which would provide misleading information, especially in the middle of the PFD, which is the most important display of all.

As to your argument that I don't want to learn about the effects of the extra hazards that a slip would create on an aircraft of this size, let me assure you that I couldn't agree more. The point is however, that the aircraft already is in a slip in a high bank angle low speed turn, and that is why I have raised this topic here. This is not about trying to "tighten the turn", this is actually about getting rid of the slip and achieving coordinated flight. According to the slip/skid indicator the yaw damper does a pretty bad job at coordinating the turn at low speeds. As the airplane accelerates into higher speed ranges turns get more coordinated because of the increasing velocity of the airflow around the rudder. Just have a look at the slip/skid indicator at low speeds and then at high speeds, using the same bank angle. The difference might surprise you.
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Old 24th Mar 2021, 17:03
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In the old days when pilots tried to do a greaser we had a captain, who if he thought you had accidentally got it right in spite of trying extremely hard, would put a large boot of rudder in during the flare and watch how you coped with the secondary effects with a highly swept wing.
Still here to tell the tail.
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Old 24th Mar 2021, 17:21
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@ BraceBrace:

True. Also true is that part of being rated on an aircraft includes for example being able to perform crosswind takeoffs and landings on slippery runways. If you read the FCOM or FCTM you won't find any information about how many inches of rudder or control wheel input you will need exactly for a particular crosswind or a particular mass. You will apply inputs as needed according to your experience, and that is what I wanted to state with my argument about being able to judge the amount of control input required as highly trained pilots, in any phase of flight.
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Old 24th Mar 2021, 17:45
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Is the ball showing a slip just during the roll-in, or also during the constant-bank, established part of the turn after stabilization? What about the rollout?
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Old 24th Mar 2021, 18:00
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Originally Posted by Timmy Tomkins View Post
On the DHC Dash 7, full rudder authority was not available until some flap was selected; are there any similar systems on the Boeing fleet?
IIRC, rudder authority is reduced at higher airspeeds, but don't recall specifics.
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Old 24th Mar 2021, 18:14
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Could this be the long-tail slip effect happening? In a turn, the airflow is curved but the fuselage is straight, so the airflow can be aligned (zero slip, or beta, angle) either at the tail or at the CG (or wherever the IMU is installed) but not both - you have to choose one or the other. So, if the inclinometer shows centered, that means there is a slip angle at the tail. Or, conversely, if there is no slip at the tail, there is slip at the CG (therefore side force, therefore off-center inclinometer). Usually negligible, but could show up in a long enough airplane in a tight-enough turn. And a basic yaw damper cancels yaw rates but not steady-state conditions of off-center yaw, so it allows this condition to persist.
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Old 24th Mar 2021, 18:23
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Airbus has automatic turn coordination, but if it happened for some reason to not work, I don't see any reason why not to use the rudder and do the job.
Sideslip should be demystified, I'm practically sure that Airbus and Boeing have very good models for simple movements around the yaw axis, why not do it in the sim ? Do a little dutch roll to see how it handles.

On the fun side, using the rudder in an A320 type will create an artificial wind on the ND. Try it and see for yourself
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