Southwest 737MAX Dutch roll rated accident

[A0283]

25th of May 2024
https://avherald.com/h?article=519ce679&opt=
Southwest Airlines 737-8 MAX, reg N8825Q flight WN-746 from Phoenix,AZ to Oakland,CA (USA) with 175 pax and 6 crew, was enroute at FL320 when the aircraft experienced Dutch Roll.

The FAA reported: "AIRCRAFT EXPERIENCED A DUTCH ROLL, REGAINED CONTROL AND POST FLIGHT INSPECTION REVEALED DAMAGE TO THE STANDBY PCU, OAKLAND, CA." and stated the aircraft sustained substantial damage, the occurrence was rated an accident.

[procede]

Since when is a dutch roll able to cause damage to an aircraft? It is generally a mildly damped eigenmotion, which is slighly annoying without a yaw damper.

This sounds more like a rudder hard over due to a PCU malfunction...

[Uplinker]

Or a pilot 'pedalling' the rudder pedals maybe ?

[A0283]

Since when is a dutch roll able to cause damage to an aircraft? It is generally a mildly damped eigenmotion, which is slighly annoying without a yaw damper.
This sounds more like a rudder hard over due to a PCU malfunction...

I am old enough to still have it (and yaw damping) as a real issue in the back of my mind. Starting with the destruction of a B707 of Braniff on the 19th of October 1959. That case a demo of DR, turning off the yaw damper to show DR, and the pilot overcontrolling. Losing 3 of 4 engines and plane crash and destruction killing half the pob. Others would follow on for example B707 and KC135 (1968, …, 2013).

Wonder which scenario this will turn out to be. Is the PCU a (contributing) cause or a consequence for example.

[msbbarratt]

Am I right in thinking that "the occurence was rated an accident" means that it is considered to have been a life-threatening event?

[A0283]

Am I right in thinking that "the occurence was rated an accident" means that it is considered to have been a life-threatening event?

Might be, but by definition, could also mean the damage was very substantial. No information on that, but loosening up engines and/or vertical fin and rudder damage come to mind as theoretical options.

I recently had a ‘conversation’ about the definitions with an AAIB. In my view it would be beneficial if safety reports would give (at least) an order of magnitude of the amount$ if monetary was the definition driver (and yes I am familiar with the issues calculating such numbers).

[msbbarratt]

Might be, but by definition, could also mean the damage was very substantial. No information on that, but loosening up engines and/or vertical fin and rudder damage come to mind as theoretical options.

I recently had a ‘conversation’ about the definitions with an AAIB. In my view it would be beneficial if safety reports would give (at least) an order of magnitude of the amount$ if monetary was the definition driver (and yes I am familiar with the issues calculating such numbers).

Thank you; I guess more will emerge; there's going to be some sort of investigation report issued in due course I'm sure.

Ah, definitions! Tricky things. What monetary aspect do you envisage that covering? The losses associated with an issue identified in a safety report actually occuring and generating consequences (diversions, crashes, etc)?

I can see that having a monetary angle is a good way of explaining to management / shareholders the value of doing the corrective work. Having solid financial benefits of "safety" made obvious would further cement corporate commitments to achieving it, which could only be a good thing.

Is that the kind of benefit you see?

[ATC Watcher]

The ICAO definition of an accident is indeed including if the aircraft sustained damage or structural failure . No monetary minimums and no further definition on the severity of the damage .it is left to appreciation of the investigative authority.

[waito]

I never thought we will face such an serious issue in modern times.

Dutch Roll seems to be almost uncontrollable manually for extended period, so Yaw Damper including all rudder components are expected on high availability.

Looking forward to learn more.

[WideScreen]

Aha, Dutch Roll.

Do the following experiment:
- Take your average TV remote (20 cm long, 1.5 cm thick and 5 cm wide).
- It does have 3 rotation axis along its symmetry axis.
- Have it in your hand and "throw" it in the air, with a rotating motion in either of the rotational axis.
- You will notice that the rotation along the shortest and longest rotational axis will be stable.
- The rotation around the middle rotational axis will be unstable and the remote starts to tumble (Before objecting/commenting, pleaseeeeeeeeeeeee test yourself, it really happens !).
- With some practice, you can catch the remote back in the hand, with just a half tumble, facing up again with the same surface, but pointing 180 degrees different.
- That is Dutch Roll.
- The cause is the tendency to strive for a minimal energy situation in the existing force field (gravitation for the remote).

For an aircraft, the tumbling becomes much more complicated, because of the forces acting on the aircraft largely varying with the position of the aircraft vs. its direction of motion. IE, the aerodynamic surfaces have a huge influence.

So, all in all, when the aircraft "rotates" over one of its axis (usually the roll axis being sensitive), the whole aircraft will start to rotate also over its other axis and the whole starts oscillating due to the complicated force fields exerted on the aircraft with its wing/tail surfaces.

What the Yaw damper does do is just adding instantly a tad rudder, to avoid (!) the other rolls. When this is done manually by the pilot, it will be too late and the compensation will introduce its own secondary roll effects as a result of the yaw, creating first and secondary effects all over the place. Connect the wires for the Dutch Roll control swapped and the airplane goes all over the place as a Russian crew found out on their test flight, after the repair of their airplane.....

Remember the demo early on in your pilot training, of the yaw effect to the opposite direction, when initiating a roll ? Largely "explained" with the added aerodynamic resistance of the ailerons, but in reality it is the above effect ;-)

Added: Nearly half a century ago, we got this demo'd in university with a box of matches, one strike side made green and the other one made red. And, it clearly showed the audience what I describe with the TV remote.

[procede]

I do not see the remote analogy, as it does not generate any significant aerodynamic forces. What you are referring to is due to moments of inertia.

Dutch roll is where yaw induces roll by increasing the lift on the wing moving forwards and decreases it on the wing moving aft. Roll then induces yaw by having the wing moving down generate more lift (and thus induced drag) and the wing moving up then generating less lift (and thus induced drag).

It will generally dampen out in a the order of 10 seconds.

As a pilot you generally should not even try to counteract it, as this will most likely result in pilot induced oscillations.

[tdracer]

My guess is that the Yaw Damper failed, the aircraft entered a Dutch Roll, and the pilot - in trying to correct it - just made it worse until things started breaking.
My analogy is pulling a trailer with a similar (or smaller) sized vehicle. If the trailer starts 'wagging' - the worst thing you can do is try to stop it with steering corrections. Your best course of action is to keep the steering wheel still and pointed in the desired direction of travel - the oscillation of the trailer will slowly damp out and stop.
I'll occasionally see a novice pulling a trailer on the highway - the trailer wagging will get so bad that they end up having to pull over and stop because they don't know the best course of action is 'no action'.
In this case, while the Dutch Roll may not damp out (or take an extremely long time to do so), it generally won't go divergent without the pilot inadvertently 'assisting'.

[Maninthebar]

My guess is that the Yaw Damper failed, the aircraft entered a Dutch Roll, and the pilot - in trying to correct it - just made it worse until things started breaking.
My analogy is pulling a trailer with a similar (or smaller) sized vehicle. If the trailer starts 'wagging' - the worst thing you can do is try to stop it with steering corrections. Your best course of action is to keep the steering wheel still and pointed in the desired direction of travel - the oscillation of the trailer will slowly damp out and stop.
I'll occasionally see a novice pulling a trailer on the highway - the trailer wagging will get so bad that they end up having to pull over and stop because they don't know the best course of action is 'no action'.
In this case, while the Dutch Roll may not damp out (or take an extremely long time to do so), it generally won't go divergent without the pilot inadvertently 'assisting'.

This works on the road as the power (force) is maintained in one axis and one axis only. Is that consitently the case where the forces are being delivered by engines whose position in relation to all 3 possible axes are changing?

[mustafagander]

Back in the day we had B707-120 series. They had a very lively dutch roll. The approved remedy with no yaw damper was to stop the upcoming wing with a little bite of aileron. An easy fix which worked very well damping it out in a few cycles. We were absolutely forbidden to even think about rudder.

[procede]

My guess is that the Yaw Damper failed, the aircraft entered a Dutch Roll, and the pilot - in trying to correct it - just made it worse until things started breaking.

My guess is that it was the yaw damper getting false yaw rate inputs and the problem only stopped when the pilots turned it off and/or the PCU broke.

[Bergerie1]

mustafagander,

It was the same on the VC10. With the yaw dampers off, if I remember rightly, the divergent dutch roll doubled its amplitude every 15 secs. The way to control it was never to rush in and never to use the rudders, but to watch the roll, wait until you had the pattern fixed in your mind and only then to firmly but gently apply a short applicaion of aileron against the upcoming wing. Any residual rolling motion could then be counteracted in the same way using smaller corrections.

During base training, we used to take the aircraft up to 35,000 ft, switch of all three dampers, induce a dutch roll with rudder, and then demonstrate the technique. The trainee was asked to watch very carefully both the rolling motion and the timing of the correction using aileron only. This was then repeated with the trainee making the corrections. But woe betide you if he got it out of synch!!

[WideScreen]

I do not see the remote analogy, as it does not generate any significant aerodynamic forces. What you are referring to is due to moments of inertia.
Yep, it's the (rotational) inertia, though in a force field. Without force field, nothing happens.

Do the TV remote test and you'd be surprised.

Dutch roll is where yaw induces roll by increasing the lift on the wing moving forwards and decreases it on the wing moving aft. Roll then induces yaw by having the wing moving down generate more lift (and thus induced drag) and the wing moving up then generating less lift (and thus induced drag).
Nop, since you could "design" these effects out and that doesn't happen.

It will generally dampen out in a the order of 10 seconds.

As a pilot you generally should not even try to counteract it, as this will most likely result in pilot induced oscillations.
Yep, in general, for an airplane, "just do nothing" and it'll stabilize, though there is no guarantee of that. It just depends on the stability of the whole.

[WideScreen]

My guess is that the Yaw Damper failed, the aircraft entered a Dutch Roll, and the pilot - in trying to correct it - just made it worse until things started breaking.
Probably.

My analogy is pulling a trailer with a similar (or smaller) sized vehicle. If the trailer starts 'wagging' - the worst thing you can do is try to stop it with steering corrections. Your best course of action is to keep the steering wheel still and pointed in the desired direction of travel - the oscillation of the trailer will slowly damp out and stop.
I'll occasionally see a novice pulling a trailer on the highway - the trailer wagging will get so bad that they end up having to pull over and stop because they don't know the best course of action is 'no action'.
With a trailer, what happens is that the trailer starts wagging the vehicle and it needs counteracting (IE steering) of that, to take care the front wheels are in the direction of movement, without that, the whole finishes pretty rapidly.
The best action is "a little" braking, at the moments the trailer "swings" back in line with the span and stop braking when the trailer swings out. Brake when the trailer swings out and it'll try to overtake the vehicle. Quite some opportunities to screw up.

Added: The same with riding off-road on an off-road motobike. When one gets "in trouble" and lets off the gas handle, you are in for the surprise that the rear end will start to overtake and within a few meters you're on the ground with (quite some) broken bones. Just stay on the gas and pull out accelerating and all will be fine. The same with riding a motobike in loose sand: Enter slowly and accelerate a little and all will be fine. Do different and your life will become miserable......

In this case, while the Dutch Roll may not damp out (or take an extremely long time to do so), it generally won't go divergent without the pilot inadvertently 'assisting'.
Yes, passenger airplanes are designed for stability, etc. The pilot can screw up, though.

[WideScreen]

mustafagander,

It was the same on the VC10. With the yaw dampers off, if I remember rightly, the divergent dutch roll doubled its amplitude every 15 secs. The way to control it was never to rush in and never to use the rudders, but to watch the roll, wait until you had the pattern fixed in your mind and only then to firmly but gently apply a short applicaion of aileron against the upcoming wing. Any residual rolling motion could then be counteracted in the same way using smaller corrections.
Yep, that is what to do (manually), when things already are "noticable".

During base training, we used to take the aircraft up to 35,000 ft, switch of all three dampers, induce a dutch roll with rudder, and then demonstrate the technique. The trainee was asked to watch very carefully both the rolling motion and the timing of the correction using aileron only. This was then repeated with the trainee making the corrections. But woe betide you if he got it out of synch!!
Out of sync will make all very happy (not).

[MechEngr]

Durtch roll is a feedback result, not a conservation of angular momentum result. The divergence in yaw results in an amplification of roll which then gets switched back to an amplification of yaw as the higher lift wing also is the higher drag wing. If the cockpit / YD response is slow it may take a while to damp out if designed correctly. If the cockpit response is out-of-phase it can get as bad as any PIO.

AFAIK, designing out Dutch roll starts by eliminating sweep of the wings and dihedral and reducing the vertical stabilizer area, which will decrease yaw opposition (wont' come straight as fast) and make for worse asymmetric engine handling. I think that swept wings for transonic transports are going to stay and dihedral is so useful on low-wing aircraft, particularly with pylon mounted engines, when landing that that will also stay.

Perhaps there will be a report about the initiating cause.

Relative to the dynamics of tossing a remote - see this video of a bistable rotation on the International Space Station: https://www.youtube.com/watch?v=1n-HMSCDYtM

[Vessbot]

Aha, Dutch Roll.
Seems you're describing inertia coupling, not Dutch roll.

Remember the demo early on in your pilot training, of the yaw effect to the opposite direction, when initiating a roll ? Largely "explained" with the added aerodynamic resistance of the ailerons, but in reality it is the above effect ;-)

If this is true, then why are there Frise ailerons?

[WideScreen]

Seems you're describing inertia coupling, not Dutch roll.
I'd not call it "coupling", it's just how nature works to minimize the total energy.....
If this is true, then why are there Frise ailerons?
To compensate a little upfront, using aero dynamic forces, it does not take out the root cause, just like with a Yaw damper, just counter-act. And, it works well, I don't argue about that ;-)

Just do the TV remote test, you'd be surprised !

[WideScreen]

Durtch roll is a feedback result, not a conservation of angular momentum result......
I don't say, its a conservation of angular momentum. And, indeed it is a kind of feedback, due to the natural "spreading" of the rotational inertial energy over the axis, in a force field.

Just do the TV remote test, you'd be surprised (and probably not believe your own eyes). For the TV remote the force field is simple. For an airplane, the force field is very complex, given all the aerodynamic influences.

[tdracer]

Durtch roll is a feedback result, not a conservation of angular momentum result. The divergence in yaw results in an amplification of roll which then gets switched back to an amplification of yaw as the higher lift wing also is the higher drag wing. If the cockpit / YD response is slow it may take a while to damp out if designed correctly. If the cockpit response is out-of-phase it can get as bad as any PIO.

AFAIK, designing out Dutch roll starts by eliminating sweep of the wings and dihedral and reducing the vertical stabilizer area, which will decrease yaw opposition (wont' come straight as fast) and make for worse asymmetric engine handling. I think that swept wings for transonic transports are going to stay and dihedral is so useful on low-wing aircraft, particularly with pylon mounted engines, when landing that that will also stay.

That rings a bell - I remember when we studied Dutch Roll way back when as an undergrad, but for the most part what I remembered was that, while it was possible to design it out, the 'cure' tended to be worse than the disease, while a (relatively) simple yaw damper gave you the best both worlds.
I recall being sent up to Vancouver, BC for a very high time 767 that was AOG due to "engine caused oscillations". Except when we really started looking, it had nothing to do with the engines, it was traced to a malfunctioning Yaw Damper (IIRC, the 767 had well north of 100,000 hours on it at the time).

[Vessbot]

I'd not call it "coupling", it's just how nature works to minimize the total energy.....

I'm not saying you called it that, I'm saying that that it its standard name.

To compensate a little upfront, using aero dynamic forces
Compensate for what?

Just do the TV remote test, you'd be surprised !
I am not surprised.

[Mr Good Cat]

So back when I started my career as a cadet on the 737-300, we would occasionally have a yaw damper failure. I don’t remember it being anything other than feeling a bit sloppy on the localiser. If it’s such a critical thing to flight safety on the 737NG, shouldn’t the checklist include the statement ‘Land at the nearest suitable airport’ if it fails? I suspect it’s PIO that have caused any PCU damage.

[procede]

AFAIK, designing out Dutch roll starts by eliminating sweep of the wings and dihedral and reducing the vertical stabilizer area, which will decrease yaw opposition (wont' come straight as fast) and make for worse asymmetric engine handling. I think that swept wings for transonic transports are going to stay and dihedral is so useful on low-wing aircraft, particularly with pylon mounted engines, when landing that that will also stay.

Dihedral is also needed for roll stability. Swept wings also help with directional stability.
You cannot get rid of dutch roll without creating an instability elsewhere. Hence the yaw damper...

[hans brinker]

-1- Yep, it's the (rotational) inertia, though in a force field. Without force field, nothing happens.

-2- Do the TV remote test and you'd be surprised.


-3- Nope, since you could "design" these effects out and that doesn't happen.


Yep, in general, for an airplane, "just do nothing" and it'll stabilize, though there is no guarantee of that. It just depends on the stability of the whole.

There have been plenty of aircraft that were certified while exhibiting dutch roll requiring yaw dampers. "Designing" it out would mean changing the whole airplane, requiring a yaw damper is much easier.

1) The force is the aerodynamic forces acting on the swept wing and tail after a disturbance

2) Dutch roll is an aerodynamic effect of the aircraft design, and your remote example is not relevant at all.

3) Changing the design would be a far more costly affair than requiring yaw dampers.

[Larry in TN]

Am I right in thinking that "the occurence was rated an accident" means that it is considered to have been a life-threatening event?
49 CFR 830.2 has the FAA definition.

Aircraft accident means an occurrence associated with the operation of an aircraft which takes place between the time any person boards the aircraft with the intention of flight and all such persons have disembarked, and in which any person suffers death or serious injury, or in which the aircraft receives substantial damage.

[waito]

Found this

​​​​​​​On Jun 13th 2024 The Aviation Herald learned that two ribs, that the stand by PCU is being mounted to, were damaged as well as the mounts of the stand by actuator. A temporary repair was done in Oakland replacing the damaged PCU, the aircraft was then ferried to Everett to replace the damaged ribs.

​​​​​​

[longisland]

Since when is a dutch roll able to cause damage to an aircraft? It is generally a mildly damped eigenmotion, which is slighly annoying without a yaw damper.

This sounds more like a rudder hard over due to a PCU malfunction...
Ever fly the 727? Especially the -100 series?

My introduction to the 727 was as a new F/E in late 1967. Sims were really just procedure trainers with no motion as I recall. So we did about 6 or so flights in the airplane. One of the demos in the airplane was a Dutch Roll demonstration. Pilot (trainee) with feet positively ON THE FLOOR. Start off with a slight dutch roll and watch it grow. We did the event in the high 20's because above 30T it really was a serious and immediate problem. This on the 727-100 series. The -200 series, because of its length was more stable. On the -100 series with one of the two yaw dampers MEL'd the a/c was restricted to FL240 or so. Higher by several thousand feet on the -200.

[1southernman]

When I hear B737and PCU in the same sentence I think UAL 585, USAIR 427 and Eastwind 517...

[msbbarratt]

49 CFR 830.2 has the FAA definition.

Thanks Larry. I followed that up and found the associated definition of "substantial damage" from LII at Cornell Law School (https://www.law.cornell.edu/definitions/index.php?width=840&height=800&iframe=true&def_id=7aa496226f9cffc104f70d588dbd6c54&term_occur=999&term_src=Title:49:Subtitle:B:Chapter:VIII:Part:830:Subpart:A :830.2),

Substantial damage means damage or failure which adversely affects the structural strength, performance, or flight characteristics of the aircraft...

I think it hinges on the use of the "adversely", which suggests that the damage has gone beyond a tolerable threshold.

[bluesideoops]

Is perhaps the more pertinent discussion here the fact that the flight crew were unable to recognise and take appropriate action to the point that the aircraft became damaged. Is this a further reflection of the fact that pilot skills are atrophying and there is too much reliance on automation?

[Fursty Ferret]

Sorry Widescreen, but Dutch Roll is a swept-wing phenomena. It’s actually not too challenging to control by hand, but crucially this needs to be done by aileron inputs. Trying to control it using the rudder is almost impossible and rapidly leads to a diverging oscillation driven almost entirely by the pilot’s inputs.

[megan]

Is perhaps the more pertinent discussion here the fact that the flight crew were unable to recognise and take appropriate action to the point that the aircraft became damaged. Is this a further reflection of the fact that pilot skills are atrophying and there is too much reliance on automationKC-135 broke up in midair as a result of dutch roll 3rd May 2013.Air Mobility Command has released the results of an accident investigation that examined what caused the May 3, 2013, crash of a KC-135 in the Kyrgyz Republic. The crew of three, en route from the Transit Center at Manas to Afghanistan on a combat aerial refueling mission, perished in the mishap.

Upon takeoff, a flight control system malfunction, the board found, generated directional instability, causing the aircraft's nose to slowly drift from side-to-side or "rudder-hunt." This condition, not fully diagnosed by the crew, progressed into a more dangerous oscillatory instability known as a "Dutch roll." The board identified that a poor layout of key information in the inflight manual and insufficient crew training contributed to the mishap by detracting from the crew's ability to act on critical information during their troubleshooting to turn off either of two cockpit switches which may have eliminated the malfunction.

Having not recognized the Dutch roll condition, the crew initiated a left turn to remain on-course along the planned route of flight and used a small amount of left rudder to coordinate the turn. The use of rudder, while in a Dutch roll, increased the aircraft's oscillatory instability. The ensuing large side-to-side movements of the aircraft varied the crew member's foot pressure on the rudder pedal which caused inadvertent fluctuations in rudder position. These fluctuating rudder movements, coupled with slight right rudder use while rolling out of the turn, compounded the Dutch roll severity and produced extreme airframe stress that caused the KC-135's tail section to separate from the aircraft. The subsequent, uncontrollable descent resulted in an in-flight explosion.

A unique combination of six factors--flight control malfunctions, insufficient crew force training, incomplete crew checklist response, use of rudder while in a Dutch roll condition, crew composition, and cumbersome procedural guidance--all came together during the flight's short 11-minute duration and resulted in this accident.

"The crew encountered a condition that they had not realistically experienced in training, and when coupled with decisions based on their relatively low recent experience levels, were presented with an unrecognized hazardous and difficult situation to overcome," the general said. "It has been the focus of our investigative team, throughout these months of hard work and travel to the accident scene in the Kyrgyz Republic, to do everything we can to fully understand the facts surrounding this tragic string of events."

What level of dutch roll training is provided these days?

[aeromech3]

Am I correct in understanding there are two Yaw damper systems on the latest B737' NGs, normally the main powered by flight control B hyd; to get the standby yaw damper to work both A and B flight control switches need to be placed in stand-by and then the damper switch activated.
This procedure might have contributed to stand-by rudder PCU damages.
'Classic' B737's have MEL for the single Yaw Damper inop and switched off with (O) & (M) procedures, don't know for Max

[sablatnic]

Dihedral is also needed for roll stability. Swept wings also help with directional stability.
You cannot get rid of dutch roll without creating an instability elsewhere. Hence the yaw damper...

Sweep gives roll stability too, four degrees of sweep equals about one degree of dihedral, but how much roll stability is actually needed? The Dutch roll could be countered by introducing anhedral, by reducing sweep and/or by adding a larger fin/rudder, but that would cost in longer gears and increased drag and weight, and the larger fin area would reduce spiral stability, but is spiral stability actually needed? Or they could be built high winged with room for enough anhedral, like the BAe 146 and many millitary transports, but that would mean having to build a stronger and heavier fuselage.

[blind pew]

Declared a mayday on a super VC10 due to a divergent oscillation which was similar to a Dutch roll. Hand flew up to around 20,000ft then plugged in the autopilot ..climbing through 25,000 we started getting a mild movement which slowly increased in amplitude. The tech log had a defect which was 4 degrees of rudder trim needed in cruise; defect had been cleared and trim was normal during climb. Having been brainwashed by BEA Trident training that the autopilot does a better job I was reluctant (frightened) to take it out.
After a ceiling panel fell down and with a clear view of the Indian sub continent (long way to fall) I thought sod it I don’t have a choice. Click calm returned. Twas a runaway autopilot yaw damper which iirc operated on one of the three rudder surfaces (split surfaces driven by individual electro/hydraulic units). There was no feedback through the controls as the system worked between the flight controls and units nor did we have yaw damper position indicators as the Trident did.
Strangely the Cabin crew hardly noticed anything whilst we were getting brown underpants..a LOT of alcohol was taken that night. Big girl’s blouses you might think.

[WideScreen]

There have been plenty of aircraft that were certified while exhibiting dutch roll requiring yaw dampers. "Designing" it out would mean changing the whole airplane, requiring a yaw damper is much easier.

1) The force is the aerodynamic forces acting on the swept wing and tail after a disturbance

2) Dutch roll is an aerodynamic effect of the aircraft design, and your remote example is not relevant at all.

3) Changing the design would be a far more costly affair than requiring yaw dampers.
The "designing it out" is only possible over a limited speed [ETC] range, because the cause and the counteract do come from different physical items. As such, the yaw damper, real-time controlling the amount of counter action is the way to go.

1): No, it also happens with non-swepped wings, even the C172 does have this effect.
2) Nop, aerodynamics plays an important aspect in the effects becoming noticeable [and even counteracting], but are not the root cause.
3) I am not against the yaw damper, not to say, it does resolves the effects, giving the airplane designers the option to optimize on other items and use the yaw damper to resolve the DR issues, let alone that "designing it out" using aerodynamics will only work on a limited speed [ETC] range.

[safetypee]

The incident as described so far appears to be an aggravation of the oscillatory roll-yaw stability mode due to a malfunction. Thus it is technically incorrect to describe it as 'DutchRoll'; see description below.

Points of interest for the 737: what are the basic (raw, un-aggravated) Dutch-Roll characteristics for this variant. What is the certification basis (beware grandfather rights) and assumptions about manual control without damping. Can this variant of the 737 be dispatched with an inoperative yaw damper?
What is the certification basis for a step or oscillatory rudder input re fin structure - cf requirements post AA accident - repetitive, cyclic rudder input. What rudder limiting is there, does this differ with pilot or yaw damper input?


HTBJ;- D. P. Davies (my emphasis)

A dutch roll is this combination of yawing and rolling motions; the yaw is not too significant, but the roll is much more noticeable and the aeroplane proceeds with a continuously reversing roll action. Not until the motion becomes very exaggerated is there any disturbance in pitch.
A more correct term for dutch roll is oscillatory stability. There is an associated quality known as spiral stability which needs to be mentioned here; it will be explained later, although the term itself is almost self-explanatory.
There are several factors which dictate the directional and lateral qualities of an aeroplane; they are all inter-related and cannot be handled separately.

On one hand we have the effects of dihedral and sweep which basically dictate the lateral qualities and on the other hand fin and rudder size and effectiveness which basically dictate the directional qualities. On the relationship of these two qualities rest the spiral and oscillatory qualities of the aeroplane, and they are always in conflict. If the lateral qualities are dominant the aero-plane tends to be spirally stable and oscillatorily unstable; if the directional qualities are dominant the aeroplane tends to be spirally unstable but oscillatorily stable. Other factors influence the aeroplane's behaviour of course but the end result, as always, is a fair compromise between these two stability requirements.

Oscillatory stability, that is stable dutch roll, can now be defined as the tendency of an aeroplane when disturbed, either directionally or laterally, to damp out the ensuing yawing/rolling motion and return to steady flight.
… remember that with a swept wing there is a marked rolling tendency with yaw; …
When an aeroplane is yawed it rolls. The fin and rudder then oppose the yaw, slow it down and stop it, and return the aircraft towards straight flight.
If the fin and rudder are big enough, the second yaw and roll are less than the first and each excursion gets progressively smaller until the motion damps right out. If, however, the fin and rudder are too small (but, please note, only 'too small' in this particular context) the second yaw and roll are bigger than the first, each overswing gets progressively larger and the motion becomes divergent, i.e. unstable.
Although the initial yaw is the trigger for this misbehaviour it is the rolling motion, in most aeroplanes, which is most noticeable to the pilot; this is why the rolling behaviour is used as the main parameter in measuring dutch roll.

Like any other form of stability, oscillatory stability can be positive, neutral or negative, which is to say the dutch roll can be stable, neutral or unstable. A dutch roll is measured by plotting bank angle against time.
Positive stability is basically safe because the aeroplane, left alone, will, either quickly or more slowly, finally bring itself under control.
Neutral stability is safe (enough) because it won't get worse, but undesirable because, if the amplitude is large, or the frequency is small, the aeroplane is tiring and tedious to fly.
Negative stability is potentially dangerous because sooner or later, depending on the rate of divergence, the aeroplane will either get out of hand or demand a constant very high level of skill and attention to maintain control.
The unstable case, however, needs to be qualified in this way: if the divergence is rapid the aeroplane is obviously unacceptable, but if the divergence is very slow then it can be tolerated. To the pilot there is no significant difference between a dutch roll which is only very, very slowly divergent and one which is truly neutral. They feel substantially the same over a fairly short period of time and a much longer period is needed to prove that the unstable case is actually unstable.
In the same way a dutch roll which is only very, very slowly convergent will feel the same as a neutral dutch roll over a short period of time.

Time, therefore, is important, and the convention is to use the time period taken to reach double amplitude (in the unstable case) or half amplitude (in the stable case) as a means of expressing the degree of stability. …
It is known that times to double amplitude down to around 50 secs. can be considered substantially to equate to neutral stability; it is also known that times up to 15 secs. constitute marked instability.
It would appear that a level could eventually be drawn around the 35 to 40 secs. mark.
Time to double amplitude, however, is not the whole story. Frequency is very important; if this falls to somewhere below 3 secs. the roll is fast enough to make it difficult for a pilot to keep in phase with his recovery on the ailerons and there is the possibility of the motion being made worse.

Dutch roll qualities vary with aircraft configuration, for a given configuration with altitude and for a given altitude with lift coefficient. The dutch roll, therefore, gets worse with increased altitude and usually, but not always, with reduced speed at constant weight or increased weight at constant speed The control of a divergent dutch roll is not difficult so long as it is handled properly.

Let us assume that your aeroplane develops a diverging dutch roll.

The first thing to do is nothing — repeat nothing.
Too many pilots have grabbed the aeroplane in a rush, done the wrong thing and made matters a lot worse. Don't worry about a few seconds delay because it won't get much worse in this time. Just watch the rolling motion and get the pattern fixed in your mind.
Then, when you are good and ready, give one firm but gentle correction on the aileron control against the upcoming wing. Don't hold it on too long — just in and out — or you will spoil the effect. You have then, in one smooth controlled action, killed the biggest part of the roll. You will be left with a residual wriggle, which you can take out, still on ailerons alone, in your own time.

Don't attempt to correct the manoeuvre with rudder; as explained, the yaw is very often suppressed and it is difficult to work out which way to apply the rudder at what particular moment and there is a good chance that the wrong rudder will be applied which will aggravate things very quickly. It is not difficult, however, to apply the correct aileron control. Further, don't attempt to squash the dutch roll flat in one fell swoop but be content with taking out a big bite first time then sorting out the remainder next time.
In a dutch roll in a turn aim for recovery at the bank angle appropriate to the turn. Don't attempt to deal with the dutch roll and the return to level flight simultaneously; fix the dutch roll first then, if necessary, recover from the turn.

The drama which surrounded the dutch roll some years ago was not really earned by the aeroplanes but was engendered perhaps more by a lack of knowledge of the subject and possibly an over-exuberance on the part of the pilots. It is a comfort to be able to say now that there are no civil jet transports flying which need be the slightest bit demanding in terms of oscillatory stability and control; most have a raw dutch roll which is only slowly unstable when there is any instability and the others are adequately protected in other ways (see the following sub-chapter on yaw and roll dampers).
The recommended drill for correction of dutch roll on ailerons alone holds good for all subsonic jet transports.

[WideScreen]

Sorry Widescreen, but Dutch Roll is a swept-wing phenomena. It’s actually not too challenging to control by hand, but crucially this needs to be done by aileron inputs. Trying to control it using the rudder is almost impossible and rapidly leads to a diverging oscillation driven almost entirely by the pilot’s inputs.
No, it's not, the swept-wing does heavily change the angular stuff of the aircraft and as such, it looks like it comes due to swept wings.

The possibility to control by hand largely depends on the stability of the aircraft. For some aircraft manual control is peanuts and for some (older ones) flying without yaw damper is a way to disaster . Ohhh, the yaw damper works on the ailerons ????? Hmmm.

Diverging oscillation happens when the counter action is out of phase, IE, applied to late, which is typically for humans, not being trained on the effect.

[WideScreen]

That rings a bell - I remember when we studied Dutch Roll way back when as an undergrad, but for the most part what I remembered was that, while it was possible to design it out, the 'cure' tended to be worse than the disease, while a (relatively) simple yaw damper gave you the best both worlds.
While "designing out" is somewhat possible, it still is patch (on patch, on patch, etc) with limitations, simply because the root cause and the patches have a different physical nature.
As such, the yaw damper.

I recall being sent up to Vancouver, BC for a very high time 767 that was AOG due to "engine caused oscillations". Except when we really started looking, it had nothing to do with the engines, it was traced to a malfunctioning Yaw Damper (IIRC, the 767 had well north of 100,000 hours on it at the time).
Not the first time, not the last time, the component initially blamed is just suffering from the results and the root cause is somewhere else ;-)

Or so to say, when "patching" seems to be very difficult, chances are pretty realistic the root cause to apply the patch is something else as originally assumed.

[WideScreen]

......
HTBJ;- D. P. Davies (my emphasis)

A dutch roll is this combination of yawing and rolling motions; the yaw is not too significant, but the roll is much more noticeable and the aeroplane proceeds with a continuously reversing roll action. Not until the motion becomes very exaggerated is there any disturbance in pitch.
A more correct term for dutch roll is oscillatory stability. There is an associated quality known as spiral stability which needs to be mentioned here; it will be explained later, although the term itself is almost self-explanatory.
I'd say a good description. And the "oscillatory stability" (around 3 axis) is exactly what I describe too ;-)

......
The first thing to do is nothing — repeat nothing.
Agree, provided the aircraft is suitable stable.

Don't attempt to correct the manoeuvre with rudder
....

Yes and no, when the rudder application is in phase with the "natural" yaw, things can get worse very fast.
When 180 degrees opposite applied (and a suitable amount), it works without notice, IE the automated yaw damper.

While Davies does have exceptional books, he does not address the root cause of the DR, but only raises aspects that have influence on the (amount of) DR.

[WideScreen]

.....
I am not surprised.
Great, what did you notice with the experiment ?

[Vessbot]

Great, what did you notice with the experiment ?
I asked first.

[GlobalNav]

Or a pilot 'pedalling' the rudder pedals maybe ?
Yeah, it must have been pilot error. Boeing designs and builds perfect airplanes.

[Bergerie1]

Good video of what it looks like and feels like:-

https://www.youtube.com/watch?v=2tgfkGiHhxs

[BugBear]

Yeah, it must have been pilot error. Boeing designs and builds perfect airplanes.

Do we look for loose/missing mounting bolts on the PCU...? Position logic would get corrupted and command the Rudder to correct for "Sensed" rudder position... That would be interesting... "...it just blew...". (Not)
Large excursions at cruise could be ...

Does the Yaw Damper have "gently" mode?

[Loose rivets]

The first thing to do is nothing — repeat nothing.
Too many pilots have grabbed the aeroplane in a rush, done the wrong thing and made matters a lot worse. Don't worry about a few seconds delay because it won't get much worse in this time. Just watch the rolling motion and get the pattern fixed in your mind.
Then, when you are good and ready, give one firm but gentle correction on the aileron control against the upcoming wing. Don't hold it on too long — just in and out — or you will spoil the effect. You have then, in one smooth controlled action, killed the biggest part of the roll. You will be left with a residual wriggle, which you can take out, still on ailerons alone, in your own time.

I have a vivid memory of Barney Potts (Eagle and then Dan) correcting a vigorous input from the BAC 1-11 training bloke. Barney corrected four times in quick succession with lesser amplitude each aileron input. One Two Three Four, as quick as you can say it. "Well, that was easy" thinks the very young self.

[Fursty Ferret]

Ohhh, the yaw damper works on the ailerons ????? Hmmm.

No, it works on the rudder. Crucially though on jet transport aircraft the oscillatory characteristics tend to be amplified by pilot input on the rudder, as the time delay between input and response leads very quickly to the pilot making the situation worse. Couple this with the fact that any rudder reversal at high speeds regardless of the magnitude is probably a bad idea, it leads into the policy that if you need to correct it, you use the ailerons.

The fact that you’re making stuff up in order to try to discredit my reply strongly suggests that your own knowledge on the subject is limited.

[EXDAC]

1): No, it also happens with non-swepped wings, even the C172 does have this effect.


I have flown quite a few different light aircraft and gliders. All exhibited adverse yaw if no rudder input was made with the ailerons. None of these aircraft, which included the C-172, exhibited dutch roll.

[WideScreen]

No, it works on the rudder. Crucially though on jet transport aircraft the oscillatory characteristics tend to be amplified by pilot input on the rudder, as the time delay between input and response leads very quickly to the pilot making the situation worse. Couple this with the fact that any rudder reversal at high speeds regardless of the magnitude is probably a bad idea, it leads into the policy that if you need to correct it, you use the ailerons.
What you describe is the danger of out-of-phase controlling and over controlling. Still the best option to counter the DR is with the rudder, very effective and harmless when done properly. There are reasons the automated DR stabilizing stuff goes through the rudder......

The fact that you’re making stuff up in order to try to discredit my reply strongly suggests that your own knowledge on the subject is limited.
Thanks for your compliment.

Actually, I originally did have a 2 lines about the ailerons included, though did remove it:

Of course, you can "counteract" with the ailerons, but, the DR started due to the aileron / banking action. Undo that action and the yaw goes away. But, hey, we did have a reason to bank, so, when counteracting the DR using the ailerons you forgo the roll intention. Better have a proper rudder input.

[WideScreen]

I have flown quite a few different light aircraft and gliders. All exhibited adverse yaw if no rudder input was made with the ailerons. None of these aircraft, which included the C-172, exhibited dutch roll.
You have to go up again in your C172 and try again.

When initiating a roll, the nose of the C172 will turn a little to the opposite direction (flight path doesn't change, except for the roll effect). For the C172 the effect is minimal, this is stable and doesn't start to oscillate, so not the ugly effects of DR, but the effect is there. Bank some 20 degrees to the left and the nose goes some 5-10 Degrees to the right. When not aware it happens, you won't notice, but is does happen. Of course, we can argue, whether to call this DR, though it is just the start for a DR (and with a C172, it is just that).

[Capn Bloggs]

What you describe is the danger of out-of-phase controlling and over controlling. Still the best option to counter the DR is with the rudder, very effective and harmless when done properly. There are reasons the automated DR stabilizing stuff goes through the rudder......

Of course, you can "counteract" with the ailerons, but, the DR started due to the aileron / banking action. Undo that action and the yaw goes away. But, hey, we did have a reason to bank, so, when counteracting the DR using the ailerons you forgo the roll intention. Better have a proper rudder input.
That second quote doesn't make sense, and I was taught exactly the opposite of all of that.

It doesn't matter how it starts, stop it by "snapping" the roll with the ailerons. Lock the rudder and don't use it to recover.

Have a read of Fly The Wing page 24.

As for Dutch Roll in a 172. Seriously?

[AirScotia]

Happened May 25th. Reported to NTSB June 7th. Is a two-week delay normal? Southwest presumably knew about the damage pretty quickly.

[olster]

This is the inherent danger of pprune and misinformation or lack of fundamental understanding. Dutch roll and recovery should be demonstrated on a swept wing transport aircraft type rating course, presumably on the simulator. It may be overlooked or down played. However as most Atpl holders should know it is oscillatory instability due to lateral stability having dominance over directional. As DP Davies, a noted test pilot of the early jet era has stated. Recovery from Dutch roll is by a series of aileron inputs of lesser magnitude. Do not apply rudder as this could lead in worst case,scenario structural failure of the vertical empennage.

[172_driver]

IIRC, the 737 (NG at least) could be dispatched with Yaw Damper U/S. The only restriction was not to use flaps 40 for landing. The natural dutch roll tendency cannot be that bad?

[olster]

172 driver you are correct in that the B737 NG could be dispatched under CAA / EASA regs without a normal yaw damper. Some operators under FAA rules do not allow this. There is another caveat apart from the Flaps 40 restriction in that areas of moderate to severe turbulence should be avoided. I once took a B737-300 on a routine flight without the yaw damper as per MEL. After landing I vowed not to repeat the experience. Even light turbulence caused a very uncomfortable ride bordering on the hazardous. You make a valid point because I do not believe that the aircraft should be dispatched without a serviceable yaw damper particularly when recovery from a Dutch roll condition is probably not trained properly.

[Vessbot]

Of course, you can "counteract" with the ailerons, but, the DR started due to the aileron / banking action. Undo that action and the yaw goes away. But, hey, we did have a reason to bank, so, when counteracting the DR using the ailerons you forgo the roll intention. Better have a proper rudder input.
In the 20 years since I've learned about the existence of Dutch roll, this is the first time I've seen the advice to stop it with rudder.

In jets, the motion is dominated by roll, which is far more noticable and identifiable. As someone else pointed out, it is easy to misidentify the yaw and put in the wrong input, or too late, or hold it in too late, and feed into the oscillation instead of damping it. (Keep in mind that this is likely that pilot's first ever use of the rudder in the type, for other than a crosswind landing. How much do you trust that it'll be well-executed?)

Better to stick with the conventional advice.

https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/615x737/dutch_roll_damping_technique_98619cd4d1b5a62ea97b4441b2525db d4b5c1150.jpg

[Vessbot]

As for Dutch Roll in a 172. Seriously?
Yes actually, in a manner of speaking. All the oscillatory modes exist in all airplanes, but it's the question of how damped it is (very) and how likely it is to be excited unintentionally (not likely). So it can effectively be ignored for this type of plane.

If you give it a rudder push to swing the nose over a big amount and let go, the yaw will oscillate a few times before it settles (like a springy weather vane). If you look over at a wing tip during this, you'll see it going up and down with each of the cycles too: Dutch roll. Unlike swept wing jets, it's dominated by yaw, so the roll is pretty small; maybe not even noticed before.

[tdracer]

You have to go up again in your C172 and try again.

I don't remember much of what we were taught about Dutch Roll, but I do remember that Dutch Roll is a characteristic of low wing dihedral.
A Cessna 172 doesn't have either one, so talking about Dutch Roll of a C172 is meaningless.

[aeromech3]

Back to the issue please: A modern airliner, with paying passengers, experiences a Dutch Roll episode at FL340; it is fitted with a computerised Yaw damper system.
Subsequent investigation reveals damage to the Standby PCU mounting and structures in the fin.
There has been a suggestion that "Or a pilot 'pedalling' the rudder pedals maybe ?" which would be absurd.
I recall the L1011 SAS in the 1970's and as such a lot of knowledge available to protect the fin structure.
Surely we don't need to wait years for this to be solved by Boeing.

[Vessbot]

Back to the issue please: A modern airliner, with paying passengers, experiences a Dutch Roll episode at FL340;

Yes and from the scant initial facts, we have no idea of the sequence of events. What came first, the PCU damage or the Dutch roll, what if any other event precipitated either of those, etc.

Ruminating over tangents (which themselves may lead to interesting if not educational discussions), is really the only thing that can happen in the thread.

[aeromech3]

1) The standby PCU sits there being un-powered manually stroked, unless the stand-by system is powered.
2) Or, it has an hydraulic lock / seizure in which case the A+B main PCU would have to fight it.
3) There is provision for input valve seizure, (lessons learnt), but not PCU seizure, as I understand it.
4) Yaw inputs to the main PCU, even in the minor yaw damper mode at FL340, would cause a major stress if the standby PCU does not follow the movement!
I am sure other Techs can add their ideas.

[Capn Bloggs]

There has been a suggestion that "Or a pilot 'pedalling' the rudder pedals maybe ?" which would be absurd.
Absurd indeed, except that a pilot "pedalled" the rudder on an A300 some years back so much that the fin was ripped off and they all died. AA587.

[hans brinker]

What you describe is the danger of out-of-phase controlling and over controlling. Still the best option to counter the DR is with the rudder, very effective and harmless when done properly. There are reasons the automated DR stabilizing stuff goes through the rudder...
Of course, you can "counteract" with the ailerons, but, the DR started due to the aileron / banking action. Undo that action and the yaw goes away. But, hey, we did have a reason to bank, so, when counteracting the DR using the ailerons you forgo the roll intention. Better have a proper rudder input.

You seem to have a habit of trying to argue with people who know better. Show us a manual/procedure for a large swept wing het that tells the pilots to correct for Dutch roll with rudder....

[BugBear]

You seem to have a habit of trying to argue with people who know better. Show us a manual/procedure for a large swept wing het that tells the pilots to correct for Dutch roll with rudder....

That plum is missing from the media...leaving the door wide open for "pilot error".

The Rudder destroyed itself, and could have killed them all... Fullstop. ....bravo crew

[AirScotia]

Am I the only one who is a little surprised that the flight experienced an uncommanded roll severe enough to cause substantial damage to the aircraft, but:

1) The pilots continued flying for just under an hour (surely they could have found somewhere to put it down between Phoenix and SF?)

2) The damage must have been seen at Oakland, because the plane remained there for eleven days. So why didn't Southwest notify the NTSB immediately? The notification seems to have coincided with the plane arriving at Everett for repair. Presumably Boeing alerted the NTSB? Why didn't Southwest report it?

[BugBear]

AirScotia...

No emergency descent? Ready to Ferry takes eleven days?

​​​​​​From 32thousand feet LA or Oakland is a tossup.
Alot of Sierras beneath the aircraft either way.

No pax interviews? Why the silence.

[AirScotia]

AirScotia...

No emergency descent? Ready to Ferry takes eleven days?

​​​​​​From 32thousand feet LA or Oakland is a tossup.
Alot of Sierras beneath the aircraft either way.

No pax interviews? Why the silence.
They landed 55 minutes later, so presumably past the mountains on the standard routing. Plenty of possibilities if they were worried about an aircraft that had done something bad and unexpected.

Do you have to wait till you've got the a/c ready for a ferry flight before you alert the authorities that the plane departed safe flight and big things broke? That's eleven days when someone else flying the same type could get the same fright in less benign conditions?

The pax presumably weren't aware that anything bad had happened. Chances are it would just seem like turbulence the pilots had failed to warn them about. They landed in the right place and nothing was hanging off the plane. But the pilots knew the plane had misbehaved in a way it was designed not to do.

Looks as if Boeing fessed up because right now, Boeing.

[BugBear]

What's the fine for deliberately overwriting the CVR to erase incriminating evidence....? The plane was grounded for repairs, then a ferry only flight to Boeing in Everett. The authority that limited the flight to ferry only, "no commercial carriage" has a "reporting responsibility" under the statute...imo

[Chris2303]

If you give it a rudder push to swing the nose over a big amount and let go, the yaw will oscillate a few times before it settles (like a springy weather vane). If you look over at a wing tip during this, you'll see it going up and down with each of the cycles too: Dutch roll. Unlike swept wing jets, it's dominated by yaw, so the roll is pretty small; maybe not even noticed before.

​​​​​​​Isn't that "the further effect of yaw" that is (should be) taught to all student pilots? It was when I got my PPL 50 years ago.

[aeromech3]

In my old neck of the woods, the local FAA office was Rome and when I needed a 'Ferry' clearance for N reg, it could take a couple of days for Rome to talk to USA, time zones etc; depending on the problem, a senior crew would need to be assigned and they could be down route; an example would be a flapless flight or one engine out ferry; in some instances procedures were already approved for the type, but there were (O) and (M) to do like removing fan blades, boroscoping the remaining serviceable engines, all takes time.
A maintenance positioning flight might have been the way around this impasse, but with structure damaged unlikely as I would presume the standby rudder PCU would be classed as inop and to position to Boeing also a definite flag.
Whilst 11 days seems extra long there must have been a lot of consultation and planning to do the repair, drawings, material, personnel+hangar space; also whether to send a team to Oakland or Ferry the 2 hr to Everett.
I don't see anything that suggests the FAA were out of the picture early on.

[Vessbot]

Isn't that "the further effect of yaw" that is (should be) taught to all student pilots? It was when I got my PPL 50 years ago.
Not familiar with that term from over here in the North Atlantic colonies, but it sounds like dihedral effect: that yaw also results in roll in the same direction. And short answer, yes it is that... over and over again, fed each repetition by its mirror effect, adverse yaw.

[IGh]

Over the past seven decades (think B707, B727, B747) , inside Boeing, such odd events were often attributed to "Discrepant Rudder".

Strictly, Dutch Roll characteristic was "undamped and divergent" on B707, B727, & B747,
under some conditions, as cited, B727-100 was so above FL250
lacking the artificial stability of Boeing's "Yaw Damper" [see Bill Cook's "Road to the 707" (https://drive.google.com/file/d/1S0wDE3LBB9vL09s9hUz7j6AU-vBAjQ7w/view?usp=drive_link) pgs 180-4].

Inside Boeing we cited the "coefficient of critical damping" under various conditions.

Now imagine your own versions of all the various sorts of "discrepant rudder"
-- when that engineered safety device eventually fails,
-- passive failure, electric rudder "soft-over", hydraulic rudder "hard-over" , Yaw Damper inop, Yaw Damper active .

During the mid' to late 1970's our industry learnt from various failures --
most important lesson came from a Lockheed military transport :
C-141-A , 59402 , 11Nov76, over north Pacific near Campbell Islands, CRZ FL410, active "discrepant rudder",
then Yaw x Roll = DIVE ! (https://drive.google.com/file/d/1HBXGkjKyYAIT8_HJXrOLtz2rODfBxvUz/view)vertical descent until about FL210
(the co-pilot told me they were spinning around a point), both these pilots were only a few years after their military spin recovery practice during UPT.

Suspected Cause-Effect Entanglement (https://drive.google.com/file/d/1DBL7rxJ9_NawsEDP42qpwMh9kCjQWLmz/view?usp=drive_link)??
Re' the NTSB's mention of structural failure of Rudder's PCU supports :
hmmm ? A fracture [ after-effect ] from that B737 upset- mishap?
or a hidden- fracture that had occurred BEFORE the mishap (https://aaiu.ie/sites/default/files/report-attachments/REPORT%202014-011_0.pdf) ?

[msbbarratt]

An interesting thing one (used to) learn on electronics degress in the analogue control system theory modules was that all oscillators are failed control systems, and all stable control systems are failed oscillators. It's quite interesting reading this thread and comparing comments to the properties of analogue control systems / oscillators, and how aircraft dynamics can be provoked into switching between the two.

A related topic is Chaos - as in Mandlebrot sets, fractals, etc. These all feature mathematical feedback of some sort, just like control systems / oscillators, but exhibit apparently random behaviours when the feedback in the system is just so. If an aircraft were driven into such a regime, there'd be no "watching and learning the pattern", because there wouldn't be one.

[punkalouver]

AFAIK, designing out Dutch roll starts by eliminating sweep of the wings and dihedral and reducing the vertical stabilizer area, which will decrease yaw opposition (wont' come straight as fast) and make for worse asymmetric engine handling. I think that swept wings for transonic transports are going to stay and dihedral is so useful on low-wing aircraft, particularly with pylon mounted engines, when landing that that will also stay.



Looks like straight-wing turboprops can have Dutch Roll as well. Have to admit, I was surprised when I read this. Anyone care to explain?

From the ATR-42 manual:

"RUDDER SHOULD NOT BE USED:
- To induce roll, except in the previous case (Aileron jam) or
- To counter roll, induced by any type of turbulence.
Whatever the airborne flight condition may be, aggressive, full or
nearly full, opposite rudder inputs must not be applied. Such inputs
can lead to loads higher than the limit, or possibly the ultimate loads
and can result in structural damage or failure.
Note: Rudder reversals must never be incorporated into airline
policy, including so-called “aircraft defensive maneuvers” to
disable or incapacitate hijackers.
As far as dutch roll is concerned, yaw damper action (if selected) or
RCU (*) are sufficient to adequately dampen dutch roll oscillations.
The rudder should not be used to complement the yaw damper
action.
(*) RCU Releasable Centering Unit"

[MechEngr]

The ATR-42 has a small amount of sweep on the leading edge ,where the majority of lift is generated, and a small amount of dihedral. I wonder if the side area of the fuselage below the wing contributes to rolling from yaw. Probably depends on where the vertical CG is.

[zzuf]

This discussion on the "Dutch Roll" is at least partially generated by differing language between the flying ops people and stability and control engineers.
To flying ops people the Dutch Roll is the oscillation they may see looking out the windscreen. To the engineers, it is also "Dynamic Lateral Directional Stability" or maybe it will include the word "oscillatory".
It is highly unlikely that any poster here would experience any aircraft which doesn't behave as a simple "mass, spring, damper" when disturbed around any axis. So, all the aircraft you fly have a dutch roll mode. All are tested, and the damping assessed during those tests, will determine if natural damping is accepted or if one or multiple yaw dampers are required (stability augmentation system). This will also greatly influence the required MEL or failure procedures.
The interaction between directional static stability and lateral static stability will determine the picture of the oscillation that the pilot views.
I have done hundreds of these test in many aircraft, ranging from large jet transports to the smallest homebuilts. The oscillation that you see may be: mainly in roll (wing rocking), mainly in yaw (snaking) or a combination. It may be damped (sometimes very heavily) or rapidly divergent (with no pilot input).
Vessbot mention viewing the path drawn by the wingtip, during a dutch roll, it can give information about the roll/yaw ratio and roll/yaw phasing.
I suppose it could be said that the dutch roll occurs because both directional static stability and lateral static stability depend on sideslip. In the case of directional static stability (and longitudinal static stability) a restoring force or moment is generated as soon as a displacement from trim occurs. There is no restoring force for a bank angle displacement - until sideslip develops. You could consider the handling characteristics of an aircraft which produced a corrective rolling moment every time the bank angle changed. Perhaps a competition aerobatic pilot would prefer that his aircraft had no rolling moments due to sideslip (neutral lateral static stability) - purely because he doesn't wish to have to consider roll changes every time he changes sideslip angle ( he would still have rolling moments due to yaw rate though).
This is obviously a simplistic look at the situation, an aircraft could require dual yaw dampers, not because of DR damping but because of a dual panel rudder to comply with control jam requirements. FBW situation is not included. The manufacture may decide to install a single YD for passenger comfort, not for damping non-compliance. For those interested a read of AC 25-7D and AC 23-8C will be informative.

[fdr]

WIde;

your TV remote has nothing to do with dutch roll, unless the TV was playing some program related to aerodynamic coupling. Your remote is an example of thew Dzhanibekov effect, A.K.A the tennis racket effect and is related to the inertial moments around the axis being different. It's Nisei math, but unrelated to the coupling that occurs with dutch roll.

Dutch roll does not require swept wings too occur, there are a number of straight wing jets that will belie the need of sweep to make for some entertainment.

[safetypee]

Adding to zzuf's informative post, D. P. Davies, (HTBJ) continues from post #41

"Yaw and roll dampers.
When an aeroplane has a significant dutch roll, that is anything less stable than a reasonably quickly damped variation, some assistance is necessary to avoid a tedious and demanding task for the pilot. It has previously been pointed out that the basic cause of a dutch rolling tendency (apart from wing sweep of course) is lack of effective fin and rudder area; the point has also been made that too large a fin area is detrimental to spiral stability qualities.
The final choice of fin and rudder area, therefore, is a compromise, as always.
If because of this the size of the fin cannot be increased the effective fin area must be increased in some other way.
On some early jet transports with manually operated rudders, the rudder tended to trail downwind in a sideslip, at least over small angles; this decreased the effect of the fin and made the oscillatory stability worse.
Boosting the rudder from zero angle resulted in the rudder remaining central in a slip thus increasing fin size and this materially improved the dutch roll.
On aeroplanes with power operated rudders (which now means most of them) the obvious step is to apply rudder against the yaw to prevent the slip starting or building up. This is exactly what a yaw damper does.
A yaw damper is a gyro system sensitive to changes in yaw which feeds a signal into the rudder which then applies rudder to oppose the yaw. With this device a dutch roll will not develop because the yaw which triggers it all off is not allowed to develop. If a dutch roll has developed with the damper off then switching it on will result in the aeroplane being brought under control. Under normal operation the damper cannot make a mistake, but applies the rudder in the correct direction and in the correct amount, thus reducing the slip angle to zero and stopping all rolling tendency.
The number of dampers required is a function of the dutch roll qualities of the naked aeroplane and the philosophy of the power control system design. If the roll is merely tedious in its demand normally only one damper is required; it is accepted that, for a damper failure en route, it is not a great hardship to continue to the destination. If the dutch roll is significantly unstable then two dampers are required so that the failure of one en route still leaves the aeroplane with some protection. In the case of marked dutch roll instability three dampers can be fitted. While it is generally true to say that the number of dampers is a reflection of the degree of instability this is not always so — some constructors fit more than the minimum demanded by the requirements so as to cater for allowable deficiencies, for example. If the power operated rudder is of the split surface design, then naturally each portion should have its own damper.
Basically there are two types of yaw damper. The early one was hooked into the rudder control circuit so that it applied rudder control through the same control run as did the pilot and its activity was reflected by rudder bar activity. While this was a comforting reassurance of its serviceability it did increase rudder control loads. To prevent this making matters worse in the event of an engine failure on take-off or a cross-wind landing the damper was switched out for take-off and landing. As this damper effectively paralleled the pilot's actions it has come to be known as a parallel yaw damper.
The later type of yaw damper is known as a series yaw damper. This hooks into the rudder control circuit effectively right at the back of the aeroplane in such a way that it does its job of moving the rudder without moving the pilot's rudder control circuit. As foot forces are not increased with the series yaw damper operative it may be used for take-off and landing.
Some aeroplanes have, additionally, a roll damper; this does substantially the same job as a yaw damper but works through the aileron controls instead.
Where this is done it is not necessarily for dutch roll damping; it can be purely for roll damping in turbulence on a type where the rolling inertias are such that this sort of damping is needed. It will of course control a dutch roll through the ailerons and can thus be equated with a yaw damper.

So ends the lesson on dutch roll and yaw dampers. It has been spelled out in some detail to emphasise the fact that with knowledge, practice and a measure of prudence there is really not much to it. The prudence bit needs to be underlined; with sweep angles increasing and the design of jet aeroplanes being stretched all the time it is likely that oscillatory stability will get worse with a heavier reliance on stability augmentation in general.
As training involves, quite rightly, a proper understanding of the basic flying qualities of the type it follows that a training captain and a pilot under training can be exposed to flight conditions inhere oscillatory stability is markedly negative. To make this sort of operation safe the excitation of dutch roll should be made gently and with care, and it is essential that the capture capability of each yaw damper, where, more than one is fitted, is known with a fair degree of accuracy. On one type presently flying, the Flight Manual drills are quite explicit and cover the extension of air brakes and an immediate reduction in altitude if the recovery of a diverging dutch roll looks like being delayed, or looks like achieving large angles of roll with associated high angles of sideslip.
Take the trouble to know your aeroplane in detail and keep in practice in dutch roll recovery if it is one of those which has a significant dutch roll; a dark and dirty night when you have a load of passengers is no time to find out whether you or the aeroplane is master of the situation."

P.S. for those who wish to debate with a range of contributors, the link below gives some advice - also applicable to ourselves - self.
Note the embedded links re 'first principles' and 'circle of competence' - relating to the above re understanding the basics.
https://fs.blog/experts-vs-imitators/

[Bergerie1]

safetypee,

Thank you for quoting DPD and HTBJ. His book was one of my 'bibles', and I also had the pleasure of flying with him on the VC10. If any of you Prooners want to get the facts from a real expert in an easily digestable form I strongly recommending reading HTBJ:-
https://www.amazon.co.uk/Handling-Big-Jets-D-P-Davies/dp/B00EKYR8WK

I also recommend listening to these podcasts from the RAeS which can be found here:-
https://www.pprune.org/tech-log/602953-d-p-davies-interviews-certificating-aircraft.html

However advanced our aircraft may become, the basics remain the same and need to be thoroughly understood. And finally, you should take the time to read the link at the end of safetypee's post about the difference between experts and imitators

[Capn Bloggs]

For those scouring their Handling The Big Jets looking for DPD's "pilot stuff" discussion on DR, it's on page 99 (Section 5); it's NOT listed in the Index! :hmm:

[Gino230]

What I'm trying to understand is how the Standby PCU became damaged at it's mounting points / mounting structure. Standby PCU should never be powered unless the FFM engages it, or it's engaged manually by placing the flight control switches to STANDBY RUD.

Even in the event of a mechanical failure, the Standby and Main PCUs are supposed to have a breakout mechanism (as I understand the system). My impression of the standby yaw damper is that it is also only engaged along with Standby Rudder.

The "rumor" is that Boeing already told another 737 operator that the damage was preexisting due to high winds encountered when the aircraft was parked. No idea if that's true or not.

There are alot of new pilots around the airline business, I am willing to bet they've never seen a dutch roll demonstration in the sim.

[BugBear]

Methinks the damage to PCU was due to a back driven process. Lord knows how much energy a back driven rudder can attain, hanging out well into the M 0.90 airstream...

If so, thankfully the rudder broke. Had PCU kept it's mounts, we might be discussing a smoking hole.
Better a loose rudder in trail than a sound one trying to kill everyone....

Airbus300, American, Queens

[Speed_Trim_Fail]

For those scouring their Handling The Big Jets looking for DPD's "pilot stuff" discussion on DR, it's on page 99 (Section 5); it's NOT listed in the Index! :hmm:

I have just read this passage with interest, which is absolutely in line with what I was taught many years ago for my first Boeing TR.

[BugBear]

I appreciate the discussion re aerodynamic anomalies. The case at hand may or may not be related... At all... What precipitated the a/c to lose stable flight at cruise? It certainly hadn't anything to do with pilots mishandling....

Spontaneous action by an automatic system that caused self inflicted substantial damage to the airframe... Seems a starting place...the flying public would be better served by a preliminary disclosure. Boeing quality, or lack of, seems to be a threat....

​​​​​it wasn't birds

[EEngr]

I appreciate the discussion re aerodynamic anomalies. The case at hand may or may not be related... At all... What precipitated the a/c to lose stable flight at cruise? It certainly hadn't anything to do with pilots mishandling....


Just guessing here: It's possible that pre-existing damage to the standby PCU interfered with normal mode control functions, including the YD. From that point on, an uncorrected Dutch Roll grows in amplitude until some mechanical linkage or mount point "hits its stops".

The story of troops marching in step across a suspension bridge comes to mind. An oscillating mode grows in amplitude by small steps until something breaks

[BugBear]

Just guessing here: It's possible that pre-existing damage to the standby PCU interfered with normal mode control functions, including the YD. From that point on, an uncorrected Dutch Roll grows in amplitude until some mechanical linkage or mount point "hits its stops".

The story of troops marching in step across a suspension bridge comes to mind. An oscillating mode grows in amplitude by small steps until something breaks

Quite... Excellent, and a follow on might be a damaged actuator biased in one direction, causing a constant overcorrection from damp neutral. The rudder control unit sensing would precipitate aggravated wear on the YD...Still, why the delay in reporting a flight critical damage?

From time of initial damage (which Boeing eventually reports), a slight but chronic YAW.

Wouldn't a deadheading pilot sitting aft constantly want to correct with his ghost pedals?

Where is the frikn FAA ??