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Southwest 737MAX Dutch roll rated accident

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Southwest 737MAX Dutch roll rated accident

Old 12th Jun 2024, 18:57
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Southwest 737MAX Dutch roll rated accident

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.
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Old 12th Jun 2024, 19:13
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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...
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Old 12th Jun 2024, 19:41
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Or a pilot 'pedalling' the rudder pedals maybe ?
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Old 12th Jun 2024, 19:46
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Originally Posted by 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...
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.



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Old 12th Jun 2024, 19:49
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Am I right in thinking that "the occurence was rated an accident" means that it is considered to have been a life-threatening event?
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Old 12th Jun 2024, 20:02
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Originally Posted by 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?
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).
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Old 12th Jun 2024, 21:03
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Originally Posted by A0283
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?
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Old 12th Jun 2024, 21:18
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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.
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Old 13th Jun 2024, 05:33
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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.
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Old 13th Jun 2024, 07:46
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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.

Last edited by WideScreen; 13th Jun 2024 at 07:57.
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Old 13th Jun 2024, 08:54
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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.
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Old 13th Jun 2024, 09:23
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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'.
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Old 13th Jun 2024, 10:30
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Originally Posted by 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'.
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?

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Old 13th Jun 2024, 11:21
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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.
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Old 13th Jun 2024, 12:03
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Originally Posted by 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 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.
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Old 13th Jun 2024, 13:48
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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!!
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Old 13th Jun 2024, 15:44
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Originally Posted by 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.
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.

Originally Posted by procede
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.

Originally Posted by procede
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.
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Old 13th Jun 2024, 15:54
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Originally Posted by 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.
Probably.

Originally Posted by tdracer
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......

Originally Posted by tdracer
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.

Last edited by WideScreen; 13th Jun 2024 at 16:22.
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Old 13th Jun 2024, 15:57
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Originally Posted by 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.
Yep, that is what to do (manually), when things already are "noticable".

Originally Posted by Bergerie1
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).
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Old 13th Jun 2024, 16:00
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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:
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