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Southwest 737MAX Dutch roll rated accident

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Southwest 737MAX Dutch roll rated accident

Old 13th Jun 2024, 16:11
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Originally Posted by WideScreen
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?
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Old 13th Jun 2024, 16:27
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Originally Posted by Vessbot
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.....
Originally Posted by Vessbot
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 !
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Old 13th Jun 2024, 16:31
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Originally Posted by MechEngr
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.
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Old 13th Jun 2024, 16:35
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Originally Posted by 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.
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).
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Old 13th Jun 2024, 16:58
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Originally Posted by WideScreen
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.
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Old 13th Jun 2024, 17:18
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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.
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Old 13th Jun 2024, 17:21
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Originally Posted by MechEngr
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...
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Old 13th Jun 2024, 17:38
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Originally Posted by WideScreen
-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.

Last edited by hans brinker; 13th Jun 2024 at 18:15.
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Old 13th Jun 2024, 18:20
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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?
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.
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Old 13th Jun 2024, 18:25
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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.
​​​​​​
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Old 13th Jun 2024, 19:33
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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...
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.
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Old 13th Jun 2024, 19:36
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When I hear B737and PCU in the same sentence I think UAL 585, USAIR 427 and Eastwind 517...
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Old 13th Jun 2024, 21:19
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Originally Posted by Larry in TN
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,

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.
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Old 13th Jun 2024, 23:03
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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?
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Old 14th Jun 2024, 02:59
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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.
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Old 14th Jun 2024, 03:18
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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
KC-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?
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Old 14th Jun 2024, 04:28
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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

Last edited by aeromech3; 14th Jun 2024 at 04:39.
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Old 14th Jun 2024, 04:30
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Originally Posted by procede
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.


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Old 14th Jun 2024, 06:27
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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.

Last edited by blind pew; 14th Jun 2024 at 07:25.
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Old 14th Jun 2024, 08:59
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Originally Posted by hans brinker
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.

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