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Southwest 737MAX Dutch roll rated accident

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Southwest 737MAX Dutch roll rated accident

Old 15th Jun 2024, 13:33
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Originally Posted by Capn Bloggs

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.

Last edited by Vessbot; 15th Jun 2024 at 14:06.
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Old 15th Jun 2024, 14:10
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Originally Posted by WideScreen
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.
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Old 15th Jun 2024, 14:20
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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.
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Old 15th Jun 2024, 14:31
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Originally Posted by aeromech3
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.

Last edited by Vessbot; 15th Jun 2024 at 15:18.
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Old 15th Jun 2024, 15:20
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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.
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Old 15th Jun 2024, 15:21
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Originally Posted by aeromech3
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.
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Old 15th Jun 2024, 15:45
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Originally Posted by WideScreen
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....
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Old 15th Jun 2024, 16:22
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Originally Posted by hans brinker
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

Last edited by BugBear; 15th Jun 2024 at 17:02.
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Old 15th Jun 2024, 17:42
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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?
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Old 15th Jun 2024, 18:52
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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.

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Old 15th Jun 2024, 19:58
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Originally Posted by 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.
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.
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Old 15th Jun 2024, 20:27
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FINE...

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
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Old 16th Jun 2024, 00:00
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Originally Posted by Vessbot
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.
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Old 16th Jun 2024, 03:44
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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.
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Old 16th Jun 2024, 06:16
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Originally Posted by Chris2303
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.
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Old 16th Jun 2024, 17:21
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"Discrepant Rudder" and Cause-Effect Entanglement

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" 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 ! 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 ??
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 ?

Last edited by IGh; 16th Jun 2024 at 20:46.
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Old 16th Jun 2024, 19:24
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Oscillators and Control Systems

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.
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Old 17th Jun 2024, 04:23
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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.
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"
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Old 17th Jun 2024, 04:50
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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.
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Old 17th Jun 2024, 06:22
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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.

Last edited by zzuf; 17th Jun 2024 at 07:19.
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