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Southwest 737MAX Dutch roll rated accident

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Southwest 737MAX Dutch roll rated accident

Old 17th Jun 2024, 08:20
  #81 (permalink)  
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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.
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Old 17th Jun 2024, 08:23
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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/


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Old 17th Jun 2024, 09:08
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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:-
Amazon Amazon

I also recommend listening to these podcasts from the RAeS which can be found here:-
D P Davies interviews on certificating aircraft

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
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Old 17th Jun 2024, 10:08
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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!
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Old 17th Jun 2024, 15:13
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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.
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Old 17th Jun 2024, 15:31
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M

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

Last edited by BugBear; 17th Jun 2024 at 15:45.
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Old 17th Jun 2024, 16:03
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Originally Posted by 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!
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.
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Old 17th Jun 2024, 21:35
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Dutch Roll

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
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Old 18th Jun 2024, 15:00
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Originally Posted by 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....
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
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Old 18th Jun 2024, 15:43
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Originally Posted by EEngr
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 ??

Last edited by BugBear; 18th Jun 2024 at 18:00.
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Old 3rd Jul 2024, 22:22
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Accident aircraft departed KPAE today on a positioning flight to KPHX. Scheduled for return to revenue service on 7/5.
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Old 9th Jul 2024, 19:20
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NTSB Preliminary Aviation Investigation Report - SWA746/N8825Q
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Old 9th Jul 2024, 23:40
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Not one to suggest over-reach by any NTSB office or official - as just an SLF/attorney - but after reading the prelim report, nothing adverse to the investigation appears to have occurred as a result of the Chief Engineer's statement during testimony before a Senate Subcommittee recently. Perhaps when the report becomes final, how his statement adversely affected something, and for what reason such effect occurred, will become known.

Of course, one could counsel highest-level caution when the context is the NTSB investigation process disclosure and comment rules. Still, it is ironic, among the few things widely agreed upon in the 737 MAX debacle and in the larger Boeing traumas, is the necessity to restore Engineering preeminence. Perhaps unavailability of some faith in the integrity of the Chief Engineer's statement to the Subcommittee is just collateral damage. Probably too testifying alongside the embattled CEO allows for no benefit of the doubt.


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Old 10th Jul 2024, 02:02
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Originally Posted by 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.
While there was damage(which obviously is significant), it sounds like this thread of 5 pages has been made under the assumption that control of the aircraft was lost in a manner similar to crashes in the 1960's. In reality, there was an unusual oscillation felt by the crew and that is it.

Misled by the "regained control" statement.
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Old 10th Jul 2024, 02:11
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Originally Posted by WillowRun 6-3
Not one to suggest over-reach by any NTSB office or official - as just an SLF/attorney - but after reading the prelim report, nothing adverse to the investigation appears to have occurred as a result of the Chief Engineer's statement during testimony before a Senate Subcommittee recently. Perhaps when the report becomes final, how his statement adversely affected something, and for what reason such effect occurred, will become known.

Of course, one could counsel highest-level caution when the context is the NTSB investigation process disclosure and comment rules. Still, it is ironic, among the few things widely agreed upon in the 737 MAX debacle and in the larger Boeing traumas, is the necessity to restore Engineering preeminence. Perhaps unavailability of some faith in the integrity of the Chief Engineer's statement to the Subcommittee is just collateral damage. Probably too testifying alongside the embattled CEO allows for no benefit of the doubt.
Which only presents the corporate vultures with a prime takeover target. They will say the share price could be a lot higher with their budget cuts, takeover the company and "unlock" shareholder value. Rinse/repeat.
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Old 10th Jul 2024, 02:20
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Punkalouver...

"Misled by the "regained control" statement."

Thinking there is no "regained" without "loss of", I see your point.
I think changing "incident" to "accident", happened post shop.
Meaning the "Incident" disclosure was premature. Why call it anything until all the criteria are met? ....picking nits here... BB


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Old 10th Jul 2024, 02:49
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Avherald should remove that FAA "dutch roll" hogwash, especially since it also mentions the later NTSB report which clearly shows it wasn't.
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Old 11th Jul 2024, 12:29
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As there is no manual reversion on the rudder system and the Pilot reports rudder pedal movements and stiffness, then I await the strip report of the Stand-by PCU as its own support bracket failed.
The report mentions the stand-by system overnight checks yet states this was the last time it was used (FDR readout); I should have expected it to be part of the pre-flight crew check, is this not so (hark back to F/E checks on B727)?
As the stand-by PCU does not have a gust dampening function, the weather front report would seem irrelevant, but the design of the rudder primary stops, rigging and function of the main PCU for gust damping mode would deserve investigating.
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Old 11th Jul 2024, 13:42
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Originally Posted by aeromech3
As the stand-by PCU does not have a gust dampening function, the weather front report would seem irrelevant, but the design of the rudder primary stops, rigging and function of the main PCU for gust damping mode would deserve investigating.
It was my understanding the the standby PCU is designed to move with the rudder when not active. Is that true?

If it is true then a defective, binding, or jammed standby PCU would transfer all rudder gust front forces to the standby PCU mounting structure. It would be the primary load path instead of the damped primary PCU.
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Old 11th Jul 2024, 16:25
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Exdac, see my post #65.
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