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Southwest 737MAX Dutch roll rated accident

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Southwest 737MAX Dutch roll rated accident

Old 14th Jun 2024, 09:00
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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.
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Old 14th Jun 2024, 09:08
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Originally Posted by 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.
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.
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Old 14th Jun 2024, 10:44
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Originally Posted by tdracer
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.

Originally Posted by tdracer
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.
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Old 14th Jun 2024, 10:56
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Originally Posted by safetypee
......
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 ;-)

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

Originally Posted by safetypee
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.
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Old 14th Jun 2024, 10:58
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Originally Posted by Vessbot
.....
I am not surprised.
Great, what did you notice with the experiment ?
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Old 14th Jun 2024, 12:56
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Originally Posted by WideScreen
Great, what did you notice with the experiment ?
I asked first.
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Old 14th Jun 2024, 17:30
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Originally Posted by Uplinker
Or a pilot 'pedalling' the rudder pedals maybe ?
Yeah, it must have been pilot error. Boeing designs and builds perfect airplanes.
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Old 14th Jun 2024, 19:52
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Good video of what it looks like and feels like:-

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Old 14th Jun 2024, 21:02
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Originally Posted by GlobalNav
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?
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Old 14th Jun 2024, 21:45
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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.
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Old 15th Jun 2024, 02:32
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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.
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Old 15th Jun 2024, 03:30
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Originally Posted by WideScreen
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.
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Old 15th Jun 2024, 07:22
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Originally Posted by Fursty Ferret
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......

Originally Posted by Fursty Ferret
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.
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Old 15th Jun 2024, 07:31
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Originally Posted by EXDAC
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).
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Old 15th Jun 2024, 08:50
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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......
Originally Posted by Widescreen
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?
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Old 15th Jun 2024, 09:52
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Happened May 25th. Reported to NTSB June 7th. Is a two-week delay normal? Southwest presumably knew about the damage pretty quickly.
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Old 15th Jun 2024, 10:12
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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.
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Old 15th Jun 2024, 10:41
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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?
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Old 15th Jun 2024, 10:53
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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.
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Old 15th Jun 2024, 13:25
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Originally Posted by WideScreen
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.


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