APC and other Dutch rolls
 

@Owain Glyndwr
I am holding that resonance definition of "dutch roll" from my automation teacher, an engineer working at the last French projects.

Iself had read conventional things about dutch roll in ATPL books but was interested by real dutch roll since 1979 from a Learjet Captain who feared it during any approach. I read and listening again.

And it happened that I decided to study automation after a background of pure and applied math, scientific informatik, and aeronautics.
One day (1985) the teacher resumed about stable/unstable systems : You know, phase planes, Lyapounov, Nyquist, Bode, sign of real part of poles, aso. And suddenly he had a thought, stopped one second, and said "Dutch roll is resonance tween the first degree system of roll of the aircraft, and action of pilot....

He ignored that at least one student from around 30 was concerned by dutch roll!
He came back to the class concerns, but the sentence was printed in my brain. I had not seen the learjet's or books' "dutch roll" in these terms. So I did nothing more during years with that declaration, despite all the respect I had for my teacher.

In my airline a MD 83 had in 1992 a dutch roll approaching NICE 05. They were very mute about it (well finished over the sea, the Captain leaved controls as he had learned as Cadet in USA. But the Airline head decided to let us discover the pleasures of dutch roll during the next off-line sim test. Nobody said they had all failed before me and I was the last one. The instructor was a former business pilot on Learjet! The first pilot started and lost 11000', we went on the back right and left... At the end the instructor said him very sad "You have seen it".

It was now to me. Suddenly I reminded to my automation teacher, and I thought that if "dutch roll" accorded to HIS definition, I could perhaps try to get it. I showed the animal,his position, speed, acceleration, counted seconds in my head, reckoned two easy differential equations in my head and piloted the result (nothing to do with the above B727 rolls described method).

And five times ago I was able to stop the "dutch roll" (in the sense of my former learjet instructor and the books) with a bank which was never more than 30°, in less than 30seconds, and never lost more than 1000 ft.

In my airline nobody asked me my method, included the instructor and the second pilot... Very sad,

I wrote it only in 1997 after I discovered that one of my former private pilot student had been killed as passenger from the Learjet which killed Baroin Sr perhaps in a dutch roll degenerating in a deepstall.Sad again.

I phoned to Leadair (LFPB) and finaly Learjet in Geneve (1998) . I told my story to the Chief Pilot, who answered they had finaly found a methodcagainst Leajet's dutch roll : ...sharing quickly the pedals... I asked me how it was possible to imagine that sequence.

On PPRuNe more recently I discovered that dutch roll was considered as a unworthy situation.

Machinbird who had written about PIO in the AF447 thread suggested me to read Mc Ruer's book about PIO and other APC.

I found there a world I had not found in aviation until that day : scientific methods and true solutions in the continuous and discontinuous domains of FBW and classical flights including the often missed rate limitations, omnipresent in FBW systems leading to oscillation (I don't remember mention in the FBW thread).

So I stay with my teacher's definition of dutch roll. I don't deny the roll and yaw reciprocity which conduced to the yaw damper conception, present on all airliners of course.

The failure of the yaw damper is the first cause of dutch roll or aggravated roll and yaw oscillations. If the pilot is not able to stop them quickly which is the most frequent case, the bank will really increase, also in the case where we still are in a first degree system in resonance with pilot's "normal" inputs...

About the stability of the "A300", the APA document refered by misd-agin is not so optimistic with the "FBW" technology version of the A300-600R. In any case iit needs its yaw-damper is functionning.

To mention too the sudden impulsion of yaw damper failure, and once again any brutality in piloting like quick reversals on rudder...

For the defense of some aircrafts major manufacturors I would like to remind that oscillation computation means are relatively recent.

It is apparent that you’ve likely never spent much time in a transport category airplane simulator – I would LOVE to hear about your “departure stall” and “stall-spin-recovery” maneuvers that you’ve done in simulators previously and that leaves me to wonder why the only “conversations on an aviation forum” that are stupid are those in which you have “been a part.”

Having my own membership card in the “professional gas-passers union” (active in the late 1960s through the early 1970s) I can say that, while the Ops Manual didn’t have a specific note in the text, a flight crew member couldn’t get through the “ground school” classes taught at Castle AFB without hearing (and I think my eardrums are still scarred) about the effectiveness of the rudder on that airplane, and how virtually ANY movement of it (powered or unpowered) was to be VERY sparingly used … and NEVER reversed!!!

I note that some of our number have concerns with a particular poster.

Rules of the forum are that we tolerate outlier posts while they reasonably don't go against the forum policies.

My suggestion is that those folks with a concern ignore such posts.

In the background, however, I shall monitor ...

I have flown the A306 for almost twenty years and never had a handling problem with it. My employer has flown it since 1990 and we have never had the prolems or incidents that AA had with theirs.I have flown a number of Boeing types and Airbus FBW and I would say that the nicest one to handfly was the A306 with the B757 the worst. The only time I had a problem with rudder on the 306 was an incident where we had an iced up pitot . This affected ADC1 and the aircraft was putting in huge amounts of rudder as it tried to co-ordinate turns as it thought it was flying at 80kt or less when in fact we were at Mn0.78.

I hope this thread ends with this post.

1. some people knew the plane's rudder would cause this to happen if ''reversed'', some didn't know. The real question and problem is why didn't the line pilots know? (and don't say they should have known, that's just a dopey excuse, like you should have known if you married ethel her cooking would give you gas)

2. Doing my bit of research, the US government was quite concerned over the role many interested parties had in trying (repeat, trying) to influence the outcome of the NTSB probable cause. Were there attempts to influence? Yes. Did they influence the final report...you decide.

3. The FAA has in the past known about problems with certain planes in certain conditions and have not passed the information along to the users of the planes. I can remember especially the problem with the F28 Fokker and so called ''hard wing planes''. An accident in Canada showed the problem and it was repeated three years later in the US...oops, the FAA forgot to tell the users.


4. There has been more than enough hatred on this thread as to ruin the bond we share in the sky. Claims and counter claims of experience and the like. One recent one about departure stalls in a transport category airplane sim really got to me. We did departure stalls in our douglas, mimicking a takeoff without proper flaps/slats, akin to the tragedy in Detroit many years ago...we called them departure stalls. We also did stalls in the approach/landing configuration and called them approach to landing stalls. We also did stalls in the clean configuration.

Don't attempt to pontificate on what airlines own wording is about unless you have flown for all airlines or approved their training programs.


Lessons that should be learned from this crash.

Engineers should think like pilots and make safeguards of every conceivable type to protect pilots from killing people.

Pilots should think like engineers and know that very few planes are as strong as pilots think they should be.

The FAA better make darn sure everyone knows more about their planes and that training and examination proves everyone knows.

And we better all know that flying can still kill you and those you are charged with protecting. Suffice to say, you should fly like a little old lady to stay out of trouble, but once you are in trouble fly like a tiger to get out...but be careful if you have a tiger by the TAIL.

And wake turbulence does kill, it caused a crash many years about (about 40) .

There may be unknown problems with you or your plane...be alert...I still think of the tragic loss with rudder hardover on the 737 as a great example...be ready, esp below FL180.


NOw, quit talking about this please

The B757 was my favorite airplanes to handfly. It flew beautifully into our most challenging airports. I loved it. It never gave me any surprises. It outperformed all of the other airliners I flew by far.

AR, re, "I’m describing the ability to not react out of panic, but rather function as you have been trained, choosing what your reaction will be – and I’ve described this in pilots as being somethink like …'scan – mentally process – feel – mentally process – scan – mentally choose a response – physically respond – scan – mentally process – feel the motion – visually confirm the motion cue – mentally process – etc.' "

Quite well put, really.

Words describing this process such as "recursive" are for me, helpful but essentially it's the same thing - constant re-evaluation of 'things' on a second-by-second basis, with calm. There really aren't very many issues/abnormals/emergencies in transport aircraft that one must instantly react/respond to at a "basal" level, so to speak.

The best thing to do in AF447's case was as you describe and then "do nothing", (which meant just keep everything the way it was because neither the airplane nor the engines care about a loss of speed indication; the energy is still there, the stable cruise flight at one altitude is there, so "do nothing" - monitor, wait, call for ECAM, etc, maintain discipline).

Likely the UAS event they experienced would have been over in less than two minutes and they'd have had a serviceable indication and as others have pointed out, a log-book entry.

Further to the point, this is what training is all about - to handle the so-called "startle factor", (I can't believe that our industry actually believes in such nonsense, but giving the notion a broad and generous interpretation, perhaps that's what automation and not staying in the books engenders in pilots who may not know their airplane sufficiently well?)

I can't remember an airliner wake turbulence crash since wake turbulence at best May have caused some control changes before separation of AA tail but wasn't the cause. SNA had a corporate jet crash behind an airliner landing about 20 years ago caused by wake turbulence. Pilots should be able to handle it with experience.

Original Quotes by flarepilot:
Not so fast!
#1. Just because you drive a Cadillac doesn't mean you can't have a wreck.
Being an engineer, I think aircraft designers and propulsion engineers do a reasonable job at incorporating safeguards of every conceivable types reasonably expected, trouble is new unreasonably types keep being invented.
Everyone should know that if you cycle something back and forth often enough and rapidly enough it will eventually break, it is called fatigue determined by the severity of the forces generated during the cycling and how many times it was cycled.
Don't you think the airlines and their pilot training organizations have more of the responsibility for this? It seems to me those closest to the issues play the biggest role and bear the most responsibility. Organizations like the FAA tend to be recorders of input and history and whatever they record is dependent on input from the front line players. Perhaps learning of aircraft automation in training today has displaced basic learning of how the airplane actually works (minus the automation) with less emphasis on the dos and don't when it is required, that is, when the autos drop out and you have to hand fly.

They have. It's called a tank and doesn't fly...it's pretty robust though and won't fall apart if you waggle the controls.

Well, I was hopeful that we could … but … I kept reading further …

I agree with you that the question you pose is proper and should be answered. If one were to look at the regulations with respect to what has to go into a training program, you should find that among other things is the necessity to include the Airplane Flight Manual in that education. Now, the issue would become, what MUST the manufacturer place into that AFM? If it included information about flight controls, and the airplane was susceptible to losing some/all of the tail structure with rapid rudder inputs/reversals, it should be clearly noted. Perhaps someone should demand that the regulator (all the regulators, actually) take another look at this requirement and how it is fulfilled.

Any time there is an accident that has considerable consequences there are always persons who make some level of attempt to influence, or ask premature questions as suggestions, or drop other such subtle hints … and I have “first-hand” knowledge of such occurrences. (more on this privately, if you'd like)

If this is factual and if the FAA had such knowledge and did not provide it to the appropriate interested persons after verifying its accuracy … I would support a public riot in front of 800 Independence Avenue in Washington.

This is probably the section that garnered the most of my interest for responding … you say you did “departure stalls” and “approach to landing stalls” and you mentioned accomplishing “clean configuration stalls” as well and because of the discussions, I am presuming you mean that you did these stalls in the Airplane Flight Simulator. I am curious – were these simulators of the older or newer versions – and for timing era reference … the older simulators I’m describing were “Pre-1980,” the newer versions are Mid-1980s, and the most modern were built after about 2005.

The specific question has to do with motion and visual system installations and, specifically, the flight data and engine data packages that were incorporated. My guess is that what you did in the simulator was likely “recovery from approaches to stall” as opposed to “recovery from an aerodynamic stall” as this was the requirement when done in the airplane – and there was no requirement to do more in the simulator than was done in the airplane … and, there wasn’t much objection because everyone understood, rather completely, that the recovery from an aerodynamic stall in a simulator could not be simulated with any degree of accuracy. So, as recovery from an approach to stall in the simulator would be only slightly different from recovering from an approach to stall in an airplane, authorizing this in a simulator seemed to be acceptable. The minor problem was that the simulator handling and performance conclusions were based on somewhat limited aerodynamic information gained from flight testing and programmed into the simulator computer. The major problem was that the response of the turbine/jet engines were not tested at approach to stall angles of attack nor for stall angles of attack, and it is suspicioned that data for the differences that would make either do not exist, or exist in only limited cases for limited applications. Therefore, these data are not incorporated into the simulator's computers. This came to light quite significantly in the analysis of the Airborne Express DC-8 accident, in Narrows, Va.

It has only been within the past 18 months that an effort, led by the FAA’s Chief Scientific and Technical Advisor for Flight Simulation, in coordination with the International Civil Aviation Organization and the Royal Aeronautical Society, using the Boeing Company, Bihrle Applied Research Inc., and CAE Electonics, Ltd., has a reasonable aerodynamic model been drafted, produced, and minimally researched as being minimally effective in full stall maneuvers in the most advanced simulator available in today’s market. This model has not been finalized nor released for system wide application.

From your lips to God’s ear.

Well …?

I guess you missed my post in which I posted an excerpt from the revision to the KC-135 Dash one, which said exactly that. I even posted the issue date, which, being 30 June 2000, would have been after you were no longer flying that airplane (But well before the AA587 crash)

Here's a link to that document.

I think we agree here on the larger question, you're just mistaken in claiming my specific information is inaccurate.

The FAR 25 spells out quite clearly, should it be read, the design standard of the rudder with respect to Va. Quite clearly our educational system is lacking. I certainly did'nt appreciate the fact until this accident.

Teldiserious, you may claim I'm a troll, and as you have done in Biz Jets, a liar. The latter claim opens you to legal proceedings should I decide to go down that route.

JT, I don't envy your job.

How to break something
 

Others have mentioned resonance as a cause for the AA587 and that was the cause in my estimation. It was not resonance of the vertical stabilizer that some here may have imagined causing the vertical stabilizer to break off, but instead, the entire aircraft oscillating in yaw around its vertical axis.

This oscillation reached an amplitude that you could never generate with a single application of full rudder. Instead it built upon the energy of preceding oscillations until the combined effect of all the oscillations plus increased angle of attack of the vertical stabilizer/rudder combination as the rudder reversed broke the vert stabilizer.

A resonant structural oscillation looks like this:
http://upload.wikimedia.org/wikipedi...-Resonance.PNG

Most of my flight time has been in what are now considered old jets with irreversible hydraulically powered controls that were connected to the stick and rudder pedals by cables and pushrods.
With irreversible controls, you do not have a clue to the forces you are exerting unless you have some sort of feedback. One rather fast aircraft that I flew used artificial feedback and changed its rudder pedal force from 2.6 lbs force/degree of travel to 11.5 lbs force/degree of travel at about 225 kt by means of an airspeed switch and a hydraulic centering cylinder. It then used the higher pedal force all the way up to its limit speed which exceeded M 2.0. The engineers were not at all concerned about us breaking the tail at that higher force ratio, but there were warnings to avoid excessive rudder deflection should the airspeed switch fail to work properly. Full travel of the rudder and rudder pedals was not restricted on that aircraft, but it was essentially impossible to exceed the intended rudder travel limits.

On the A306, the variable rudder travel limits protect the aircraft against a single rudder input overstress. Unfortunately, that system was not (and is still not) able to protect against an oscillatory rudder input where the input frequency approaches the natural tail wagging frequency of the aircraft. This system is also a mechanically controlled ,irreversible, hydraulic powered flight control with artificial feel. The problem is that the force and travel required to activate the rudder to its limits were minimal, and thus it was easy to excite a yaw oscillation by relatively small repetitive rudder inputs. Not the best design, but now that everyone is aware of the hazard, it is unlikely to bite again.

Incidentally, the simcrash posted by SMOC was almost certainly caused by resonance with the natural frequency of the simulator on its base.
http://virtualmystic.files.wordpress...6/simcrash.jpg
This could then have been prevented by a software change to attenuate near-resonant frequency inputs.

Hi roulis

Whilst I hold to that I got from my professor and all the reports I read on the subject as a young man!

Look roulis, this is mainly semantics and I don't wish to prolong the debate. I learned a long time ago that if you express your arguments in a firework display of Greek letters and differential equations then 95% of your audience will not understand you and the other 5% will not believe you, so let me say my piece once in plain English and then go back to lurking.

All aircraft have a dutch roll mode but not all aircraft have a dutch roll problem.

Lateral stability has three parts (OK, solutions to the equations of motion if you must ...)
a) Roll damping - always a subsidence
b) Spiral stability - either a subsidence or a divergence but never an oscillation
c) Dutch roll oscillation which may be damped or undamped; described by three parameters - frequency, damping and roll/sideslip ratio.

All of these are excited by manoeuvering the aircraft or by turbulence.


There I think he was wrong, if only because one can't get resonance in a mode with only one degree of freedom. Dutch roll is an oscillation that can be, and is, triggered by sideslip with no pilot action necessary.

If the dutch roll is well damped then there is no problem, but if not and particularly if the roll/sideslip ratio is high then if turn coordination is not perfect applying aileron can start off an oscillation which yes, does affect the roll response. In extreme cases it can generate a hesitation or even a partial reversal in the bank angle response. At high AoA if the aircraft rotates around the fuselage axis then sideslip may be generated anyway unless some rudder is applied at the same time.

In other cases it becomes difficult to maintain straight and level flight.

If you can solve two differential equations in your head whilst at the same time flying an airplane then you are a better man than I :D But in the end, were you doing any more than applying the process that Chris Scott (I think) described earlier - applying corrective aileron at the correct time in the roll?

Yeah - all good stuff isn't it

And I will stay with mine, but as I said it is mostly semantics

Like I said, no resonance in first degree systems....

Errrr - what FBW technology on the A300?

Well we didn't have today's computing power, but we were calculating dutch roll contributions to lateral behaviour half a century ago :ok:

Owain Glyndwr, replying to roulishollandais (dutch roll):
...were you doing any more than applying the process that Chris Scott (I think) described earlier - applying corrective aileron at the correct time in the roll?

For those who didn't see it, there was a recent Tech Log thread entitled "Mystery of Yaw Damper", which is worth a read in the context of dutch roll. This is the post OG is referring to, and provides a quick link to that thread -

http://www.pprune.org/tech-log/52252...ml#post8028984


On this thread, the robustness of the A306 VS/fin has been regularly called into question by a minority of New-Worlders, whose patriotism blinds them to the fact that - since the pioneering Comet 1 disasters - European airframes have a record at least as good as their American counterparts. This misconception was characteristic also of all our AF447 discussions, specifically in relation to the integrity of the A330 VS/fin.

The discussion between AirRabbit and A Squared have made me wonder afresh about the B707/KC-135 airframe. Here's part of an earlier post of mine, which failed to provoke a response:

Quote from tdracer:
"I don't seem to recall anyone ever claiming that the 707 airframe wasn't robust."
When I did my base training with AA at DFW in 1975, I was surprised that no attempt was made to demonstrate dutch roll characteristics and recovery at altitude, even though the a/c was equipped with only one yaw damper. (BTW, I'm not suggesting that dutch roll recovery by the pilot would involve any use of rudder.) Four years earlier, my VC10 conversion had included several full demonstrations (up to about 40 degrees of bank) and recovery. The VC10 has(d) 3 independent rudders, each with a yaw damper.
One possible interpretation was that the a/c was not inclined to serious dutch roll at altitude - we all know that it would on the approach. On reflection, I'm wondering if the B707 airframe, specifically the vertical surfaces, may have been merely adequate for the regs? Has anyone got a copy of Davies to hand?

Why do so many North Americans have such dumb confidence in the robustness of Boeing's products, and such a jaundiced perception of Airbus's?

Thanks for correcting my posting Chris - one loses track of who said what & where!


Short answer: Davies doesn't mention structural capability so far as I can see. Boeing paper on use of rudder as published by IFALPA says that all Boeing aircraft meet the regs with a bit to spare, but don't say if that applies to just current range or to all designs past and present (KC135 excluded!)

Davies has quite a bit to say about dutch roll and recovery therefrom, also some interesting remarks on rudder bar travel cfd rudder angle. (p263 in my 1976 edition)

On the road at the moment so don't have access to the library, but Davies was the certification pilot for the 707 in the UK. He refused to give it a tick until Boeing did something about the fin/rudder. Forget the exact details of the issue to hand.

Brian

IIRC it was to add a bit of fin area (ventral fin?) Mystery of the Yaw Damper thread again I think refers