NTSB Recommendation re Airbus Rudder Travel Limits

[BOAC]

Barit - again I do not agree - you are also involving the 'dynamics' of a body in this

the mass and dynamic response of each pilot's arms and legs

. A basic PIO is totally pilot induced, and caused by deliberate control inputs which are incorrectly timed/phased, due either to pilot response, aircraft reaction or control damping. The correct 'cure' is not to 'let go' but to 'freeze' the controls until the PIO dampens (normal stability). You and Machin seem to be describing some sort of event whereby a pilot's body is 'flailing around' and moving the controls (I've met pilots like that). I could, I suppose, envisage a situation in severe turbulence where a pilot was flying with one hand on a yoke stick (unlikely!) and thereby being forced to make aileron inputs through the acceleration forces on the arm, but that is an unlikely event. It would be most unlikely in pitch and yaw. Yours is not a classic PIO and more relevantly neither can I see any way in which the yawing motion of 587 could in any way have induced such rudder application.

[Mad (Flt) Scientist]

Such 'g'-induced inputs to the flight controls through accelerations acting on parts of the pilots body would seem to be out of the realm of this accident. Assuming the pilot was properly strapped in etc., the idea that he was under forces so great as to practically lose control of his legs is ... difficult to comprehend. Indeed, such force levels probably themselves would be enough to take the aircraft close to, if not beyond, design loads.

I won't deny that such effects are plausible under the right circumstances - single arm control of a handwheel or control coumn would seem most likely. And I've seen cases where there appeared to be difficulty in the pilot applying precise control in multiple axes due in part to 'g' forces on body components.

But as this was in a high performance jet aircraft conducting rapid rolling testing under high 'g' levels, with a centre column/single arm scenario, and EVEN SO the pilot at no point was rendered unable to control the aircraft (he simply couldn't achieve the perhaps over-ambitious "with no fore/aft stick input" we were trying to achieve) I really can't see that the AA587 scenario gives any scope for such inputs.

Agree 100% with those stating this scenario is in any case not PIO, and certainly not in the classical sense.

edited to add: one classic example of the kind of inadvertent control input being discussed is the carrier catapult launch case, where if hand is kept on the control column to 'g' force on the pilot arm may induce a back stick application which is unintentional. Even the weight of the control column, sans pilot, has to be considered and properly balanced. But, again, these are 'g' levels outside any conceivable large transport aircraft's capability

[bearfoil]

The F-18 requires the pilot's right hand to be grasping the handhold on the forward canopy hoop, and OFF the stick at cat-stroke. Until the boss sees his right fist, there is no launch. The a/c takes itself off, and the pilot assumes control with a pos. climb.

To see a real PIO google F-22 PIO

bear

[misd-agin]

PBL - thanks for posting the link to the Hess Report.

Interesting reading(re-reading?) it.

[mike-wsm]

Could anyone with specific type experience clarify some points, please?
1. At what point would the Yaw Damper have been engaged?
2. Was there an additional Turbulence Damper and was this used?
3. What effect do pilot inputs have on damper performance?
Many thanks!

[Machinbird]

PBL
Thank you for the references. Lets see if this old dog can learn a few new tricks.
I am going to quote a paragraph from one of the references you provided: From the introduction to "Aviation Safety and Pilot Control" ISBN-10: 0-309-05688-8

Aircraft-Pilot Coupling

The committee has adopted APC phraseology for two reasons. The first is to remove the presumption of blame implicit in the term "pilot-induced"; although it is often difficult to pinpoint the cause of specific APC events, a majority of severe APC events result from deficiencies in the design of the aircraft (especially with regard to the FCS) that result in adverse coupling of the pilot with the aircraft. The second reason for referring to APC events instead of PIOs is to expand the focus of the term to include other extreme, unwanted PVS motions that, although not necessarily oscillatory,still derive from inadvertent pilot-vehicle interactions. (Note: PVS=Pilot-Vehicle System).

Unfortunately, I'm going to now quote some writers who use the PIO term.

The Hess report (PBL's first link in post #137)(Group Chairman's Factual Report -Addendum Two) concludes pretty conclusively that AA 587 was a PIO (APC) accident:

This report concludes that activity consistent with a lateral-directional pilot-induced oscillation (PIO) was evident in the moments before the crash of AA 587. The lateral-directional PIO was likely accompanied by a similar oscillation in the longitudinal axis. There was a high probability of rate saturation of the aileron and rudder actuators during the oscillations. It has been demonstrated in ground tests that this saturation can create additional time delays in the flight control system and require increased wheel and pedal forces of the pilot, both of which could contribute to the severity and duration of a PIO.
The sensitivity of the rudder/pedal control system of the A300-600 aircraft could constitute a control system characteristic conducive to a PIO. One necessary (but not sufficient) condition for the pedal/rudder sensitivity to serve as such as characteristic would be a demonstration that, in its absence, PIOs do not occur. One such demonstration occurred in AA 587, itself. That is, in the first wake vortex encounter, pedal inputs were minimal and sustained oscillations did not occur.

The Hess report looks at rate limiting of actuators and does not look at forces on the aircrew causing undesired control inputs.

. A basic PIO is totally pilot induced, and caused by deliberate control inputs which are incorrectly timed/phased, due either to pilot response, aircraft reaction or control damping. The correct 'cure' is not to 'let go' but to 'freeze' the controls until the PIO dampens (normal stability). You and Machin seem to be describing some sort of event whereby a pilot's body is 'flailing around' and moving the controls

Flailing around is too severe a term although there are situations where that has happened. Everything you do with the controls in a cockpit requires a stable reference. We can compensate for slow, momentary or expected forces on our bodies to a great extent, but if we are dislodged from our "perch" by unexpected motion that changes the distance between us and the controls we manipulate, then we may induce unintended control input. Unfortunately all cockpits are not perfectly designed to avoid unintended control input from aircrew motion. The resulting control motions can either stabilize or destabilize the aircraft.
Only problem with freezing the controls is that in most aircraft there is no good way to do that when you are being bounced around. Perhaps on a Cessna you can grab the column forward of the yoke where it penetrates the panel and stop things from moving in pitch, but on most other aircraft, if you are being moved by accelerations, it is very hard to freeze the controls.
Barit1 is correct, the most positive way to break a positive feedback loop is to release the controls. (But on the Sageburner F-4 PIO accident that act precipitated the final g overshoot.YouTube - F-4 PIO).



HarryMann

Nevertheless, this is rather naughty...

The Airbus rudder system does not get harder to move as airspeed increases and thus would be rather prone to such a PIO.

.
Well Harry, I hope you mean it was naughty of Airbus to design it that way. The bird I used to fly found it prudent to increase rudder force per degree almost 5 fold when accelerating through ~250 knots.
If I understand the Airbus travel limiter properly, the rudder pedal stroke gets shorter as airspeed increases but the speed of rudder response and the force to actuate remain unchanged. The breakout forces change and the gradients change slightly between Airbus models but do not change in-flight.

I am very glad to see that the NTSB and EASA appear to be moving in the direction of a better standard. That some of the existing designs can induce an amplifying oscillation (APC) has already been demonstrated.

[misd-agin]

"I am very glad to see that the NTSB and EASA appear to be moving in the direction of a better standard. That some of the existing designs can induce an amplifying oscillation (APC) has already been demonstrated."


Unfortunately, and to me surprisingly, they didn't come to that conclusion after the AA 587 crash.

[BOAC]

Originally Posted by machinbird

forces on the aircrew causing undesired control inputs

- I'm still intrigued by these thoughts - how do you envisage the motion of 587 itself causing the co-pilots legs to push on the pedals - do you see him sliding around in his seat? Why did the first wt encounter not do the same?

587 to me exhibits a simple, classic PIO. The only query I have is why did it start.

Out of interest how did you conclude that the F4 pilot 'let go' of the stick?

[p51guy]

The statute of limitations was probably one reason the rudder problem wasn't addressed until now.

[Machinbird]

BOAC

Out of interest how did you conclude that the F4 pilot 'let go' of the stick?

I read the accident report. The PhotoTheodolite footage clearly showed the motion of the stabilator relative to the fuselage. The accident board made the conclusion from that information. Unfortunately the aircraft was trimmed nose up from the turn to align with the course. I should add that the stability augmentation earlier disengaged-in those days it was an electrically held switch that selected it.

[bearfoil]

To me, the correct exit from "oscillation" involves not "letting go", that's counterintuitive and not recommended. The answer is to not "over control", and that is why I think the F22 Raptor youtube is the best example. The F-4 vid looks more like an overloaded wing(s).

The Raptor has (highly) powered controls, far stronger than any human's legs or hand (arm). The (test) pilot was using what he thought were correct inputs, but his powers of observation weren't in tune with elevators that can change pitch 90degrees at 350 knots in a heart beat at 35 tons. He was behind the a/c, and out of control, without a runway to stop his boneheaded stickwork, he would have been dead.

Each change in pitch the jet made, he corrected with WAY too much stabilator.

It was HIS oscillation, no one else's. That's why PILOT induced oscillation. Each one of the accompanying videos on the you tube link has a different explanation, float, bounce, etc.

Aircraft Coupled/Pilot terminology looks like PC newspeak for situations that went by many different names in the past, and doesn't really do the endeavour much good.

Please don't "Let go of the controls".

[PBL]

Originally Posted by bearfoil

Each change in pitch the jet made, he corrected with WAY too much stabilator.

How do you know it was the pilot moving the stabilator? Answer: you don't. Further answer: it is obviously the FBW control system moving the stabilator in accordance with (a) what it "thinks" it should do, and (b) pilot inputs.

Originally Posted by bearfoil

It was HIS oscillation, no one else's. That's why PILOT induced oscillation.

So you don't think a FBW control system does anything other than mimic pilot control inputs? Astonishing.

Originally Posted by bearfoil

Aircraft Coupled/Pilot terminology looks like PC newspeak for situations that went by many different names in the past, and doesn't really do the endeavour much good.

The National Research Council committee thought that such situations had at least as much to do with the design of the control laws as they did with the pilot, and produced the Bode and Nyquist plots to prove it.

But you can look at the stabilator motion and know it was the pilot! What a talent! I wish I had it!

PBL

[Mansfield]

It was HIS oscillation, no one else's.

A statement like this suggests to me that you have never experienced a PIO.

The APC term was developed in the hope of removing the stigma associated with the phrase "pilot-induced oscillation". These have nothing to do with someone's powers of observation being out of phase with the aircraft dynamics. Rather, they are associated with with the control-response feedback loop, which is very much driven by the control system capabilities.

Reference AD 2006-23-15, which addresses the roll PIO issue on the 757. This problem was often initiated by the 757's peculiar ability to dump lift off the upwind outboard flap with a slight crosswind and a slightly raised upwind spoiler. The AD required, among other things, the installation of a roll damper.

Landing on runway 27 at BOS several years ago in an airplane that had not been so modified, I managed to find this little cliff while retarding the power to flare. The resulting PIO that I produced used three roll oscillations and three mainwheel contacts before the FO manually deployed the spoilers and killed the event. We did not damage the airplane, thank goodness, and we were very fortunately on a ferry flight.

Subsequent discussion with the FAA flight test engineer working with Boeing on this problem revealed that during the flight test program executed in search of this issue, a JAA test pilot had also created a PIO following a similar wing drop. The Boeing test pilot in the right seat froze the controls and killed the oscillation.

No doubt this is the correct response. But I have to tell you that, in between main wheel contacts, freezing the controls doesn't immediately cross one's mind. Particularly the rudder, which proved quite useful in keeping the nose pointed down the runway. Of one thing I can be absolutely certain: letting go of the controls was not an option.

The FAA was quite concerned that they had only documented eleven cases of this problem, yet the airplane had been flying for twenty years before the first Boeing Flight Operations Technical Bulletin in 2002. Boeing was concerned enough to mount a fifty odd hour flight test program. But the elephant in the room was obviously that many more of these had taken place...and not been reported.

The authors of Aviation Safety and Pilot Control were attempting to introduce a terminology that would allow such events to be more readily reported and discussed within the community.

The committee has adopted APC phraseology for two reasons. The first is to remove the presumption of blame implicit in the term "pilot-induced"; although it is often difficult to pinpoint the cause of specific APC events, a majority of severe APC events result from deficiencies in the design of the aircraft (especially with regard to the FCS) that result in adverse coupling of the pilot with the aircraft.

Continuing to perpetuate uninformed notions about these events leads to the misinterpretation of the manufacturer's guidance (my case) and, much worse, the under-reporting of them. Read the book, which is quite good, or research PIOs through other venues.

Somehow, we need to move beyond the "everybody else is an idiot except the guys in my class, or airline, or generation..." style of discussion. It does no one any good.

[bearfoil]

Mansfield

Thanks for your reply, I've had more pilot initiated turbulence than I will admit to.

I am not alone, and if my post has offended anyone I have no excuse.

I watched two a/c take off for formation once. One pilot was civilian, the other was a decorated instructor at Top Gun. Their first pass down the runway was odd. The civil pilot was moving along like he was on rails. The Navy guy couldn't maintain his altitude, his distance, or his a/s. He'd start with a slight manouveur, followed by a series of "corrections" that made it worse, until he gathered himself and did fine until his next "small" mistake. I call that PIO. Another instructor noted how she preferred civilian flight to Navy. She said the military teaches "hamhandedness", and the best way to fly is gently. She is a 777Captain.

bear

edit for pth. H---er was flying #2, (Navy)

[protectthehornet]

navy pilots vs civilian...come on...

I'e flown with great military pilots and crappy ones....I've flown with great civilian pilots and crappy ones.

and who was following who in the formation flight?

I flew formation once with another plane...my altitude didn't change according to the altimeter...the other plane LOOKED like it was all over the sky...much more to this sort of thing than meets the eye.

[stepwilk]

"H---er was flying #2 (Navy)..."

Would that be Bob Hoover? He certainly wasn't Navy.

[protectthehornet]

I ask myself and you...if you had to fight a dogfight, would you rather have a 777 captain, or a naval aviator.

But, if you had to land a 777, wouldn't a naval aviator do just fine?

hamhanded indeed.

[bearfoil]

If it was me, I'd trust both dogfight and 777 landing to me, I was passing along the young lady's opinion, not mine

At the time, and after we saw H**er's performance, she made sense.

(Sorry for adding too many empties, stepwilk)

As long as your asking, JR, maybe you can help me get a handle on the debate over
computer v. Pilot. It seems to me that if FBW is what you have, you might be better off with a cadet flying rather than a stick and Rudder type. If it Stalls, you're just screwed, either way, but with Captain Super flying, you'd still crash. Is it that important to have someone to blame? The cadet can blame the fbw, If yer Auto trips out, leaving you blind, thin and confused, it's "The Pilot's Fault", by virtue of your hours.

bear

[stepwilk]

I'm confused. You say you "watched two a/c take off in formation once," and that apparently one was a klutz. Hinker, or Honker, or maybe Humper.

Now you're saying that you're simply "passing along the young lady's opinion" of the two-ship? Is there a post I missed?

[bearfoil]

stepwilk

The lady and I were watching together, she made the "hamhanded" crack.

sorry, and my apologies for misspelling H**er's name.

bear