NTSB and Rudders

[PaperTiger]

The NTSB, having postulated about inappropriate control inputs, finally comes back to mentioning the yaw damp(en)er.

From the original release (12 Dec):. . [quote]The yaw damper also initially failed the pre-flight check, but it too was reset by a mechanic and the problem was reported resolved, according to the NTSB. . ."Investigators are going to want to get a good handle on how effective the reset was," McKenna said. "They need to make sure the reset is valid and didn't just have the appearance of fixing the problem." <hr></blockquote>

OK maybe I'm paranoid and this was just an eerie coincidence, but would such 'sensor confusion' be limited to just this airplane, or endemic to all A300s ?

[ 09 February 2002: Message edited by: PaperTiger ]</p>

[Cosmo]

RatherBeFlying . .Wishing to not take part in hypothesizing about the accident, due to lack of qualifications, I merely want to point out that a 32 pound resistance doesn't seem like much. I think that even an untrained individual can easily press around 300 pounds (150 kgs) with their legs.

Cosmo

[Raas767]

I went through AA upset training as did every American Airlines pilot. The people who initiated and taught this to the line pilots all had technical backgrounds, many line pilots are former military test pilots and quite a few pilots I have flown with are Aero Engineering graduates. The point is that nowhere in the training do they advocate such agressive use of a flight control unless it is absolutely necessary. That being said, however, no information was ever presented, nor has informnation ever been presented to me during my traing or my career to sugest that rudder use can cause a vertical stab to fail. As pilots we all know that airplanes can be overstressed in certain situations but with load limiters I for one was not aware that I could overstress an airplane by going full left then full right with the rudder.. .In any case. The wake encounter behind the 747 could not possibly have necessitated the use of the rudder that aggresively. The rudder moved but I doubt it was pilot induced.. .I guess you learn something new everyday in this business.

[BOING]

I doubt the A300 crew could have applied so many rapid rudder reversals so quickly. A more likely possibility would be an inappropriate flight system response to wake turbulence followed quickly by the pilots attempt to correct the flight system error. A matter of unfortunate bad timing. This is probably going to be a difficult accident to determine an exact cause despite all of the information that should be available from a modern FDR.

I am sure the wiring systems on "electronic" aeroplanes are designed and maintained to the Nth. degree but anybody who has ever touched a Lucas car electrical system must shudder at the idea of keeping all those (miles?) of electrical wiring and thousands of electrical connectors in good condition as an aeroplane ages. All it would take is one connector to be corroded by a spilt Coke or a blow from a badly loaded freight igloo and Bingo!

By the way, what happened about the insulation failures that were reported a while back on some modern wiring?

[Ignition Override]

Raas 767 put it in a nutshell. On the one hand, I've never seen warnings about sudden full rudder deflection at high speeds anywhere, other than the fact that any pilots knows that even partial rudder would frighten the passengers and might hurt pax or crew (and spill very hot coffee).

Our Aircraft Operating Manual clearly describes rudder restriction mechanisms with increased restriction from 170-300 kts, and a different restrictor to reduce unwanted side slip characteristics, engaged with slats/flaps 0 or 5 (on the dash 30 and 40 series Douglas).

[ 14 February 2002: Message edited by: Ignition Override ]</p>

[arcniz]

There's a little bit of an off-odor to these suddenly promulgated Agency and Airframe Mfr. proclamations of "everyone knows that too much rudder will knock off tails".

Available information critical to safety travels fairly efficiently in aviation. Pilots as a group are serious people who do not mumble or stay silent when safety issues come up for discussion. And they have plenty of time, over the years, to talk endlessly with one another about the short list of aerodynamic gotchas for cruise flight.

The craftsman - apprentice supervisory system for bringing pilots along, combined with endless reams of data and technical tools for rehearsing every aspect of flight convey the impression that all the bad news is on the table for everyone to see. The important things one also prepares for in a simulator. So we thought.

More than a few aviators are astonished that passenger-bus aircraft would be constructed with such a fundamental and accessable succeptibility.

If it were a well-known truth that known a given aircraft model could be near-certainly ripped out of the sky by a simple tap-dance on the rudder, wouldn't it have made sense to program some sensitivity to that phenomenon into the simulator routines - or, better yet, into the actual airplanes? The seriouness of this tail-strength issue - as it is now revealed to us - would seem to merit a red light about 2 feet wide that would come on somewhere up front saying "WHATEVER YOU DO, FOOL, DON'T PUSH THAT RUDDER AGAIN!"

From the historical lack of structural cautions, printed warnings, etc. about this, the lack of sim-training to address it, the lack of feedback mechanisms in the aircraft to detect and alert about it, one wonders how this info has gotten so little attention over the years.

And one also wonders if the existing yaw-damper control program and rudder control programs in the Airbus know about this constraint and implement specific fail-safe interlocks to prevent bang-bang rudder action of a nature that would overstress the tail.

I bet they will in the not-distant future.

[sky9]

I have checked my Boeing Training Manual for the 767/757 and there is nothing about applying too much force however there is a comment about jammed flight controls which states:. ."If any jammed flight control condition exists, both pilots should apply force to try to either clear the jam or activate the override feature or shear out feature(the rudder has a shear out feature)". .I suspect that we are seeing the start of blame transfer. "The pilots are dead lets blame them".

[Wino]

The only thing that this investigation proves is that dead pilots have bad lawyers.

There is no way on gods green earth That Sten Molin or Ed States would have used the rudder pedals like a stairmaster which is exactly what we are talking about here. Both fine Aviatiors had been at AA for 10+ years and had long careers before AA. When AA quoted their experience they were only quoting time at AA, not the 1000s and 1000s of hours they had to get in the door at AA. Both gentleman were consumate professionals.

But because dead pilots have bad lawyers, Airbus is gonna blame the pilots. So this investigations once again proves that dead pilots have bad lawyers.

E X T R E M E L Y P I S S E D O F F <img src="mad.gif" border="0"> <img src="mad.gif" border="0"> <img src="mad.gif" border="0">

[RatherBeFlying]

I have real trouble imagining any situation that would call for four full rudder reversals in four seconds save air combat where somebody is shooting at you and you have no other ideas <img src="eek.gif" border="0">

So let's help out the NTSB with a poll. If anybody has ever performed a series of rapid rudder reversals, can you let us know. Or you can e-mail the details to the NTSB investigation at [email protected]

[Belgique]

Just another Theory

1. Clues:

a. AA587 + <a href="http://www.iasa-intl.com/folders/Safety_Issues/others/AA_587_Survey1.rtf" target="_blank">eight prior</a> uncommanded yaw incidents (U/Y) + a bent FEDEX rudder actuator rod (with associated fin disbonding)

b. An anecdotal reputation (particularly amongst F/A's) for the A300 being a tailwagger.

c. early design, early software

d. Composite fin more prone (than metal) to sudden failures due to its axial strength, load-bearing capabilities and "strength only through integrity" characteristic.

e. five recorded rapid high-throw rudder deflections at 255kts during and immediately after the second wake encounter (before fin failure) on AA587 (assuming this was the filtered DFDR read-out). The A300-600 rudder can move at a rapid 39 deg/sec.

f. neither autopilot nor Control Wheel Steering engaged.

g. There was an audible "rattle" from aft (as recorded on the AA587 CVR) after the second wake encounter (just prior to fin failure).

2. Assumptions: (intuitive, logical, recommended and standard practices)

a. A pilot would not (normally) make a large rudder input at 255kts (about 1.9Vs) [see Boeing/Airbus Guidance at:. .<a href="http://www.boeing.com/commercial/aeromagazine/aero_03/textonly/fo01txt.html." target="_blank">http://www.boeing.com/commercial/aeromagazine/aero_03/textonly/fo01txt.html.</a> Any rudder inputs are commended only at lower (near or at stall) type speeds.

The FAA guidance for pilots is transparently a bit of an irrelevant butt-covering exercise - but it does give us the heads up that an oscillatory rudder (see Farley above) certainly can break a fin. Happy with that - because that oscillatory motion is equivalent to aerodynamic or system induced "flutter". Flutter has always been associated with imminent catastrophic failure - as it often tends to be oscillatory and divergent. The nature of flutter is such that it all happens too quickly to enable recognition and speed (or configuration) changes. It's best avoided altogether (through design).

b. Pilots had no time in which to react - either appropriately or inappropriately (about 7 secs for recognise and respond/react). They would not have recognised the fin loss and the full power response probably did not materially affect the outcome.

c. The uncommanded yaw can happen on approach (i.e. slow and configured) or at FL310 so that common factor gives us a further clue. It doesn't appear to be a factor on/near the ground (i.e. you need a speed range).

d. As not all uncommanded yaw instances resulted in disastrous failure, there must be a trigger and exacerbating factors (i.e. present for AA587). We know of one - wake turbulence. Possibly another - the preflight yaw damper reset by maintenance. We can possibly refine this to "a characteristic (of design) plus an unserviceability" in order to complete the chain. Or maybe it's all predicated by design plus circumstance......no u/s required.

3. Givens:. . a. The "yaw damper reversed" theory (both Miami 99 and Farley above) is a tempting hook upon which to hang one's hat and it may indeed be the reason why the yaw damper needed a reset.

b. The rudder trim switch had been subject of two AD's, the second one of which called for a lengthened switch shaft and a wiring change. That gave three rudder-trim switch maint mischoice possibilities (plus the wiring change). There was also an aileron trim AD for inadvertent actuation (both switches being close adjacent on the 408VU panel - increasing the possibility of a maint "Murphy")

c. Intermittent wiring problems have been known to cause similar flight control erratics....but oscillatory?? Maybe not, that's more likely a systemic interaction.

d. The Loral DFDR had been previously found unsatisfactory by investigators because it recorded only a filtered sampling of data (as per that displayed in the cockpit). The FCS is in fact capable of moving the rudder more than twice in the time that the DFDR records one motion. This may help explain the "rattle" (lateral fin-rocking) recorded on the DFDR and yet indicate that the pilots could not have possibly moved the rudder as fast as the FCS was capable of doing (39 deg/sec) - but without it being recorded.(See <a href="http://www.aviationnow.com/avnow/news/channel_maint.jsp?view=story&id=news/raa21114.xml" target="_blank">this article</a> regarding AA587’s DFDR deficiencies (low sampling rate). And see also [url=http://www.srg.caa.co.uk/publications/CAP455_airworthiness_notices.pdf]Loral_800_"obsolescence"

4. Theory: Considering all of the above and extrapolating into plausible explanations, here's an hypothesis:

a. Static port positions are chosen by designers in order to minimize local pressure disturbances throughout the speed and configuration range - so that inputs to the Air Data Computer (ADC) are as solid and error-free as possible. However they tend NOT to consider what happens under yawed flight. In a steady-state yaw (asymmetric, engine-out, OEI) the port/starboard discrepancy is soaked up by the fact that there are ports on both sides of the fuselage and the two pressures are joined in a Y Junction before being presented to the ADC. If, however, the yaw angles become much greater, there is both an asymmetric blanking and a pressure change (venturi) effect caused by both the rapid reversal of yaw and the extent and periodicity of that yaw. The actual data (incremental pressure changes) presented at the static port has a long way to travel and it can become both erroneous and out-of-phase - simply due to the dynamics of the situation (and perhaps even due to the sampling rate of the ADC / and its subsequent input rate to the FCS). PEC (position error correction for static errors) is normally applied to the ADC only for balanced flight in the envelope - for normal configurations.

b. Consider further the effect of yaw if the static ports are placed in an optimal error-free position for balanced flight - but just happen to be at a point of curvature on the fuselage where the venturi effect on the sensed pressure, under yaw, will be very significant (i.e. perhaps producing large local pressure drops).

c. Airspeed is Dynamic Pressure (but is sensed at the pitot as TOTAL Pressure (static plus dynamic). The static port pressure is deducted to give you the indicated airspeed (i.e. the dynamic only). If you need a yard-stick for the significance of static port pressure errors on the sensed airspeed, take note that trapped water freeze-blocking a static line (in a climb above freezing level) will cause an ASI to wind down from 220kts to zero in just under 2800ft of climb (equivalent to 9mbs Hg). It would also cause the VSI to zero and the altimeter to freeze - but that's irrelevant here. I'm just pointing out that quite a small static port pressure discrepancy can have a large effect on ADC-sensed airspeed. Those sensed airspeeds control yaw-damper action and rudder ratio limiting - at any one point in time.

d. So why (and how) might this be significant? In prior instances of U/Y there may have been a minor out-of-phase disagreement between the FCS and the ADC. As pointed out, this could be caused by the rudder limiter's throw (and/or yaw damper input) being misset by the flawed ADC airspeed info. A yaw requiring an 18% displacement throw of the rudder might end up being much greater because the rudder limiter is permitting (courtesy of the ADC and FCS software) up to 16 degrees full throw (instead of the 10 deg appropriate to the actual airspeed at that instant). That much greater (or simply inappropriate) corrective yaw causes another ADC sensing error and the tail goes into "wag" mode (familiar to A300 backenders). Eventually the damping effect of that large fin normally causes these rudder-induced overshoot oscillations to dissipate. But you need a trigger for this to "kick off" - some initial yaw. atmospheric disturbance, pilot "stretching" with feet on rudder pedals (done it myself), climb/descent through an inversion, wind-shear, turbulent air, wake turbulence etc etc. Help me out here.

e. But why did AA587 self-destruct? I think it may be as simple as the first wake encounter starting the tail wagging the dog and the next wake encounter really confusing the issue and leading to an instantaneous fin overload (the rattle of that semi-detached fin indicating that lugs had sheared and that the fin was on its way). These fins and rudders on the big twins are quite powerful because they have to accommodate OEI any old time. Courtesy of the . .system that tempers that power, the FCS is quite capable of breaking the fin off - both due to it being a composite structure (that won't just partially fail/fatigue crack) and because the range of rudder-throw action is large. At 250 kts it shouldn't exceed 9.3 deg, but if you fool it into thinking it's at 165 kts, you can have the whole 30 degrees of throw. Obviously that's going to be enough to break it at the higher speed. I'm not suggesting that it went full-scale deflection, just that induced static errors caused the ADC to continually re-schedule the FCS to permit inappropriate deflections for the actual airspeeds - leading to an oscillatory flutter as each deflection proved inappropriate - yet gave rise to the need for a prompt further deflection (and thus becoming oscillatory). That's not overswing (per the FAA release), it's system error - where the rudder's power is pitted against the inherent dampening of the fin. At a certain point it will become exponential and destructive. Because of this classic A300 tail-wag, the sensed static port pressure was never on . .the money and the ADC/FCS interaction was in full disagreement (after the first wake-induced yaw) when the second wake turbulence encounter hit at just the wrong angle and broke a fin lug, starting the fin's rudder-driven death-rattle.

f. Why the bent FEDEX rudder actuator rod? Sometimes the fin wins and the periodicity of the feedback to the rudder is sufficient to bend the actuator.

g. Or perhaps it wasn’t the FCS computer’s fault at all, perhaps it was a cross-wiring or a reversed hook-up of the static ports or a hydraulic servo valve. Whatever it was, if it was a hardware flaw, it may have been that way for a while. Sometimes you need a significant divergence (such as that caused by wake turbulence), before you cross a trigger-point for feedback mayhem. Think of the microphone analogy. If you place an open mike in front of a speaker that’s in the same circuit, as long as the volume is way down there’ll be no feedback squeal. Wind it up a little and it will suddenly be deafening.

5. No doubt someone will now tell me that yaw-induced static port pressure errors aren't of sufficient magnitude to start this ball rolling....

or that this is all accommodated in a cunning ADC/FCS program patch of narrow notch (or broadband) filtering.. But as I said, it's just a theory that seems to fit the bill (and, if correct, one flaw which should be easily fixed). I don't think that it's a fin strength composite) problem. Whether or not this static port theory holds water, it's still likely to be an FCS computer-derived (and driven) flutter that arises from a sensing problem. Just keep in mind that the rudder has two movements. Rudder deflection resolves the yaw (or is intended to) and then (in an ideal world) centering sends the rudder to parade rest, calmly zero'd in trail. But in my theory, there was always a bunfight going on back there and a simple recentering was rarely on the cards; sometimes the yawing gave cause for alarm, but normally the static port error wasn't significant because the yaw wasn't massive and so the natural damping of that large fin meant there were only a few wags at most. But the potential was (and is still) there for an A300 to eventually meet its wake encounter match. One is left wondering what the "break-out" yaw angle/rate is (that might "dud" the static port pressures and set this ball rolling).

[Belgique]

And while we're at it, also think about the effect of some trapped water in one side of the static system - and what effect it might have (on the ADC) during yaw gyrations.

[arcniz]

Belgique - Masterful!

Your lucid and clearly communicated static port theory is the first I have seen that explicitly addresses the "second failure mode" of the independent rudder-travel limiting mechanism seemingly failing to keep rudder deflections inside the stipulated limits for the aircraft speed.

The AA587 aircraft might have ridden through the wake events ok if the extreme deflections of rudder had been limited according to the speed vs throw angle rules - even if the First Failure Mechanism for rudder oscillation were present due to design instability, wiring faults, or both.

One suspects the static system value data has considerable mechanical and/or electronic "buffering" in it so that responses to individual port changes are averaged over a time interval of seconds, at least. Because it is the baseline reference for barometric calculations, fast response to static pressure variations would just add noise to the gauges and controls.

If this is the case, a pressure change from the first wake event (and related slip?) might have created enough distortion of the averaged static pressure that the computed airspeed for rudder limiter purposes was scaled back toward the 165K mark where full rudder travel is allowed. The inter-wake interval leaves time for the (probably fairly slow) mechanical travel interlocks to reposition for a substantially wider rudder travel window. Then comes wake bump number 2, which sets off the First Failure Mechanism on a cycle of uncommanded rudder oscillations. Because the rudder travel window is inappropriately wide at that moment, those swings do the tail in short order.

I sure wish someone would start measuring how "normal" A300's wiggle their tails in the course of a working day....per my comment on the "Pilots want A300's grounded " thread.

[hobie]

systemsguy ......

"I sure wish someone would start measuring how "normal" A300's wiggle their tails in the course of a working day....per my comment on the "Pilots want A300's grounded "

Why not select 10% of the fleet spread around the world and do this monitoring Now!!! ......

[Few Cloudy]

Two valid points run through this thread;

- Pilots would not kick rapid full throws of opposite alternating rudder,

- If they did it could cause a structural failure.

Both valid points I think.

They both refer to a healthy aeroplane, however. If the rudder/fin were structurally damaged, causing a full rudder swing and yaw, a pilot might well be brought to put in an opposite input - he doesn't have time to analyse the reason for the initial swing, just corrects the flight path. Depending on the failure, this could aggravate the problem. The rudder might be about to swing to the other side anyway, making the handling just as unpredictable as the Air Alaska MD-80 free floating stabiliser case.

COnclusions for me are;

- I sympathise with the pilots, who were probably in an invidious position,

- I regret that another piece of useful 'how to destroy an airliner' knowledge has been made available to self destuctive terrorist individuals.

[IcePack]

This all seems to me an excuse for not making the aeroplanes strong enough in the first place.. ."It is not the aeroplane it must be ze pilots."

[ignoramusextremus]

What a buch of Chuck Yeagers. I haven't seen so many comments by so many experts in a long time. Way to go aces! How about we grow up a little and realize the investigation is not over. Oh I almost forgot, if it is a "cover up" you probably will be branded a conspiracy theory "type". So much for all this babbling, Ladies.

[ 11 February 2002: Message edited by: ignoramusextremus ]</p>

[Few Cloudy]

Ah!

That's that nice man who calls the pilots mice on the UAl attempted Hijack thread. What a star!. .Who is this fellow guys? Must have a brain bigger than Pooh Bear...

[crazytwon]

alright, being new to the flying feild, i dont really know all the stuff that everyone is talking about. however i did find many of the comments to be well founded.

what my question deals with is how exactly does wake turbulance (when encoutered by a medium to large size aircraft)? Since i have never flown a large aircraft (unless you would like to consider my piper warrior one <img src="wink.gif" border="0"> ) i have not encountered wake turbulance.

Also, when an aircraft encouteres jet "upset" at low to medium altitude, what are the basic manuvers to perform to correct the situation (ie. do you use the rudder to correct the situation at a high angle of attack instead of using the alierons for fear of stall?)

just to clearify, i am a student pilot (only a few hours in as of yet) and am working on my ppl here in the u.s. so if i dont make any sense...please understand i am rather new to this!!! thanks!

[ignoramusextremus]

Actually, yes. I have a brain much bigger than the bear. I am just tired of all the whinning going on, not to mention the lack of professional attitude. By the way Post-Tanker, I graduated Magna Cum Laude from my university. Pilots are their own worst enemies.

[ 11 February 2002: Message edited by: ignoramusextremus ]</p>

[Shore Guy]

What a profoundly bitter individual....

Coming on the forum is a choice, and, oh yes, what does PPRUNE stand for?

Let's move on.