Justin

-----Original Message-----
From: justinmcgrane [mailto:justinmcgrane@sbcglobal.net]
Sent: Wednesday, August 13, 2003 5:32 AM
To: safety@iasa-intl.com
Subject: A300 Fish Tailing?

Dear Sir

What does the term "fish tailing" mean for the A300?

Justin

You will find reference to it in an extract from an official report on this page (also here). Also search on the IASA site search engine (or Google) for AA587 and you will find other amplifying references under

Fish tail

Fish tailing

Fish-tailing

Fishtailing

The term is a graphic analogy that describes the official and anecdotal (flight attendant and passenger) evidence of an A300’s frequent fish-like characteristic (left/right wagging of its tail under certain flight conditions). It has been suggested that it is a flight-control feedback loop that permits either an external excitation (such as a wake encounter, atmospheric or clear air turbulence) or an internal input (pilot’s feet on pedals or hydraulic glitch) to start a yaw cycle. By design, such a yaw cycle should be damped by the great stabilizing influence of the tall vertical fin – and the yaw dampers. However if a corrective input is (or becomes) slightly out-of-phase, the stabilization might be slowed (i.e. it will overshoot the null position slightly, but to an ever decreasing degree – and finally sort itself out in accordance with the Law of Diminishing Returns). However if the initiating force is unusually high, a divergent phugoid might result in a destructive amplification of that process (and this may well have been the case for American Airlines Flight 587). It may just depend upon the size of that initial excitation (or subsequent input – such as the second “bump” felt on exiting the other side of a preceding aircraft’s wake). If the corrective rudder input becomes out-of-phase (for some reason), the yawing will be progressively amplified. The same effect can be seen at air displays where fighter and Jet Trainer type aircraft pilots would introduce a rudder pedal input at just the right moment to accentuate the aircraft’s yaw to a visually alarming degree. I incorporated just such a fish-tailing in my low-level act. The F-4 Phantom was one of the best exponents of this - as seen in its crowd-pleasing, high angle-of-attack, high yawing Phantom “wing walk”.

As to what (other than an inept pilot) might cause (induce or reinforce) this feedback loop, well that is the big question. It is apparently a question that Airbus isn’t eager to answer (nor one that has been clearly asked). FEDEX had a rudder actuator break in its hangar (at Memphis I think it was) during maintenance. The FAA then brought out an airworthiness directive that addressed the possible lack of harmonization that could cause such an event. So, being under no air loads in the hangar, that would point to it being a flight control system induced hydraulic glitch. It’s also been theorized (by me) that water trapped in the static pressure lines could cause erroneous pressure signals to be transmitted to the Air Data Computers (and onwards from there to the yaw dampers and rudder limiters). Why would that be? Well usually static pressure is sensed on each side of the aircraft in order to iron out any anomalies introduced by yawing (which can expose the static port on one side to a suction and the other side’s port to a dynamic inflow). If however the arrangement to overcome (i.e. neutralize) this latter phenomenon is compromised by water trapped in the system’s “designed in” low points, water (having inertia) is going to flow aft on one side and forward on the other during each yaw cycle. The net effect of this would be contrary to the desired effect (of neutralizing the “suck and blow” during yaw that’s felt by the port and starboard static ports). The CADC (Air Data Computer) would be converting erroneous analog signals to digital data and, due to its high sampling rate, might then create the conditions for a yaw damper to start the destructive cycling.

A few more FAQ pointers here:

a. Ques: How would water become trapped in the system? Answer: During an aircraft wash, as a result of ice melting (in flight or on the ground), failing to install bungs in the static ports while on the ground. However, also see b.

b. Ques: Even if you install bungs whilst the aircraft is being washed or in heavy rain whilst parked overnight, can water still get in? Answer: Yes it can. Normally static port bungs are angled downwards so that water can’t flow uphill. However water flowing down the fuselage and over the bungs can be sucked up into and through the bungs by capillary action – in particular if the atmospheric pressure is changing quickly (which is often the case during heavy rain).

c. Ques: But aren’t the bungs solid rubber? Answer: In my experience they have a thin capillary (i.e. a hollow shaft) running up the centre because pneumatic instruments must be allowed to “breathe”. That’s why you will always see, whilst the bungs are still in, the atmospheric pressure changes (that have occurred since the aircraft was parked) reflected on the standby non-digital altimeter in the cockpit. The reason for doing this is that large rapid pressure changes (such as bung removal) might otherwise be injurious to the pneumatic bellows and delicate gearing that you find in instruments or sensors that are pneumatically plumbed direct into the static pressure. In fact a person blowing into a static port or pitot tube can destroy delicate flight instruments (or at least require them to be recalibrated).

d. Ques: But aren’t the static lines checked for water? Answer: They are, but infrequently (probably only at major servicings – and I doubt that quantities removed would be recorded). A small amount of water would have little effect (and in modern aircraft, unlike unpressurized airplanes, cannot freeze when the aircraft flies above freezing level – which is a wholly different pitot-static dilemma). Once the amount of accumulated water built up to the point that it filled the line at a low point, then you have the inertia problem that I spoke of above. And of course, the more water, the greater will be the inertia effect. And it is possible that a static line low point might be situated short of where the port and starboard lines are plumbed (i.e. Y’d or T’d) together. That opens up the possibility of only the “weather” side having its line’s low point full of water. Think about that (for its effect upon what is sensed airborne (during extreme yaw) at the ADC or its transducers). The trapped water phenomenon might also explain why some aircraft exhibit the characteristic at some stage - but at others just don't (i.e. depending upon whether water is present in sufficient quantity - or isn't).

e. Ques: What is a transducer? Answer: In a digital system you sense analog data (i.e. ambient air pressure) but must at some stage do an analog/digital conversion in order to get a signal that the ADC can process for its computer data input. ADC’s have transducers either at the ADC or somewhere in the static lines. Logically they would (in my opinion only) be located somewhere DOWNstream of where the port and starboard lines came together. Within the ADC there are tables for PEC (pressure error corrections that accommodate slight errors due to static port position errors that change with changing IAS). These PEC tables may well introduce another factor enabling a rogue feedback loop.

f. Ques: So why hasn’t anyone rung alarm bells if they’ve found amounts of water in the static lines? Answer: I’m not sure how often the lines are checked, but there are usually drain-taps located at the low-points (in fact that’s why there are designed-in low points – so that the water will pool there and can be drained). If the work is done at a contractor, I doubt that they’d even record the fact that water was found in the lines. If you think about it, I doubt very much that there would be any snags recorded by a pilot or engineers that would cause them to investigate water in the static lines. AA587 had a yaw-damper self-test glitch prior to start on its final flight and that was “fixed” by resetting the system (which probably means cycling the breaker). Intermittent glitches are often made to just go away by such system reboots or resets.

g. Ques: What are the outputs from the ADC that might be affected? Answer: Well obviously rate-of-climb, airspeed, altimeter, transponder height ATC reporting (Mode Charlie), Machmeter, rudder limiter, yaw-damper (and systems various that utilize these signals – autopilot being one). But at this point you need to start considering that digital data systems have sampling rates (rates at which they take up and convert their analog inputs). These sampling rates can be the “fly in the ointment” Think of it as similar to a sudden alcohol-induced loss of hand-eye sensory coordination that causes you to stick your fork-full of food into a closed mouth.

h. Ques: Which outputs might cause the feedback condition? Answer: Well obviously the magnitude of any yaw-damper correction to a yaw upset is going to depend upon the airspeed sensed by the ADC. Static pressure changes influence this greatly. For example, if both static ports were to freeze over, the pilot’s airspeed indicator would wind back to zero over a further climb of about 2500ft (but he’d not see that climb because the altimeter and VSI would be stuck). Now consider that instead of freezing, the pressure was cycling (due to the rush back and forward of that water in the static line’s low points). What might that do to the ADC’s outputs during its sampling intervals? Would the yaw-damper system become confused?

Other Reasons for Feedback Loops are given here

You may be aware of some of these discussion threads below. The one reproduced below is my examination of just one rogue hydraulic possibility (for the quaint yaw-damper actuator set-up to have produced the feedback loop).

link one (multi-page)

In particular the Wino, Belgique and Overtalk posts

Reliable Redundancy

Patrickal

Quite right. And the "cleared" problem need not have been electrical. It might have been, in the case of AA587, mechanical and related to the 3rd servo-actuator (the one that serves up the output for both yaw dampers). Yaw damper actuators spend 99.999% of their time moving through a very limited travel and so probably pick up quite distinctive wear patterns. Perhaps it's only at "those other times" that travel outside their normal comfort zone detects a bit of stiction (or dirty hyd fluid or corrosion/erosion) and causes the FAC to hiccup. Those other times? Pre-start BITE checks and the dynamism of wake encounters or CAT.

Fascinating design arrangement that one. A soft-drive (#2YD) and a hard-drive (#1YD) act through a singular servo-actuator valve. Hardly fail-safe or fail-operational if a momentary intermittency in one causes the system NOT to trip, but to cycle between the two yaw damper outputs. I wonder if that might induce rudder oscillations?

During the NTSB's Public Docket (just finished) they very vaguely referred to a failure mode of those three rudder hyd actuators as "force-fighting" and reassured everyone that because of that FEDEX hangar failure of an actuator they'd implemented a 1300 hour synchronization check. Now that's about as reassuring as Firestone introducing a 10,000 mile safety check on their quality radial tyres. It doesn't make that failure mode evaporate. Until force-fighting was mentioned, I thought that that possibility might just have been a figment of my imagination.

Also interesting that the Airbus chappie gave, as a reason for having three rudder actuators connected to a single rudder panel, the logic that "how else would you provide for each of the 3 hyd systems to be available and cover a dual failure (in the uncontained engine failure case)". But no-one seemed to think it relevant to query whether it constituted "reasonable redundancy". That term was introduced by Dr Loeb in the NTSB Public Docket on the 737 rudder hard-overs. Eventually Boeing has had to introduce a 737 rudder fix. I personally find much similarity between the 737 rudder valve's shenanigans and the misconduct of the A300-600's rudder over the many incidents culminating in AA587. The similarity of the 737's actuator and the dual acting A300 yaw damper servo actuator is very striking. I'm suggesting that it may be the quintessential Achille's Heel.

Below (in a few links) is the story of two 747 uncommanded yaw incidents. Just reflect upon a similar malfunction involving NOT the life-saving split panel rudder arrangement in the 747 incidents, but the single panel scenario - with a number of FAC driven authorities fighting for rudder command and control (and perhaps passing the ball between them). The dismissive comment in the 747SP incident report is very telling when you think of AA587. It says:
"System redundancy had operated as required to limit the effect of the upper yaw damper anomaly" Here they are referring to the split rudder on that a/c.

A few links
one

two

three

four (the final two posts)

In Daze of Yaw

Not much is being said about AA587 since Airbus imposed its will upon the last hearing. The pilot error theory

is being supported by the NTSB and endorsed by the FAA. The glitch is more likely (IMHO) to be in the original

design (or introduced later by a software patch)._

And I still like my original theory (below):

So, (in part) addressing these selected quotes:
A. "A good look at the DFDR data does not show rudder movement consistent with a mechanical problem." and
B. "Even the AA flight 903 incident occurred in violent weather and an inflight upset that may have been a stall

or may have been something else."

In Daze of Yaw
The AA587 explanation may be as simple as water getting into the pitot-static lines every time an A300-600

aircraft is washed (or is parked in torrential rain) - and introducing a pressure change damping effect that

cannot be accommodated by the CADC’s pneumatic “expectations” (particularly in yaw). Why would that (i.e.

trapped water) make a difference? It’s not as if it might freeze in the lines and cause glaring static errors

(in airspeed, altitude and rate of climb). The errors might be much more subtle than that. In fact, in normal

flight they might be hardly noticeable. But in strongly yawed flight it could be entirely different. Why would

that be?

Explanation

_The input to the CADC’s is (ideally) purely pneumatic pressures being mildly differentiated by height and/or

speed variations. The CADC as a sensor is capable of picking up very minute pressure changes and rates of change

of pressure variation (as well as temperature increments of course). It feeds the resolved heights, rates of

climb and (of particular interest and significance) current airspeeds to a range of systems – including the yaw

damper and rudder limiter. _If _water is inadvertently introduced into the static lines (say) you then have a

damped and laggard hydro-pneumatic response in all the variables that feed pressure changes into the air data

computers (which will in turn misinterpret these “damaged” inputs and thus apply incorrect outputs). Normally

there will be four or more static ports (holes) – with a couple each side (of the airplane) to guard against

blockage (of a singular one). The siting of those holes is designedly where, in normal symmetric flight, the

smallest PEc (pressure error correction) requirement is generated. Some aircraft have the static ports right up

the front on either side of the nose and some aircraft have them way down back, just forward of the empennage.

Being thus located either well forward or well aft of the aircraft’s vertical (i.e. yaw) axis, the effect of

yawing needs to be cancelled out (and that’s why you have the ports uniformly on the port and starboard sides of

the fuselage). Works well, but how will water trapped in the lines affect the sensed pressures?

When the aircraft yaws due to turbulence (or pilot-pedal input) any airflows injected dynamically into the

static lines are self-canceling because the port and starboard static ports are Y-pieced together. Net effect on

the CADC’s sensed pressures? Not a lot, although extreme yaw can cause airspeed indications to fluctuate wildly.

But what if water has collected at a low point in the static lines – perhaps before the port and starboard

pickups are Y-pieced together?? (even if only on one side). In a sharp yaw to port any water in a starboard line

would tend to flow aft (and water on the port side would tend to flow forwards). What will be the effect of that

on the air-pressures sensed by the CADC and, more importantly, its outputs? At this point I should also

acknowledge that some modern systems (but probably not the A300) do incorporate air pressure transducers that

convert what is sensed to a digital signal that then goes to the CADCs. But whether those signals were valid or

not would depend on the location of those transducers. Because the CADC senses both quantum change and trends,

there will be at least a lag and more likely a contrary signal generated by the adverse flow of water affecting

the line pressures. Any correction fed to the rudder actuator by the yaw damper might then be inappropriate in

both magnitude and direction. So where is this error headed? Will any initial wake-induced yaw be damped out or

magnified by the yaw damper in this scenario? Will the erroneous CADC signal cause an initial significant yaw to

overshoot equilibrium and, in fact exacerbate the L/R yaw cycle? (i.e. "set the ball rolling" for some

rudder-supported extreme yaw cycling).

My theory says that YES, if the initial externally-triggered yaw is large enough, it could do that. If an

inappropriate correcting rudder deflection is commanded because of water trapped in the static lines and the

wrong airspeed being sensed - and the yaw correction consequently overshoots significantly, then we are into a

rudder-inspired undamped phugoid around the yaw axis. And beyond that “threshold of significant external yaw”,

because of the great stabilizing influence of the large vertical fin, any out-of-phase rudder control inputs

would be amplified and rapidly approach the point where something has to give (structurally). And that is what

I’m guessing may have happened in AA587. It might explain why much anecdotal evidence of tail-wagging is quite

irregular – possibly because static line water trap drains are cleared out on (just guessing) each C service (or

might be even more regularly)._ Then the aircraft's unalarming (but irritating) tail-wag phenomenon would

disappear overnight without comment. That would explain why particular airframes don't get (and keep) a bad

reputation.

So the theory really says: "You'd need to unluckily combine a waterlogged static system and a wake turbulence event (with

its large amplitude yaw “kick-off”) to end up generating an AA587 event".

In my scenario is there any scope for pilot complication or compounding of the wildly yawing initial scenario

(prior to structural failure)? The easy answer is: “Of course”. No pilot is going to sit there and allow the

airplane to swing wildly from side to side. He will at least attempt to counter the yawing with judicious rudder

pedal inputs (at least I would). But whether he could be successful (or just get out of phase himself and

exacerbate the situation) is the real quandary. Would the other pilot be aware of any such intervention on the

part of the PF (pilot flying)? Not really – and the whole thing would happen so fast that there’d be little

chance for anything further than the gasped expletives heard on the AA587 CVR.

How could you test this theory? The easy answer would be to “water” an A300-605 and go out and see whether

atmospheric disturbances or pilot pedal input could bring about any strange unexpected yawing responses. Another

approach would be to monitor the static-line water-traps across the fleet, measure the amounts of water found

and note whether aircraft washes (or heavy rinses) increased this amount.

Addenda
To specifically address this input by ALPA: (particularly the observation in blue)
<<"As I understand it, the A300 -600 was being hand flown at the time of the upset._ So the autopilot

would not have been engaged._ The A300 is not a fly by wire airplane._ It is conventional hydraulic controls so

the FCS electronics would be very limited._ The yaw damper would be active but the rudder

swings far exceed the authority of the yaw damper (it is mechanically limited)._ So, based on the DFDR, I do not

see any airplane issues based on flight control malfunctions.">>

Apart from the sampling issue with the AA587 DFDR meaning that the number and size of AA587's rudder deflections were not determinable, you

would not need max deflection rudder authority for a yaw-damper-actuated rudder to achieve large yaw amplitudes.

It's the timing of the actuator inputs that would be critical. I used to do a low-level jet aero display that

incorporated extreme yawing cycles on a flyby. Rudder authority was such that the last thing you wanted to do

was to stall the vertical fin - so you needed a keen sense of timing when "walking" the rudders somewhat

gingerly so as to achieve peak yaw in each direction. It is quite possible to achieve extreme yawing angles

(talking about > plus/minus 30 degrees here) by just getting the timing right. In addition to that, do not

disregard my opinion above - that any pilot (PF) would instinctively intervene (and perhaps disastrously) once

an identifiable cyclic yaw was underway.

Why wouldn't this problem afflict every airplane model? What is it about the A300-600 that makes it

different? Good question. Let’s see now:

1. Static Port placement? (and mayhap one that sucks in water flow-by when the atmospheric pressure is varying –

see *more below on this)

2. Early design that didn’t accommodate any real safeguards against a feedback loop in the FCS

3. Deficient safeguards for aircraft washing?

4. Airborne airflow patterns that allow rainwater to be entrained into static ports (or melting ice on the airframe to flow down/along and to be sucked in)

5. A CADC design that has sampling rates so high that you could say it was “hair-triggered” and over-responsive)

6. Designed-in low points in the static lines that allow water to pool and completely (rather than partially) obstruct the static lines (and so maximizing the hydro-pneumatic damping of air pressure sensing under rapid yaw.)

7. Insufficiently frequent specified water-trap draining intervals – i.e. allowing the water to accumulate.

8. Servicing manual deficiencies that don’t specify the water traps to be checked/drained after aircraft washes.

* My first experience with the ability of static ports to take in water was a long time ago. Imagine a rubber

bung that’s concentrically hollow to allow the static system to accommodate atmospheric pressure changes while

parked (and so allow air-fed flight instruments to “breathe” properly). Even though the bung is designedly

inserted upwards so that the hole faces downwards, water flowing down over it (in torrential rain) whilst parked

can still get sucked up that hollow rubber tube by capillary action. We proved that after a nasty mass incident

back in 1974 (or thereabouts). So you can get trapped water in the lines. In the case referred to here there was

no static port or line heating and that trapped water froze causing all pneumatic flight instruments to be lost

during a climb above freezing level. Not really applicable to AA587 but water in the lines can still have an

effect as outlined in the theory.

Explanation: “and so allow flight instruments to “breathe” properly” Pneumatic instruments can be destroyed (or

at least require removal and recalibration) if subjected to large pressure transients (like blowing orally into

a static port or obstructing a tube and removing a solid bung in a much changed atmospheric pressure days later

etc.) They are best left not 100% sealed in other words).

It seems pretty clear (to me) that Overtalk is referring to the possibility that accumulated water trapped in the static lines could induce the air data computer to feed faulty airspeed signals to either/both the yaw damper and rudder limiter via the Flight Augmentation Computer. Water in the static lines would dampen the ADC inputs considerably and create the potential for an out-of-phase condition to develop (particularly following on from the yaw caused by the externally applied gross stimulus of a wake encounter).. Closed loop feedback is the most likely cause of this accident - despite all the obfuscation by Airbus, the FAA and NTSB about pilot inputs. Overtalk is admitting that there may have been belated pilot intervention attempts that may not have helped (and may indeed have exacerbated the condition). That is quite different to a pilot-input inspired event.

Unlike roll and pitch, the problem in the yaw circuit is that the very large vertical fin is going to provide a powerful stabilizing force - but that can be subverted by mis-timed FAC signals to the rudder (as caused by faulty feeds from the ADC). In my view the theory is credible and might explain the A300's tail-wagging proclivities. For AA587 it was likely to have been a case of an unfortunate conjunction of a wake encounter, water in the static lines and a PF who was hand-flying and tried his best to calm the beast that was suddenly unleashed by the wake encounter.

While Overtalk and I both agree to what is the most likely_"late event" in the causal chain (a divergent rudder oscillation caused by a control loop with negative dynamic stability), I believe we diverge in our opinions as we progress backwards in the causal chain to what may have been the cause of this divergent oscillation of the control loop.

Overtalk favors the Air Data Computer inputs to the Flight Augmentation Computer, whereas I believe it was induced by hi-frequency pulsing in the hydraulic system. The reason I can't "stretch" to Overtalk's theory is because the characteristics of air data are well-known in the flight control world, and have been since even before the advent of digital computers as flight control system controllers. I think the entire community of flight control engineers would be aghast if we were to find out that Airbus did not filter out hi-frequency, oscillatory effects from the air data inputs to the control laws. It is THE FIRST consideration you make once you decide to use an airspeed parameter in your control loop computation.

The most typical, and easy, solution is a simple low-pass filter, so named because it only allows the LOW frequencies to pass thru to affect the control law. Low pass filters are typically characterized by a "break frequency", which is the design point at which inputs at that frequency begin to be attenuated (rejected). [Low Pass Filters are even discussed in the CVR analysis in the accident factuals.]__For airspeed inputs, the filter break frequency_is usually somewhere around the 8-10 Hz range. Any oscillation in the digital airspeed signal higher than this frequency is filtered, and will never drive the control law, for the specific reasoning of avoiding divergent oscillation caused by excessive phase lag. Such filtering is inherent to ALL flight control system designs that use airspeed programming in the control laws, and this is why hi-freq oscillations in airspeed do not plague the worldwide fleet. It would be a VERY regular occurrence (on Airbus as well as other airplanes) if such filtering did not exist....and yes, water in the lines would exacerbate such oscillations.

It would be even easier than performing a flight test to see if Overtalk's theory holds water (pardon the pun). If Airbus would simply reveal their design specifics of the FAC control laws with respect to airspeed inputs to the rudder control law, any one of a multitude of controls engineers could easily perform a frequency domain analysis, and tell you if there was any potential for oscillating airspeed signals to get into the closed-loop control law.

The other issue I have in accepting this theory is lack of abundant "smoking gun" evidence. Yes, we have a_fair amount_of tail wagging events in the Airbus history file. However, one cannot assume that this is all due to faulty air data processing without some hard evidence that points in that direction (it could just as easily point to my theory). However, this is where I believe my theory (rudder hydraulic_de-synchronization)_shows ample smoking-gun evidence:

1) The existing AD on de-synchronization is the biggest smoking gun! But it goes deeper:
2) The FedEx hangar event (with airspeed=0) was a clear rudder oscillation that lead to mechanical failure. The test being performed was, indeed, the test required to attempt to detect the de-synchronization problem. I'd say_they found it!
3) The AA587 subject airplane had a history of rudder system related write-ups.
4) The subject airplane had a FAC preflight test failure right before the doomed flight. It is my understanding that this pre-flight test specifically seeks to verify proper operation of_the rudder servo control loop.

But again, even my theory could be dispelled by Airbus coming clean on their design details. To dispel Overtalk's theory, one only needs to know the control law filtering specifics on airpseed signals coming from the ADC to the FAC. To dispel my theory, one only needs to see actuator system frequency response test data, both under normal conditions, and under the conditions described as "de-synchronization".

In the past, I have amply described how the word "synchronization" is a direct reference to a closed-loop control system's amount of phase lag.

So...there's my summary! Nothing personal....just a difference of technical opinions. :-)

Belgique: Does the expression "control law" apply to the older technology Airbus 300/310 rudder systems as much it does to the fly-by-wire A-320/330--- systems?

I have no engineering training but find the comments on this topic quite interesting, and wish I could better understand some of the complexities without a graphic flow chart in a flight manual. In my company's flight ops magazine, the Internet and in "Aviation Week & ST" magazines, the words "control law" seem to be used only in connection with the A-319/320 etc, but never with the 757/767 generation.

Control Laws

Ignition Override

Any time that you have digital data feeds inputting into a flight control system (yaw damper) you must have "control laws". These accommodate (and/or negate) the type of dangerous feedback loops described by OVERTALK.

It is almost unthinkable (but not unbelievable) that in an early design, such as the A300, an unforeseen anomaly may have been allowed to creep into the system. But then again, would they have been testing for an unlikely "sometime" occurrence (such as an accumulation of water in the static lines) to create a feedback loop - and only once excitated by an external influence (such as a wake encounter). But it cannot be ruled out - basically on the grounds that I doubt that Airbus would have watered the prototype's static lines and tested for it. They'd have needed to go seek out an external excitation (such as a wake encounter, stall or similar upset) whilst having a significant amount of water trapped in the lines. See my point there?

Neither can the possibility of hydraulic feedback loops be ruled out. Both possibilities are far more likely to have caused the AA587 accident than a pilot pedalling into a destructive yaw cycle. It's just that the non-aerodynamic FEDEX hangar incident points more to the actuator having broken because of a straight hydraulic anomaly.

unlikely occurrence?

Belgique says:
"But then again, would they have been testing for an unlikely "sometime" occurrence (such as an accumulation of water in the static lines)"

Rather than "unlikely" I might have said "unanticipated". Probably no manufacturers would water a static line and go out and flight- test for such a condition. However in my experience some aircraft are more likely than others to draw water into their static lines. It would be interesting to know:

a. How much they normally find in an A300 (and whether records are kept)

b. Whether they check it after an aircraft wash (most companies wouldn't, they'd just trust that taping over the ports would ensure that no water got in).

c. Whether water can accumulate inflight (due to flight through heavy precipitation - or be drawn in on descent as airframe icing melts.

d. What parameters American Airlines use for mandatorily chucking the bungs in (overnight parking only - or at all other times?)

I think I'm right in saying that low points are normally provided as water traps (with drains fitted) at some point before the port and starboard static lines are Y'd (or T'd) together. Water accumulating in those low points MUST affect the air pressure being sensed by the transducers further downstream in the lines (or at the CADC itself) - particularly during rapid yawing, when the water flows would be in opposite directions on the port and stbd sides..

I'll leave it up to your imagination as to what effect this hydro-pneumatic damping might have upon the static pressures being sensed at the CADC (and more importantly what effect a pulsing pressure [that's usually fairly constant] might have upon the airspeed outputs being provided to other aircraft systems - including most importantly the yaw damper). This is where "sampling rates" start to confuse the issue and you'd really need to go out and conduct experiments to get to a bottom line.