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. :-)