RatherBeFlying
Think you've misunderstood. Not talking about yaw being sensed by a vane - that's normally a gyro function. The yaw that was sensed was probably quite correct - it was the corrective rudder inputs that may have been continually inappropriate. Why?
What I'm looking at is the airspeed inputs to the yaw dampers and rudder limiters being in error. The reason why they can be in error is fairly clearly explained in that post - but to
summarize:. .Imagine that there's water trapped within that static system and you enter the first wake encounter induced yaw cycle. Water has inertia and will be affected by the yawing.
Flowing adversely (and even out-of-phase) within the static system, the instantaneous pick-offs by the ADC are going to lead to inappropriate rudder input corrections - simply because the sensed airspeed will be wrong (and thus the rudder ratio limiter is mis-set). Because the airspeed derived from a false static pressure can be quite grossly in error (example given) you will have a series of non-rhythmic corrective yaw inputs that will each prove to be "unfinished business". A minor yaw disturbance will result in a diminishing yaw cycle - because of the overpowering damping influence of the vertical fin (which is why the fin is there in the first place). However a larger initial yaw might result in a divergent (or at least self-sustaining) oscillation.
I'm suggesting that this is what has happened with AA587. Unfortunately that cycle was interrupted at just the wrong time (and angle) by the second wake encounter - and that's what broke an attachment lug. After that first lug broke, the fin was soon rocking laterally and that was the death-rattle heard on the CVR (as the rudder was driven into a frenzy trying to compensate for the fin's lateral rocking as it danced its Detachment Dance).
The bent/broken FEDEX rudder input actuator rod. .The event they had in their hangar was related to the hydraulic synchronization issue (hydraulic pressure pulses from different sources getting out of phase-sync). The mechanic who initially heard the "banging" did say it was oscillatory in nature, although there was no one watching the rudder at the time. Remember that only one actuator is moved by the
yaw damper. If it was this one that’s bent? Well then it’s just about case solved. I would then submit that there might have possibly been a battle between a rudder limiter that had run amuck (courtesy of the ADC’s flawed pressure inputs) and a yaw damper that was properly doing its job, as it saw it.
As these threads are very perishable, a page of informed surmise will be maintained <a href="http://www.iasa-intl.com/folders/Safety_Issues/others/aa587/ruddersnapfinoff.html" target="_blank">here</a> (with links of interest at the bottom).. .Feel free to contribute.