Just another AA587 Theory
Quote: The A 310 manual states -
In the event of loss of both systems switching circuits located in each FLC provide a signal to retract the variable stop actuator to low speed position.
Does this hold true for the -605, and could the systems have failed with no announcement being produced on the flight deck?
To quote the a300-605R flight manual:
"2 independent rudder limiting systems controlled by the FLC, progressively decrease the maximum rudder travel from +/- 30 below 165kt to +/- 3.5 above 310kts. If both systems fail, whatever the aircraft speed, the mechanism returns to the low speed position (+/- 30 rudder travel available).
If the system does not return to the low speed position, an ECAM warning (indicating the rudder travel system is jammed in the high speed range) is activated when flaps are at 20 or more."
. Assume that a software(or design) glitch existed such that when large pressure transients are experienced (such as in a wake vortex) a rudder limiter system (RLS) trips itself into self-test (or hands over to the other RLS system - OR BOTH [see below]). Because their function is directly speed-related and their purpose is to progressively inhibit rudder travel, their one essential driving input must come from the CADC. But if the CADC's input data (and outputs) is in a state of flux because of transiting the wake vortex, you may see a situation where RLS1 hands to RLS2 (which is feeling the same strain about its own credibility and attempts immediately to hand-over back to RLS1 etc etc).
Now why would this occur? Some of these systems (and early systems in particular) were designed to hand-over and go into a self-test mode before declaring themselves failed (similar to a BITE check self-test on power-up). Great to have systems capable of mutual back-up - but not so great if they were to share a common glitch that could be brought about by an environmental (large short-term pressure transients) that had them swapping authority back and forth (or attempting to do so whilst simultaneously attempting to self-test). And what might be happening if they are handed and/or forced to accept authority whilst in that self-test? Why would they be? Perhaps because self-test is yet incomplete and as far as the FLC is concerned, they have not declared themselves failed or off-line and are therefore deemed available. Or perhaps they are passing their own self-test and are available (but only until back in operational authority)....
What we are talking about here is a scenario where both RLS systems are simultaneously experiencing the same "induced" failure mode (or something that is being self-interpreted as a failure - and which has them (or courtesy of the FLC) trying to hand-over to each other. In this weird interregnum you may indeed have the aircraft FLC (or pilots) trying to correct for sudden yaw but having to contend with excursions through this configuration in green [If both systems fail, whatever the aircraft speed, the mechanism returns to the low speed position (+/- 30 rudder travel available)]. Why wouldn't a failure necessarily be annunciated? In many systems, in order to stop nuisance failure alerts caused by transient conditions, failure attention-getting is latched out either for a finite period or until an “off-line” self-test mode is completed. In the scenario that I have envisaged and described here, because the failure is a commonly held susceptibility to CADC data-flux, neither system gets to complete its self-check before it is called upon [by the other] to take over authority.... therefore no alerts and RLS systems that are dangerously making excursions into and out of (+/- 30 rudder travel available). End result? Inappropriate rudder deflection travels that in turn generate a requirement for correction (but those corrections are also overtravelled….). See the destructive continuum?
With a little further examination this type of mutual excitation failure may be able to be induced by other “inappropriately sensed failure” parameters (ticking faults?) and explain the FEDEX hangar failure and other instances of tail-wag. It may indeed cause a yaw-damper system to trip in protest (before start-up - say).