Both wsherif1 and RBF have good points.
wsherif1 however seems to be missing the fact that the A300 received two wake “hits” - and the real damage was likely to have been done by the second - and for sound reasons.
Unfortunately at the time of that second “hit” the FCs was still responding to the yaw upset caused by the first. It’s like that old pugilistic “one-two” punch set. The first fist (the “right”) puts him off balance and the second (“left uppercut”) follows through at just the correct moment, catching the punch-drunk fighter dazed, off-balance and off-guard. We know that the A300 has a factual and anecdotal history of tail-wagging. However unless that involves external forces and reaches extreme amplitudes or gets nastily out-of-phase, the outcome is simply “noteworthy”, although discomfiting. In AA587’s case the characteristics of the FCS corrective rudder displacement) and the natural dampening stability of that large vertical fin meant that the amount, rate (and direction) of side-slip at the instant of that second hit was wholly adverse - and enough to break the fin off. No need to involve any pilot inputs here, in fact because of the tight timings it’s unwise to do so. But that’s not to say that attempted pilot intervention could NOT have happened. If I was sitting in the seat as PF and felt that the FCS had lost the bubble, I’d be attempting to stop the yaw nonsense (for pax comfort). That doesn’t mean that I’d be laying on a bootful. To the contrary, I’d be tromping on both pedals equally and simultaneously and trying to dampen out oscillations related to any excess pedal travel - in the hope that natural damping would stop the yawing. That’s just about instinctive for any pilot.
If the FCS has a built-in software (or CADC induced) flaw that causes any restorative rudder inputs to overtravel inappropriately, then that would explain both its reputation for tail-wagging and the AA587 outcome. Why so for AA587? Well think of it as a wake-induced yaw that went “a Bridge too Far” and entered dangerous, normally untravelled, territory. Let’s say that, after the first wake encounter, at that IAS a 5 degree rudder displacement would have been appropriate to assist the fin’s natural yaw dampening. But for some (programmed) reason the FCS fed in 10 degrees and at a higher rate than might have been ideal. The end result is an overshooting instability and an even greater amount of slip or skid out the other side - at which point the second wake “hit” occurred. For FCS failure modes see “servo control desynchronisation” and “yaw inertia” in my earlier posts on this thread (pg3).
If it had been another type aircraft (with a less rigid metal fin and rudder incorporating some flexing "give") the dynamics of the side-stresses could have been soaked up by the structure - and possibly without any permanent deformations. But it was an A300 and its vertical fin attachment is via a set of six composite attachment lugs dissimilarly mated to metal brackets on the fuselage hull. If it had been a metal-to-metal attachment and the metal upper attachment lug-set had been firmly part of a fin-embedded metal structure, then the shear of an excessive side-force may not have caused the failures in tension seen just above those composite lugs (it's called "load-sharing"). So it’s back to the composite engineers’ admissions that the composite matrix lay-up is necessarily set so as to give maximum strength in certain force directions - unlike metal. The first wake “hit” engaged an essentially yaw-static airplane. But when the second wake-hit occurred, at just the wrong moment, it engaged with some opposing FCS-induced yaw-dynamics. That combination was dynamic enough to exceed the fin’s ability to withstand lateral stresses and an attachment failure occurred. In fact it probably exceeded ultimate design load and any inherent weakness due to manufacture, repair or prior incident damage would have been minimal factors. In other words any A300 might have suffered a similar failure in the circumstances - and that’s the big worry. The one disquieting aspect of structural composites (as against metal) is that they need a continued 100% integrity. In other words, once you have a composite failure affecting load-bearing integrity, it’s likely to become progressive. The other anxieties about whether or not a visual inspection is sufficient to detect significant delamination or disbonding and the effects of resin aging and moisture ingress? They just add to the disquiet.
My spies tell me that all concerned in the investigation are now leaning heavily towards pilot intervention as being a satisfactory explanation for AA587. Even those who
don’t believe that explanation remain convinced that it was an unholy stroke of bad luck that caused all those factors to come together and fail AA587’s fin. Be that as it may, I myself will be thinking twice about traveling the structural composite routes. Catastrophic failure is not supposed to occur, no matter how fast a pilot may (or may not) have been pedaling his wares.
