A Pro Flt Control Design Engineer's Opinion
IMHO you could have a sideways hammer-blow from a mature wake vortex - but whether it would be laterally sharp (and hard) enough to cause a rudder movement that would snap one, two or three of its actuators? Hard to say - but I would doubt it.
But then again, you have to ask: "So when did the rudder actuators snap and the rudder separate from the fin? Did actuator breakage and rudder hinge detachment occur simultaneously? Was it whilst the vertical fin was still attached to the fuselage?”. In my lay view it would have to have been - because the detached fin's short fluttering descent to the water would not have provided sufficient force to cause the rudder to wholly detach..
Bit of a mystery really when looked at from the angle of rudder detachment timing and forces required. Maybe Rainman would have a view. Obviously the NTSB would have sucked on this aspect already.
Rainman says in reply
unctuous wrote:
>>Did actuator breakage and rudder hinge detachment occur simultaneously? Was it whilst the vertical fin was still attached to the fuselage?<<
Not only would I say yes, but I also think this is another sign of an undamped oscillation that supports my theory of what happened on this airplane. A standing wave oscillation (wake vortex, in this case) that stimulates a closed-loop control system that has negative phase margin is going to cause larger, additive hinge moments in the rudder control system than if it were not out-of-phase. That's because if the control system is 180 degrees out of phase (directly opposing) the aerodynamic load, then the hinge "feels" the force of the aerodynamic load, and it also feels the force its control system is exerting against it, in its out-of-phase attempt to control the airplane.
As the oscillation continues, each new "peak" reaches a new (higher) peak in surface load and therefore hinge moment. If the control system is truly 180 degrees out of phase, then at the point of fracture, the rudder control system would have been pushing one way (say left) with all its force, and the vertical stabilizer would have been pushing the opposite direction (right) with the full force of the aerodynamic response. Rudder breaks and flies off in one direction, tail-fin breaks and flies off in the other.
I still would like to see someone push a big question to the front of the media on this accident: Airbus, could you please publish the "normal" frequency response of the rudder control system, and your "estimated" frequency response of the same system while subjected to your known "rudder synchronization" failure mode described in the subject AD?
Why is this data not available? If there is no smoking gun in this data, not only will I shut up, but other professionals with my knowledge would also comment on that data, and they could pass their own, independent judgment.
Rainman