I’m sorry if this has already been mentioned, but just to be sure:
On the B747-400 we have frequently practiced the volcanic-ash scenario losing not only (all of) our engines (possibly explains the RAT on the B748
), but also our IAS indications (due clogged Pitot-Static lines). As a result, flying the aircraft becomes a bit challenging; not as much due to the lack of airspeed indication (you can always use GS or AOA [MCDU] as a fair replacement) or the lack of engines (although that does pump up the adrenaline a bit); the main problem is the yaw-damper(s): since rudder deflection of the yaw-damper (both function and max; rudder ratio changers) is IAS related, an IAS near zero will result in max rudder deflection ... and I don’t know whether the rudder/vertical-stabilizer can take that kind of punishment at Mach .8x (the B744 might .. since it is still a heavy-metal aircraft; the A330 though …). Not hard to see the analogies of blocked PS lines due volcanic-ashes and severe icing.
As bearfiol rightfully put it: the rudder may be allowed to shear of, the design of the vertical stabilizer should be such that it won’t ... indeed, no aircraft will fly without vertical stabilizer.
Not only check the design of the VS-rudder; also check the logic of the yaw-dampers…