Spiral instability.
With regards to large jet transport aircraft, can be induced by too large of a vertical stabilizer, as was found out by Boeing many yeras ago when they were trying to figure out a way around DP Davies' insistance that the B707 be improved, otherwise it would
not be allowed onto the British civil register.
A 39 inch ventral fin was the first action, then the ventral fin was discarded and the vertical fin extended, whereupon spiral instability was found.
Enter, stage left, the hydraulic guys.
Improved rudder power system design solved the problem to a large extent, and the taller vertical fin could remain
However, if the yaw damper failed in flight with these early airplanes, best you should find a place to land...right now, least you found yourself in moderate/severe turbulence, and the very unstable dutch roll tendancies of the airplane became apparent at higher altitudes...uncontrolable alternating fifty degree plus banks will get your attention in a heartbeat.
