Originally Posted by
ShyTorque
...The answer is to mandate that by design that all helicopter tail rotor control systems are equipped with a centreing device for the servo valve mechanism, to prevent a servo runaway in the event of the pilot's flight yaw controls becoming disconnected, such as now appears to have occurred here. With such a device fitted, the servo is automatically biased to move to a fixed, central position enabling controlled flight to still be available and a safe landing of some sort to be carried out...
This wouldn't help if the failure was beyond the servo, i.e. the shaft to the tail rotor. There is also the down side of adding weight and complexity.