What does runaway failure mean (position difference between APPU and FPPU) on A320? I've been reading the description and operation of Flap systems. WTBs will stop and hold the transmission whenever SFCCs receive some failures.
A runaway failure of an actuator is a situation where the actuator deflects maximally into one of its stops and stays there. Particularly hydraulic actuators can be subject to these: if the servo valve cannot close due to for instance dirt being stuck in the valve, oil flow will cause this type of failure. Electrical actuators commanded by a servo loop can fail in this mode as well.