How can I switch off the yaw damper in an Airbus A350? In the A320, this can be done by switching off the FAC, but the A350 doesn't have a FAC switch; it has prim and sec instead of FAC and ELAC.
If you prevent the yaw damper function to be calculated it will switch off. As yaw damper is calculated using lateral data from ADIRUs and specific FBW lateral accelerometers you need to switch off all these; the ADIRUs by push buttons and the lateral accelerometers by specific breakers. On some Airbus you still have an emergency AUTONOMOUS yaw damper unit, some models are fully electrical and use up to the battery power, so you pull the relevant breaker, but some models use the hydraulic power to generate their own electricity so you need to switch off the corresponding hydraulics, leaving active the only hydraulic that the autonomous yaw damper does not use. On the other hand note in direct FBW mode the yaw damper is degraded but still functional, so being in direct mode is not sufficient to inhibit the yaw damper. But by putting off the yaw damper most probably you will be in direct law.