Yaw Damping is provided by two completely independent dampers which can be operated singly or simultaneously. The prime function of the system is to minimise Duch Roll by providing automatic rudder displacement proportional to and opposing the amount of yaw experienced.
One yaw damper controls the upper rudder the other the lower. Each yaw damper has an associated coupler which operates as a rate gyro and senses yaw. The damper system then provides the necessary rudder movement to oppose and damp out the yaw. Rudder displacement resulting from yaw damper input is limited to 5 deg to prevent full rudder being applied in the event of a yaw damper malfunction.
The lower yaw damper is powered electrically from the Essential Radio Bus, upper yaw damper is powered from the No 2 Radio Bus.
Two yaw damper warning flags on the rudder and elevator position indicator are biased out of view when the respective yaw damper is engaged. Some aircraft have green lights instead of the warning flags, lights are green when the damper is engaged.
Each yaw damper controls it's associated rudder through a transfer valve on the rudder power unit. The upper damper uses system B, the lower damper system A. The loss of either hydraulic system pressure will result in the loss of the associated yaw damper. If this occurs a loss damper disengaged warning will NOT occur.
The only common circuitry between the yaw dampers and the autopilot is an interlock that requires at least one yaw damper to be on in flight before the autopilot can be engaged.