YAW CONTROL
Yaw control is accomplished through dual rudders and yaw dampers. The rudders are hydraulically powered, the upper rudder by system "B" pressure, the lower rudder by system "A" pressure. As a backup, the lower rudder can also be operated by the standby hydraulic system, which powers a separate rudder actuator. If all hydraulic power is lost there is no rudder control. Full system "A" pressure is provided to the lower rudder when the flaps are extended. When the flaps are up pressure to the lower rudder is decreased through action of the rudder load limiter. Illumination of the rudder load limiter light on the FE's panel indicates that the pressure to the lower rudder is not proper for the inboard flap position. The upper rudder always operates at reduced system "B" pressure. Therefore, the pressure changeover monitored by this light is applicable only to the lower rudder. Both rudders have anti-balance tabs, which move in the same direction as the rudder. Artificial feel for rudder inputs is provided through a spring-loaded roller and cam assembly.
The rudders are trimmed by positioning the rudder trim control on the control stand. The rudder pedals will reposition during trim input. As with the ailerons, rudder trim is available only when hydraulic power is available.
YAW DAMPERS
The tendency to "Dutch Roll", at high altitude and high airspeed, is counteracted in the 727 by a yaw damper system for each rudder. The lower rudder yaw damper does not function when that rudder is powered by the standby system. The yaw dampers receive electrical signals from the rate gyros. As the nose moves left or right, the rate gyro senses a yaw. This signal is sent to the yaw dampers , which direct the rudders opposite to the direction of the yaw. Rudder movement caused by the yaw dampers is not transmitted to the rudder pedals and does not interfere with pilot input to the rudders. A yaw damper test switch on the Captain's forward instrument panel allows testing of the yaw dampers before taxiing. The yaw dampers are designed to be used continuously. Therefore, the guarded yaw damper engage switches on the center forward instrument panel are normally ON at all times. On the Captain's forward panel is a rudder-elevator indicator. The yellow YID flag will appear if the respective yaw damper system loses electrical power or is turned off. If one or both yaw dampers fail, airspeed and altitude restrictions are imposed. These restrictions are listed in the Limitation section of the Flight Manual.
