737 Operations Manual
A rudder pressure reducer is connected to the A system hydraulic line upstream of the main rudder PCU. Hydraulic pressure to the rudder is reduced when the airplane climbs above 1000 feet AGL. Hydraulic pressure returns to normal when the airplane descends through 700 feet AGL, or if B hydraulic system depressurizes, or whenever the N1 difference between the left and right engines exceeds 45%.
Rudder Trim
The RUDDER trim control, located on the aft electronic panel, electrically repositions the rudder feel and centering unit which adjusts the rudder neutral position. The rudder pedals are displaced proportionately. The rudder trim indicator displays the rudder trim position in units.
Yaw Damper
The yaw damper system prevents unwanted (Dutch) roll and provides turn coordination. The yaw damper coupler receives inputs from the yaw rate gyro and the air data computer. It then provides inputs to the rudder through the main rudder PCU. At higher airspeeds the amount of yaw damper rudder deflection decreases. No rudder pedal movement results from yaw damper operation.
The yaw damper uses hydraulic system B pressure only. If hydraulic system B pressure is lost the yaw damper system is inoperative but the YAW DAMPER switch remains in the ON position until the B FLT CONTROL switch is positioned to OFF or STBY RUD. Then the YAW DAMPER switch disengages and the amber YAW DAMPER light illuminates and the YAW DAMPER cannot be reengaged.
Copyright . The Boeing Company. See title page for details.
9.20.10  June 4, 2004

Flight Controls -System Description
Yaw Control Schematic (before RSEP modification)

VALVE
Yaw Control (after Rudder System Enhancement Program (RSEP) modification)
Yaw control is accomplished by a hydraulically powered rudder and a yaw damper system. The rudder is controlled by displacing the rudder pedals. The yaw damping functions are controlled by the yaw damper coupler (YDC).
Rudder
The rudder provides yaw control about the airplane’s vertical axis. The A and B FLT CONTROL switches control hydraulic shutoff valves for the rudder and the standby rudder.
Flight Controls -System Description

737 Operations Manual
Each set of rudder pedals is mechanically connected by cables to the input levers of the main and standby rudder PCUs. The main PCU consists of two independent input rods, two individual control valves, and two separate actuators; one for Hydraulic system A and one for Hydraulic system B. The standby rudder PCU is controlled by a separate input rod and control valve and is powered by the standby hydraulic system. All three input rods have individual jam override mechanisms that allow input commands to continue to be transferred to the remaining free input rods if an input rod or downstream hardware is hindered or jammed.
A rudder pressure reducer (RPR) is connected to the Hydraulic system A line upstream of the main rudder PCU. A rudder pressure limiter (RPL) is incorporated in the Hydraulic system B part of the main rudder PCU. Both the RPR and RPL limit hydraulic pressure to the rudder when full rudder authority is not required. Hydraulic pressure to the rudder is limited when the airplane climbs above 1000 feet AGL. Hydraulic pressure is returned to normal when the airplane descends through 700 feet AGL, or if B hydraulic system depressurizes, or whenever the N1 difference between the left and right engines exceeds 45%. This function limits full rudder authority in flight after takeoff and before landing. The Yaw Damper Coupler (YDC) controls both the RPR and the RPL respectively, for Hydraulic system A and Hydraulic system B of the main rudder PCU.
The main rudder PCU contains a Force Fight Monitor (FFM) that detects opposing pressure (force fight) between A and B actuators. This may occur if either system A or B input is jammed or disconnected. The FFM output is used to automatically turn on the Standby Hydraulic pump, open the standby rudder shutoff valve pressurizing the standby rudder PCU, and illuminate the STBY RUD ON, Master Caution, and Flight Control (FLT CONT) lights.
The standby rudder PCU is powered by the standby hydraulic system. The standby hydraulic system is provided as a backup if system A and/or B pressure is lost. With the standby PCU powered the pilot retains adequate rudder control capability. It can be operated manually through the FLT CONTROL switches or automatically. (Refer to Chapter 13, Hydraulics, Standby Hydraulic System)
An amber STBY RUD ON light illuminates when the standby rudder hydraulic system is commanded on. The standby rudder system can be pressurized with either Flight Control switch, automatically during takeoff or landing (Refer to Chapter 13, Hydraulics, Standby Hydraulic System) or automatically by the Force Fight Monitor. The STBY RUD ON light illumination actives Master Caution and Flight Control warning lights on the Systems Annunciation Panel.
　
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