(3)  
When FLT CONTROL B LOW PRESSURE warning lamp is illuminated on the Pilot’s Overhead Panel (Fig. 5).

C.  The rudder pressure reducer does not interfere with yaw damper operation, which makes inputs to the half of the main PCU powered by the B hydraulic system. The rudder hydraulic system is monitored by the yaw damper coupler, and failure of the rudder pressure reducer is indicated by illumination of FLT CONTROL A LOW PRESSURE warning lamp on the Pilot’s Overhead Panel (Fig. 5). The yaw damper coupler also has BITE on the rudder pressure reducer (Ref 22-12-01, D/O).
15. Operation
A.  The rudder control system is operated by two sets of pedals, one for each pilot. Pedal movement is transmitted to forward quadrants and jackshafts by rods. The two forward quadrants are interconnected by a bus rod. Cables transmit forward quadrant motion to the aft quadrant located in the vertical fin. Rotation of the aft quadrant in turn causes rotation of the rudder control torque tube.
B.  The rudder control torque tube, when rotated, provides simultaneous input to the rudder power control unit, the standby rudder actuator, and to the rudder feel and centering mechanism. In normal operation the power control unit is powered by hydraulic systems A and B (Fig. 9). Control input to the power unit occurs when a torque tube-mounted crank rotates. The crank is connected by a rod to the input linkage of the power unit. As the input linkage moves, the control valve opens and ports hydraulic fluid to the actuator cylinder. The actuating piston then moves and causes rudder deflection. Nominal rudder travel is 26 degrees in both directions. 
C.  If hydraulic systems A and B are not available to power the rudder, then standby power may be turned on, and the rudder operated with the standby actuator. Rudder pedal input positions a servovalve in the actuator which ports fluid to the actuating piston. Movement of the piston closes off the ports when the desired rudder travel is reached.
D.  In addition to operating the rudder power unit and the standby actuator, the torque tube moves the centering cam in the feel and centering mechanism. When the camshaft is rotated it displaces the spring-loaded cam follower.
E.  If rudder trim is adjusted with the power control unit depressurized, the drag of the unpowered rudder system will allow the system to remain out of center. This will force the rudder feel and centering mechanism cam follower out of the cam detent. Then when hydraulic power is applied, the rudder pedals and rudder will rapidly move to the newly trimmed position.
508 
27-21-0 Page 16  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  May 01/99 

K14924 K15127
520  Rudder Hydraulic System Schematic 
May 01/99  Figure 9  27-21-0 
Page 17 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

Rudder Pressure Reducer  501 
27-21-0 Page 18  Figure 8 (Sheet 1) BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  May 01/99 

RUDDER AND RUDDER TRIM CONTROL SYSTEM - TROUBLE SHOOTING
1.  Rudder and Rudder Trim Control System Trouble Shooting Chart
TROUBLE  PROBABLE CAUSE  ISOLATION PROCEDURE  REMEDY 
Insufficient rudder or rudder pedal travel *[1]  Foreign objects interfering with rudder pedals and forward quadrants Rudder pedals and/or forward quadrants out of rig Binding, worn or defective parts in forward quadrants Binding, worn or defective parts in rudder pedal assembly  Foreign objects interfering with aft rudder quadrant, torque tube in fin or input linkage to power control unit Fwd quadrant stops incorrectly installed  Insufficient clearance between input rods or linkages and cranks or quadrants near full travel positions  Check for foreign objects in area of rudder pedals and forward quadrants Check rudder pedal and/or forward quadrant rigging. Refer to 27-21-0, Rudder and Rudder Trim Control System -Adjustment/Test Check forward quadrant parts Isolate rudder pedal assembly and check terminal and bellcrank. Refer to 27-21-31, Rudder Removal/Installation Check for foreign objects in area of aft quadrants, torque tube and PCU input linkage Check if quadrant is contacting stops before required travel is obtained With rudder pedals near full travel position, check for clearance between rods and cranks, etc. Fwd stops should be contacted before linkages run out of travel  Remove foreign objects Adjust rudder pedals and/or forward quadrants as necessary Replace worn or defective parts Replace worn or defective parts Pedal AssembliesRemove foreign object or interference Correct improper installation Remove interference 

*[1]  All trouble shooting to be done with power on, unless otherwise stated. Provide or remove rudder hydraulic systems power per 27-21-0, Rudder and Rudder Trim Control System -Maintenance Practices.
　
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