11.  
Rudder Power Control Unit (Fig. 5)

A.  The rudder power control unit moves the rudder right or left when actuated by rudder pedal input. It also provides wind gust snubbing when the airplane is parked. The unit is located in the vertical fin (Fig. 1). The body is fixed to fin structure and the power head to the rudder. It is separately powered by hydraulic systems A and B. Either system acting alone will provide full rudder control.
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G07245 G07290
526  Rudder Feel and Centering Mechanism Schematic 
Aug 15/69  Figure 4  27-21-0 
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Rudder Power Control Unit Schematic  545 
27-21-0Page 12   Figure 5 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Aug 01/74 

B.  Power control unit yaw damper function is to minimize dutch roll during manually and automatically controlled flight. The yaw damper provides rudder displacement proportional to the yaw rate, and opposing the yaw direction of the airplane. The yaw damper input is in series with the pilots' input and is not felt at the rudder pedals.
C.  Pilots' input, by rudder pedals or trim wheel, is transmitted through cables and linkages to the power control unit input crank. The input crank rotates and moves the control valve which ports hydraulic pressure to the actuating cylinder. The amount of control valve movement is also governed by the yaw damper input. The pilots' input and yaw damper input are combined algebraically by the summing levers, which connect to the primary and secondary control valves. The power control unit piston rod end is attached to the rudder. Therefore, extension or retraction of the rod moves the rudder left or right. An external summing lever attached to the actuating piston provides feedback to return the control valve to neutral and to stop the rudder at the desired position.
12.  Standby Rudder Actuator Shutoff Valve
A.  The standby rudder actuator shutoff valve is located on the aft bulkhead of the main wheel well (Fig. 7). The unit is mounted on the standby system modular unit by four bolts. The valve is electrically operated from a 28 volt-dc source, and consists of a motor spool sleeve and cartridge. The cartridge containing the spool and sleeve fits into a cavity in the modular package. The electric motor is splined to a cam which converts rotary motor action into linear spool travel in the sleeve. The valve is equipped with a manual override lever and position indicator. The manual override allows the valve to be positioned with electrical power off. The valve is on when the lever is in position 1, and off when the lever is in position 2.
13.   Standby Rudder Actuator
A.  The standby rudder actuator (Fig. 6) positions the rudder when hydraulic systems A and B are not available. The actuator consists of a bypass valve, control valve, and the actuating cylinder. Standby hydraulic power for the standby actuator is controlled by system A and B switches on the overhead flight controls panel.
B.  Hydraulic pressure supplied from the standby system operates the actuator. Rotation of the input crank will position the control valve to port fluid to the actuating cylinder.
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G07297G07311
 Rudder Standby Actuator  500 
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501  Standby Rudder Actuator Shutoff Valve 
Aug 01/74  Figure 7  27-21-0 
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14.  Rudder Pressure Reducer (Airplanes incorporating SB 27A1206)
A.  A rudder pressure reducer is located on the A hydraulic system supply line to the rudder power control unit, which reduces hydraulic system pressure supplied to the rudder power control unit to a specified range. The rudder pressure reducer is located in the upper left-hand side of fuselage STA 1104. System pressure of 3000 psi at the inlet port of the pressure reducer unit is reduced to 1400 psi at the outlet port during conditions other than those listed below.
B.  The rudder pressure reducer will command full system pressure in the following situations:
(1)  
Ground operations and takeoff to 1000 ft above ground level (AGL)

(2)  
On landing below 700 ft AGL
　
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