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 22-00-01
 ALL  ú ú 03 Page 35 ú Jun 20/89
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A

757
MAINTENANCE MANUAL

 (i)  Interconnecting Linkages with dual (primary and secondary) output rods.
 (2)  
Manual control is accomplished by a cable system from the pilots' rudder pedals to the aft quadrant. Autopilot rudder control is only used for directional guidance during landing rollout. Autopilot control signals from the FCCs operate the autopilot rollout guidance servos, which provide an input to the aft quadrant.

 (3)  
Automatic stability control is by the CSEU yaw damper modules, which provide yaw damping and turn coordination through the yaw damper servos.

 (4)  
Control inputs are conditioned by feel and centering, and by the trim and ratio changer. Feel, centering, and trim devices are on the aft quadrant. The ratio changer servo is controlled by the CSEU rudder ratio changer modules. All inputs are translated to move each PCA control valve. The control valve causes the PCA to extend or retract, which moves the rudder. Each PCA is powered by a separate hydraulic system.

 (5)  
Rudder surface position is indicated on the control surface position indicator on the lower EICAS display unit status page (Ref 31-41-00). The rudder position transmitter on the rudder provides an input to the indicator.

 (6)  
An amber RUDDER RATIO light is on the panel P5. Faults in the rudder ratio changer turn on the light.

 (7)  
A rudder trim control switch is on the aft end of the control stand panel P8. The trim control drives the rudder trim actuator, which recenters the rudder.

 AB. Rudder Control Conditioning Devices
 (1)  
Rudder Feel, Centering, and Trim

 (a)  
The rudder feel, centering, and trim device is beside the aft quadrant in the vertical stabilizer. Feel and centering is accomplished by a cam and spring loaded cam follower. Feel force increases as the cam follower is forced up the cam's slope by rotation of the aft control quadrant. Centering force is a result of the cam detent being forced to align with the cam follower after input forces to the aft quadrant are removed.

 (b)  
Rudder trim is accomplished when the trim actuator rotates the feel cam to establish a new center position. As the feel cam detent shifts, the cam follower moves and provides an input to the PCAs, which then repositions the rudder. The rudder trim switch on the control stand provides electrical inputs to the trim actuator.

 (2)  
Rudder Ratio Changer

 (a)  The rudder ratio changer is above the aft quadrant in the vertical stabilizer. The mechanism changes the output control linkage movement in proportion to aft quadrant and CSEU rudder ratio changer module inputs. Rudder displacement is increased at low airspeeds to maintain rudder authority.
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

 22-00-01
 ALL  ú ú 01 Page 36 ú Mar 20/93
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A

757 MAINTENANCE MANUAL
 (b)  A hydraulic actuator responds to inputs from the CSEU rudder ratio changer modules. The rudder ratio changer modules provide an output in proportion to airspeed. The actuator repositions output lever arms to vary the ratio of control rod input to output. The yaw damper system servos also connect to the rudder ratio changer output rods that drive the rudder PCAs.
 AC. Yaw Damper and Rudder PCAs (Fig. 22)
 (1)  
Yaw Damper Assembly

 (a)  
The yaw damper assembly is above the rudder ratio changer assembly. Two yaw damper servos respond to commands from CSEU yaw damper modules to counteract undesired yaw motion. The yaw damper modules provide an output in response to yaw rate and turn coordination (roll) inputs.

 (b)  
Summing linkages combine the output of the two yaw damper servos with the output of the rudder ratio changer. The final control input to rudder PCAs is with two rods, a primary input rod and a secondary input rod.

 (2)  
Rudder PCAs

 (a)  Three PCAs are above the yaw damper assembly. The PCAs translate control lever inputs to rudder surface movement. Each PCA uses a separate hydraulic system.
　
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