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A

757
MAINTENANCE MANUAL

 (2)  
Inputs to the RRCA consist of hydraulic pressure from the left hydraulic system and electrical signals to the solenoid valve and EHSV. Outputs from the RRCA are hydraulic pressure to the middle power control actuator (PCA) and mechanical drive to the left ratio changer crank.

 N.
Yaw Damper Summing Lever (Fig. 8)

________________________
 (1)  
The summing levers are located directly above the ratio changer mechanism. The levers sum control inputs from the ratio changer together with yaw damper servo inputs and outputs to the PCA control rods.

 (2)  
The torque tube has four connecting lugs for the primary and secondary control path summing levers and two crank arms for the yaw damper servo input linkage and the yaw damper bias pogo.

 (3)  
The yaw damper pogo is parallel with the yaw damper servo inputs. It provides a ground point for the control linkage in case of yaw damper servo input linkage failure and is slightly biased to remove linkage backlash. Pogo breakout force is 22.5 to 32.5 pounds (100 to 144 newtons). Rig pin R8, at the lower end of the pogo, fixes the amount of bias.

 O.
Yaw Damper Servos

_________________
 (1)  
Two servos are mounted side-by-side on the vertical fin aft spar. Their purpose is to provide rudder input commands for dutch roll damping and gust load relief. Left and center hydraulic systems power the servos (Ref 22-21-00).

 P.
Power Control Actuators (PCAs) (Fig. 9)

______________________________
 (1)  
The three rudder PCAs are interchangeable. Each PCA is controlled by primary control path inputs and the middle and lower PCAs receive secondary control path inputs. The upper PCA is powered by the right hydraulic system, the middle PCA by the left system (through the RRCA) and the lower PCA is powered by the center system.

 (2)  
The PCAs are mounted parallel in a vertical row above the yaw damper summing lever, between the vertical fin rear spar and rudder front spar. The PCA cylinder base is mounted on a trunnion block connected to the vertical fin rear spar; the rod end is mounted to the rudder front spar.

 (3)  
On each PCA, there are two relief valves that operate at 3500 psi and reset at 3200 psi. Two anti-cavitation check valves and one inlet check valve have 8-psi cracking pressures in the free flow direction. A check valve in the return line (from each side of the piston) prevents cavitation. A compensator with a volume of 4 cubic inches and a 110-psi cracking pressure acts as an accumulator and a relief valve. It keeps pressurized fluid in the PCA when it is without power. This compensator also supplies leakage make-up fluid. A 250 micron filter, located in the pressure line, removes unwanted contamination from the hydraulic fluid before it enters the PCA.

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A

757
MAINTENANCE MANUAL

Not Used
Figure 7

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A

757
MAINTENANCE MANUAL

324BL 
SEE A  OUTPUTS TO PCAs 
A  INPUT CRANK  SUMMING LINKAGE  SEE  B 

SHEAR
PIN INPUTS FROMYAW DAMPER RATIO CHANGERSERVOS (2)
YAW DAMPER POGO
SEE C
FWD
A
YFMM-27-21-0059
A
Yaw Damper Summing Lever Figure 8 (Sheet 1)
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