TO YAW INTERNALS
DAMPER
SAME ASUPPER PCA
ENGAGEPRESSLOWER
SOLENOID DIFFARM
RUDDER
SERVOPCA EH SERVO
SOLENOID
SNSR  CMDVALVE LEFT YAW DAMPER SERVO ACTUATOR
PRESSERVO
REG
TO
DETENTFILTERRUDDER
ENGAGESCREEN CONTROL
SERVO VALVEINTERNALS
INTERNALSSAME ASHYD ARMRELIEF VALVE
SAME AS
RIGHTRIGHT
ROLL &

ROLL &ROLL &
AUTOPILOTAUTOPILOT
YAW

YAWYAW26V AC AUTOLANDAUTOLAND
ROLLOUTROLLOUT
AUTOLAND

GUIDANCEGUIDANCE26V AC
SIGNALSSIGNALS
SIGNALS - L SERVOSERVORETURN
ACTRACTRR & C R & C SERVO POSN
ACT RES LVDT ACTUATOR
26V AC
26V AC
RETURN

RETURN
PRESSURE
LEFT AUTOPILOT CENTER AUTOPILOT

RUDDER CONTROL
CRANK POSN
ROLLOUT GUIDANCEROLLOUT GUIDANCESURFACE POSN LVDT SYSTEM INPUT
SERVO ACTUATOR SERVO ACTUATOR RIGHT AUTOPILOT ROLLOUT GUIDANCE SERVO ACTUATOR
Rudder and Rudder Trim Control System
Figure 12 (Sheet 3)

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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757
MAINTENANCE MANUAL

 (5)  Trim Control Switch
___________________
 (a)  
The rudder trim switch in the flight deck provides input to the trim actuator. Rotating the trim knob arms the trim motor circuit and selects either the extend mode for left rudder trim or the retract mode for right trim. The switch is powered by 28 volts ac from the left bus.

 (b)  
Movement of the trim switch through the 5° position removes the system ground. Movement through the 15° position arms the trim motor circuit. Rotation through the 25° position closes the control switch and selects the extend (left trim) or retract (right trim) operation. The switch has a mechanical stop at the 30° position.

 (6)  
Trim Actuator

_____________
 (a)  
The trim actuator is driven by a self-contained electric motor. The motor drive is controlled by the rudder trim switch or by activation of an internal limit switch. An electromagnetic brake prevents actuator overrun and internal mechanical stops prevent overtravel in case of a limit switch malfunction.

 (7)  
Trim Position Indicator

_______________________
 (a)  
The trim actuator has a self-contained position transmitter driven by a gear train. The transmitter sends a signal to the indicator which is summed with a feedback signal from the indicator's rebalance potentiometer. When the potentiometer shaft is in the proper position, the feedback signal cancels the transmitter signal and the indicator displays the present trim position. When changing the trim position, the transmitter signals are not cancelled by the potentiometer signal, resulting in a difference signal. This difference signal causes a dc amplifier to drive the indicator dc motor, pointer-driven roller and rebalance potentiometer in the correct direction to cancel out the transmitter signal and display the new trim position.

 (8)  
Yaw Damper Summing Lever (Fig. 13)

________________________
 (a)  The summing lever combines the displacement inputs from the manual control system and the yaw damper system. These inputs are series summed and the resultant displacement is the input to the rudder PCAs. The bias pogo provides a ground point for the summing lever in case of a failure in the yaw damper servos and output linkage.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A

757
MAINTENANCE MANUAL

OUTPUT TO PCA
INPUT CRANK
A
YAW DAMPER INPUT LINK
A  INPUT CRANK  AXIS "B"  TORQUE TUBE DUAL PATH 
INPUT FROM RATIO CHANGER YAW DAMPER BIAS POGO CENTERING POGO INPUT AND BIAS  AXIS "A"  PRIMARY PRIMARY INPUT OUTPUT 
UP  COMPENSATING INPUT CRANK THERMAL  SECONDARY OUTPUT 
　
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