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Yaw Damper Rudder Position Indicator
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 A.  The Yaw Damper rudder position indicator displays rudder movement induced by Yaw Damper coupler commands.
 B.  Whenever the Yaw Damper system is engaged, the Yaw Damper rudder position indicator reflects the movements of the rudder. The position transducer on the rudder power control unit senses rudder displacement from neutral and provides a signal to the indicator.
Rudder Position Transducers
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 A.  A linear position transducer mounted on the rudder power control unit supplies a rudder position feedback signal to the Yaw Damper coupler. The transducer is a variable reluctance transformer which is excited by 26 volts ac. The magnitude of the output signal varies directly with the length of the input stroke and the phase of the signal changes with the direction of the stroke from the center null position.
Rudder Transfer and Solenoid Valves
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 A.  The engage solenoid and the transfer valve control hydraulic flow for Yaw Damper control of the rudder power control unit.
 B.  The Yaw Damper engage solenoid is mounted on the rudder power control unit. It is a solenoid actuated valve which is energized when the Yaw Damper is engaged; it then completes hydraulic power to the transfer valve.
 C.  The transfer valve, also mounted on the rudder power control unit, converts the Yaw Damper electrical signals into hydraulic flow to move the rudder. The unit consists of a torque motor which moves a jet pipe assembly that regulates the hydraulic flow to the control valve.
Yaw Damper Operation
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 A.  General (Fig. 2)
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 ú 02 ú BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.  Page 4Mar 15/99 

A 737-300/400/500MAINTENANCE MANUAL

Yaw Damper System Block Diagram
Figure 2

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 COUPLER  ú ú 03 Page 5 ú Mar 15/99
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 (1)  
The Yaw Damper may be turned on by placing the Yaw Damper switch to ON. Hydraulic power must be available and the "B" flight control switch ON. Two seconds after the yaw damper is ON the YAW DAMPER light extinguishes. The engagement is monitored by a power monitor circuit in the AFC accessory unit (or IFSAU). If ac power is lost for over 2 seconds, the YAW DAMPER light illuminates and the Yaw Damper switch returns to OFF.

 (2)  
The yaw rate gyro senses oscillations in the yaw axis. The signal is filtered, programmed by the ADC for changes in speed, and amplified. It is then applied to the transfer valve. The transfer valve ports hydraulic pressure to move the Yaw Damper actuator. The Yaw Damper actuator output is summed with rudder pedal input to move the main actuator which controls the rudder, but there is no mechanical feedback to move the rudder pedals.

 (3)  
As the Yaw Damper actuator moves, the LVDT supplies the feedback signal to cancel the signal from the yaw rate gyro. When the oscillation stops, the feedback signal returns the rudder to the original position. The feedback signal is also applied to the position indicator; however, during normal operation, the signals are of insufficient value to be monitored by the position indicator.

 (4)  
AIRPLANES WITH THE IFSAU; The connections between the IFSAU, the Yaw Damper coupler and the TE flap up limit switch supplies these benefits:

 (a)  
The yaw rate gain will increase 29% during the flaps down conditions to decrease the dutch roll step.

 (b)  
The Yaw Damper high frequency gain will reduce 50% to decrease the 8 Hz structural vibration.

 (c)  
These changes supply better airplane performance and passenger comfort.

 

 B.  Operating Modes_______________
 (1)  
General

 (a)  The Yaw Damper system provides damping of movement about the airplane's yaw axis by shaping, amplifying, and coupling rate gyro and Yaw Damper actuator position signals to control the rudder through the hydraulic actuator. Yaw Damper operation is confined to synchronization mode, engaged mode interlocks and logic, and engaged mode operation.
　
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