L19617
Yaw Damper System Functional Schematic DiagramSep 01/00 (With Ride Quality) 22-12-01 Figure 2 (Sheet 2) Page 2C/2D BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

L19625
500 Yaw Damper System Functional Schematic DiagramMay 01/00 (With Ride Quality) 22-12-01 Figure 2 (Sheet 3) Page 2E/2FBOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

L19632
Yaw Damper System Functional Schematic DiagramSep 01/00 (Without Ride Quality) 22-12-01 Figure 3 (Sheet 1) Page 2G/2H BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

L19639
Yaw Damper System Functional Schematic DiagramSep 01/00 (Without Ride Quality) 22-12-01 Figure 3 (Sheet 2) Page 2I/2J BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

L19657
500 Yaw Damper Systems Functional Schematic DiagramMay 01/00 (Without Ride Quality) 22-12-01 Figure 3 (Sheet 3) Page 2K/2LBOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

2.  Yaw Damper Coupler
A.  The YDC is installed on the electronic equipment racks in the electrical and electronic
compartment. See figure 1.

B.  The YDC is a dual processor architecture utilizing an active channel (Lane 1) and monitor channel (Lane 2) to drive and monitor the yaw damper actuator and other outputs from the YDC. The dual-lane architecture provides redundant signals for monitoring the yaw damper and rudder pressure reducer systems.
C.  The YDC provides control and monitoring of the yaw damper system. The YDC senses airplane motion in the yaw axis using dual solid state quartz rate sensors located inside of the YDC. The YDC provides control signals to the yaw damper actuator solenoid valve and transfer valve, provides 26 VAC 400 Hz excitation to the actuator position transducer, and receives actuator position signals from the yaw damper actuator position transducer. The YDC automatically disengages the yaw damper engage switch when its monitors detect a malfunction in the yaw damper system. The YDC disables the yaw damper actuator in response to manual or automatic disengagement of the yaw damper engage switch. The YDC illuminates the autopilot disengage warning lamps when the YDC is performing ground maintenance functions. 
D.  The YDC provides control and monitoring of the main rudder PCU pressure reducer (RPR) system. The YDC provides control signals to the RPR solenoid, provides pressure sensor excitation, and receives signals from the pressure sensor to measure A system hydraulic pressure to the main rudder PCU. The YDC interfaces to Radio Altimeter #1 and Radio Altimeter #2, if installed, to determine when the airplane transitions above 1,000 feet and below 700 feet to activate and deactivate the RPR solenoid. The YDC interfaces to the Flight Control System B pressure switch and deactivates the rudder pressure reducer solenoid when B hydraulic system is depressurized. The YDC interfaces to the Flight Control System A Low Pressure Light and provides illumination when A system pressure to the main rudder PCU is below 500 PSI or when full pressure is commanded to the RPR and system pressure is below 2,200 PSI. There is a 5 second delay from when the YDC detects a change in system pressure to when the YDC turns on or off the low pressure light.
500 
May 01/00  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  22-12-01 Page 3 

K06871
Yaw Damper System Flight Deck Interfaces  500 
22-12-01  Figure 4  May 01/00 
Page 4 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

3.  Component Description
A.  Yaw Damper Engage Switch, Solenoid, and Disengage Light
(1)  
The Yaw Damper Engage switch is located on overhead panel P5. It is a solenoid held switch. When the engage switch is placed to the ON position, and if the yaw damper engage interlocks are satisfied, the solenoid holds the switch in the engaged position and the YAW DAMPER light extinguishes. The yaw damper is then in the yaw damping mode.

(2)  
Whenever the yaw damper is disengaged, the amber light labeled YAW DAMPER, on overhead panel P5, illuminates steadily. The MASTER CAUTION lights and the FLT CONT master caution annunciator light, located on the light shield, also illuminate.

(3)  
Failure of the YDC or selected interfacing systems causes the yaw damper to automatically disengage.

B.  Flight Control B Switch
(1)  The flight control B switch is located on the pilot's overhead panel (see figure 2). It is used to active the B system hydraulics. For airplanes LN 213 and beyond and airplanes with SB 737-22-1009, the Flight Control B switch must be in ON position for the yaw damper to remain engaged. The yaw damper actuator will not engage until the yaw damper engage switch is placed to the ON position and B system hydraulics is pressurized. The YDC is designed to remain engaged when the B system hydraulics is depressurized, although the yaw damper actuator will be disabled.
　
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