(a)  
Two Air Data Computers (ADCs)

 (b)  
Three Inertial Reference Units (IRUs)

 (c)  
Two Modal Suppression Accelerometers

 (3)  
The YDMs monitor system operation and indicate system faults through automatic and manually initiated system testing. Yaw damper fault indications consist of the following:

 (a)  
Upper and lower yaw damper INOP lights

 (b)  
EICAS display unit annunciation

 (c)  
Control Display Unit (CDU) annunciation

 E.  Rudder Power Control Modules
 (1)  The rudder power control modules (RPCM) use electrical command inputs from the yaw damper modules. The electrical input commands control hydraulic flow to yaw damper actuator pistons, which provide a mechanical output. This output is connected in series with manual and autopilot rudder commands supplied to rudder power control actuators, which actually move the rudder.
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747-400
MAINTENANCE MANUAL

 (2)  The maximum rudder authority for each rudder power control module is |4 degrees. The yaw damper command is zero while the airplane is on ground. The yaw damper command is provided to the rudder power control module via an easy-on/easy-off function to reduce transients during ground/air transitions. If a system fault is detected, the RPCMs are disengaged.
 F.  Modal Suppression Accelerometers
 (1)  The four modal suppression accelerometers provide the yaw damper modules with lateral acceleration data. The accelerometers provide inputs that suppress lateral flexing and body bending. There are two modal suppression accelerometers associated with each YDM.
 G.  Yaw Damper Component Locations and Details (Fig. 1)
 (1)  
Flight Deck

 (a)  The captains and first officers overhead panel (P5) contains the yaw damper controls and indicators. Each channel (upper and lower) has a separate yaw damper ON switch/light and INOP indicator. Panel (P9) contains the left and right Control Display Unit (CDU). This unit displays all fault messages associated with the yaw damper system.

 (2)  
Passenger Oxygen and Yaw Damper Panel Assembly

 (a)  
The ON lamps use 0-5 volts ac. The INOP indicator lamps use 28 volts dc provided by the master dim and test system (Ref 33-12-00/001, Master Dim and Test). Case cooling is forced ambient cooling air. Three electrical connectors are on the backside of the panel.

 (b)  
The ON switch/light controls engagement of the YDS by controlling 28 volts dc to the solenoid valve on the rudder power control modules. When the switch/light is first pressed, the ON legend is turned on. Pressing the switch/light a second time turns off the YDS.

 (c)  
The INOP indicator illuminates under the following conditions:

 NOTE: Yaw damper INOP is delayed for 15 seconds if it is
____ activated by either YDM faults or after removal of hydraulic pressure (for annunciating servo loop faults only).
 1)  When the yaw damper module associated with the INOP is not installed (the opposite module must be installed and functioning properly).
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747-400 MAINTENANCE MANUAL
YAW DAMPER
UPPER LOWER
PASSENGER OXYGEN AND YAW DAMPER CONTROL PANEL (P5) ON ON
ww SEE A
INOP INOP
aa
YAW DAMPER ENGAGE SWITCHES
PASSENGER OXYGEN AND YAW DAMPER CONTROL PANEL
A
RIGHT CDU (P9)
SEE BLEFT CDU (P9)
SEE B
UPPER YAW DAMPER MODULE (E1-2)
a  a SEEC
LINE
SELECT
KEYS

LOWER YAW DAMPER MODULE (E2-2)
INIT DEP
BRT
REF

RTE
VNAV  SEE C
ARR EXEC
FUNCTION FIX LEGS HOLD PROG
AND MODE
NAVMENU RAD ABCDE
KEYS
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