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747-400 MAINTENANCE MANUAL
 (f)  Autopilot/Flight Director System Interchannel Data (AMM 22-15-00/001) covers the general description and operation of cross-channel data buses. Digital data is transmitted between the three FCCs.
 C. Yaw Damper System (AMM 22-21-00/001) (Fig. 3)
 (1)  
The Yaw Damper System (YDS) provides commands to the rudder that will dampen oscillations due to dutch roll and gust induced sideslip. The system also provides turn coordination and suppress turbulence induced structural body bending modes.

 (2)  
The YDS consists of a control panel with system engage/annunciator switches, two yaw damper actuators (YD ACTs) which are part of the Rudder Power Control Modules, and linear variable differential transducers (LVDTs), two Rudder Power Control Modules (RPCMs), four Modal Suppression Accelerometers, and two Yaw Damper Modules (YDMs).

 (3)  
Each YDM receives system engage-disengage commands from its associated control panel switch. Each YDM also receives data from two Air Data Computers (ADCs), two modal suppression accelerometers, and three Inertial Reference Units (IRUs). The YDMs send status information to its control panel switch and all three EFIS/EICAS Interface Units (EIUs), maintenance information to the two Central Maintenance Computers (CMCs), and electrical control commands to its associated YD ACT. The YD ACT translates the electrical commands to mechancial outputs which are combined with other RPCM mechanical inputs to drive the associated rudder.

 (4)  
This section covers the general description of the Yaw Damper System. This section also covers the component details and operation of the Passenger Oxygen and Yaw Damper Panel, Yaw Damper Modules, YD ACT electrohydraulic servovalve and solenoid valves, Rudder Power Control Modules, and Modal Suppression Accelerometers.

 D. Automatic Stabilizer Trim System (AMM 22-22-00/001) (Fig. 4)
 (1)  
The Automatic Stabilizer Trim (AST) System receives commands from the FCCs to trim the stabilizer. The stabilizer is trimmed to reduce elevator loads. The automatic trim mode is only active when the autopilot is engaged.

 (2)  
This section covers the general description and operation of the AST System.

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Yaw Damper System
Figure 3

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747-400
MAINTENANCE MANUAL

 E. Mach/Speed Trim System (AMM 22-24-00/001) (Fig. 5)
 (1)  
The Mach/Speed Trim System is a software mode of the Stabilizer Trim/Rudder Ratio Modules (SRMs). The SRMs receive Air Data Computer Airspeed/Mach data to provide trimming commands to the stabilizer. This improves the airplanes Mach speed stability as Mach number or computed airspeed changes. The mode is active when electric, manual, and automatic trimming commands are not present.

 (2)  
This section covers the general description and operation of the Mach/Speed Trim system.

LEFT
STABILIZER
TRIM CONTROL
MODULE

RIGHT
STABILIZER
TRIM CONTROL
MODULE

Automatic Stabilizer Trim System
Figure 4

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