The yaw damper (YD) system provides control signals to drive the rudder servo for dutch roll damping and turn coordination. The YD system has one linear actuator installed in the vertical stabilizer that moves the rudder. The YD system operates independently of the Automatic Flight Control System (AFCS). YD movement does not move the pilot pedals.

The left Flight Guidance Computer (FGC) supplies its calculated data for a fail-safe yaw damper and provides the drive signal for the YD linear actuator. The data concentrator unit (DCU) is also connected to the YD linear actuator to monitor its position and for independent shutdown.

Controls for the YD are on the flight guidance panel (FGP). YD annunciations are displayed on the primary flight displays (PFDs).

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The yaw damper system has one linear actuator. The actuator is in the vertical stabilizer. The linear actuator contains the following parts:

• A brushless DC motor
• An electrical limit switch
• A servo brake
• Two position sensors - linear voltage differential transducers (LVDT), and
• A precision gear train

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The yaw damper system has a linear actuator that moves the rudder. The yaw damper receives inputs from FGC No. 1.

When the YD is engaged, the signal from the flight guidance panel (FGP) YD switch releases the servo brake.

The main processor of the FGC calculates the position of the yaw damper and sends it to a servo loop in the FGC. The main processor gives a yaw damper command calculated on rates, accelerations, and attitudes. This command goes through the airspeed-controlled magnitude limiter before the servo loop. When the airspeed is not correct or not available, maximum airspeed values are set in the FGC to make sure that safe commands are applied to the servo loop.

The data concentrator unit (DCU), channel A, monitors the feedback signal (YD POSITION B OUT) for the yaw damper position. If the yaw damper position is more than the permitted position (for the airspeed), the DCU uses the servo linear actuator (SVL) relay to remove power from YD linear actuator.

There are monitors in the input/output (I/O) processor software. The FGC I/O processor compares the feedback of the servo linear actuator position sensor 1 (SVLPS 1) against the servo linear actuator position sensor 2 (SVLPS 2). If the difference between the values of SVLPS1 and SVLPS2 is not less than 0.03 inches, the yaw damper will disengage.

A large error signal or an internal position at the mechanical stops causes the processor to supply a cutout signal. The actuator command then slowly moves the yaw damper to its center position.

The FGC uses the following external sensors for the yaw damper:

• The air data system (ADS) for air data information.
• The inertial reference units (IRUs) for the yaw rate and roll attitude, and pitch attitude.

The FGC receives data from the ADCs through the I/O concentrator (IOC).

The IRS data goes via the IOC to the FGC where the yaw is calculated and monitored.

If the autopilot is engaged, the YD is automatically engaged (if not engaged before). If the yaw damper disengages, the autopilot will automatically disengage. The pilot can override the yaw damper command with the rudder pedals.

FGC No. 2 receives YD engage feedback for YD status and to report a fault condition. FGC No.2 also receives a RUDDER POSITION IN signal from the DCU for the reasons that follow:

• As a provision for the YD, and/or
• For the internal use of the autopilot (AP) control law

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The YD engage pushbutton on the FGP supplies the rudder YD engage signal to the yaw damper linear actuator and the FGC. If the FGC finds no unusual attitudes or rates and the AP/YD DISC bar is in the up position, the yaw damper can be engaged.

The primary flight display (PFD) shows a green YD annunciation when the yaw damper is engaged. The PFD shows a yellow YD annunciation when the yaw damper disengages.

If the FGC senses a yaw damper failure condition, the FGC automatically disengages the yaw damper. The PFD shows a yellow YD annunciation. The YD annunciation flashes for 5 seconds and then becomes stable.

If the FGC becomes defective, the DCU supplies a discrete signal to the yaw damper to automatically disengage. The PFD shows a yellow YD annunciation when the yaw damper disengages it. The YD annunciation flashes for 5 sec and then is removed.

The YAW DAMPER FAIL caution message will show when the YD becomes defective.

The YAW DAMPER OFF status message will show when the YD is not engaged.

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The yaw damper system has interfaces with the systems that follow:

• Automatic Flight-Control System (AFCS)
• Engine Indication and Crew Alerting System

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Component Location Index
IDENT	DESCRIPTION	LOCATION	IPC REF
A127	YAW DAMPER LINEAR-ACTUATOR	PANEL(S) 341CR,ZONE(S) 340	22-22-01

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