(2)  
Engage Interlocks and Logic (Fig. 4)

(a)  
Prior to engaging the yaw damper system, the yaw damper disengage warning light is illuminated. Before the yaw damper engage switch will remain engaged, the monitor must be good from the 115-volt ac power supply and the flight control B switch must be on.

(b)  
The yaw damper engage switch remains in the ON position after holding the switch to ON for at least 0.1 second. This allows voltage through the holding coil and the logic circuit timer to complete a path to ground as provided by the logic OR gate circuit.

(c)  
The yaw damper engage relay is energized by a ground through the logic AND gate after going through the 2-second timer. Two sets of contacts apply power to the interlock circuit and to the rudder power system. An additional set of contacts open to extinguish the warning light.

(d)  
Transferring between 115-volt ac generators causes the monitor to momentarily invalid. However, with a 2-second decay timer in the engage switch circuit, the yaw damper engage relay remains energized.

(3) 
 Engaged Mode

(a)  When the yaw damper coupler is engaged, processed rate gyro signals at summing point 2 produce a valve amplifier output. This output is applied to the series yaw damper actuator transfer valve to command movement. Movement of the yaw damper series actuator results in rudder movement and an output from the position transducer.
591 
22-12-0 Page 4  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Oct 20/83 

EFFECTIVITY
YAW DAMPER COUPLER PART NO. 4030952-902

505  Yaw Damper Block Diagram 
Feb 20/90  Figure 2 (Sheet 1)  22-12-0 
Page 4A/4B 

BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
EFFECTIVITY
YAW DAMPER COUPLER PART NO. 4030952-906

505  Yaw Damper Block Diagram 
Feb 20/90  Figure 2 (Sheet 2)  22-12-0 
Page 5 

BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

H19388
Yaw Damper Pictorial Diagram  591 
22-12-0  Figure 3  Feb 15/80 
Page 6 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

(b)  
Position transducer output is demodulated and applied to summing point 2 to cancel the processed rate gyro signal when the corresponding change in rudder position is appropriate for the rate of yaw attitude change. Position transducer output is also applied to control the integrator. The integrator lag is large enough so that the integrator output, which is applied to summing point 2, does not attenuate yaw damper action in the dutch roll frequency spectrum.

(c)  
Rate gyro signals are processed in the rate gyro filter. The rate gyro filter consists of a demodulator, amplifier, and washout network. Filtering accomplished in these circuits eliminates steady-state signals and unwanted high frequency signals. Consequently, the yaw damper coupler does not oppose normal turn maneuvers and does not respond to airplane vibration and bending. Frequency response is tailored to minimize the airplane dutch roll. The rate gyro filtered output is routed to summing point 2 through Q-potentiometer in the CADC. Rate gyro signal gain is varied by this potentiometer to compensate for changes in airplane dynamics as a function of airspeed.

C.  Yaw Damper Test (Fig. 5)
(1)  The yaw damper indicator, located on the center instrument panel, is a full time operating meter. Whenever the yaw damper system is engaged, the yaw damper indicator reflects the movements of the rudder. Actuation of the yaw damper test switch to either the left or right applies a voltage to the yaw damper coupler rate gyro torquing coil which torques the rate gyro and simulates airplane movement. The coupler provides the properly related voltage to rudder power unit for rudder deflection consistent with direction selected. The yaw damper indicator responds to this action as a result of an output from the position transducer. If the test switch is placed and held in L, left (or R, right) position, the bar on the indicator should deflect left (or right) momentarily and return to center. Upon switch release, the indicator bar should deflect right (or left) momentarily and return to center.
8.  Yaw Damper BITE Operation (Fig. 6)
A.  The BITE system simultaneously checks the overall operation of each electronic unit and its interface with other components. Each series of tests can be completed in spite of individual test failures.
B.  The BITE portion utilizes both analog and digital circuitry to test a high percentage of the computation and logic circuits. Manually initiated automatic go/no go testing checks dynamic circuits for gain and frequency and mode sensors for proper threshold levels. Each test is initiated with a single push button switch and evaluated by one of two colored indicators. These PASS and FAIL indicators provide the test status of each test without requiring any meter movement interpretation.
　
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