(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 to ground as provided by the time delay gate S1.

 (c)  
The Yaw Damper engage relay is energized by a ground through time delay gate switch S2 after a 2-second delay. Two sets of contacts apply power to the interlock circuit and to the rudder power control unit actuator solenoid. A third set of contacts open to extinguish the warning light.

 (d)  
Transferring between 115-volt ac generators causes the monitor to momentarily be invalid. With a 2-second delay timer in the engage switch circuit, the Yaw Damper engage switch remains engaged. The engage relay is de-energized until 2 seconds after the monitor is valid. This delay allows the Yaw Damper coupler to synchronize with the new reference voltage. The Yaw Damper light will be on during this time.

 (4)  
Engaged Mode Operation (Fig. 3)

 (a)  
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 1 through a step gain in the ADC. Rate gyro signal gain is varied by this step gain to compensate for changes in airplane dynamics as a function of airspeed.

 (b)  
When the Yaw Damper coupler is engaged, processed rate gyro signals at summing point 1 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.

 (c)  
Position transducer output is demodulated and applied to summing point 1 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 thru switch S7 and summing point 2. The integrator lag is large enough so that the integrator output, which is applied to summing point 1, does not attenuate Yaw Damper action in the dutch roll frequency spectrum.

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 ú 06 ú BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.  Page 9Mar 15/99 

A 737-300/400/500MAINTENANCE MANUAL
 9.  Yaw Damper BITE Operation (Fig. 3)
_________________________
 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.
 C.  BITE Test Preparations
 (1)  
The built-in test equipment (BITE) provides a self-test of the Yaw Damper coupler and its interface with other equipment. It is accomplished using switches and pushbuttons on the front panel of the coupler. A pass or fail light will illuminate to indicate test results. The entire series of tests can be performed, even if one test fails.

 (2)  
Preliminary flight compartment setup involves closing the Yaw Damper circuit breakers on the P18 panel, turning on hydraulic power and engaging the Yaw Damper. Assure that flight control surfaces are clear before performing Yaw Damper BITE.

 (3)  
In order to perform the tests, the BITE OFF/ARM switch must be placed in the ARM position. This provides voltages to the test circuits and a ground through the AFC accessory unit (or IFSAU) to illuminate the A/P disengage warning lights.

 (4)  
Before actually performing BITE, a check of the BITE system must be accomplished as follows:
　
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