(c)  
The yaw damper engage switch hold-in solenoid is energized by a ground through the YDC. A set of relay contacts in the autopilot accessory unit open to extinguish the yaw damper warning light when 28 volts is applied to the relay from the yaw damper engage switch contacts.

(d)  
A 2-second delay timer in the YDC allows the yaw damper engage switch solenoid to remain energized during transfers between 115 volt ac generators.

(3) 
 Engaged Mode

(a)  When the yaw damper coupler is engaged, its output is applied to the series yaw damper actuator transfer valve to command rudder movement. Movement of the yaw damper series actuator results in rudder movement and an output from the position transducer.
C.  Yaw Damper Test
(1)  
Following application of 115 VAC power to the YDC, after it was off for more than 2.5 seconds, a series of self tests and system tests are performed to verify that the YDC hardware, software, and disconnect mechanisms are operating. The power up self tests are completed within 12 seconds and verify correct operation of the YDC hardware, software, and configuration of the airplane program pins. Following the YDC self test, when the yaw damper engage switch is placed to the on position, the YDC performs a test of the engage switch solenoid to verify that it can be disconnected electrically. Following the engage solenoid test, and upon application of B system hydraulic pressure, the YDC performs a test of the yaw damper actuator to verify that the actuator will not move with the actuator solenoid de-energized. After these tests are complete, the actuator solenoid is energized and the yaw damper system is fully operational. If the power up self test or the engage switch tests fail, the yaw damper engage switch will not hold in the ON position. Failure of the actuator solenoid test will cause the yaw damper to disconnect when B system hydraulics is pressurized. If any of these tests fail, it is necessary to correct the system malfunction and to perform YDC BITE to reset the YDC monitors before the yaw damper system can be engaged.

(2)  
Application of 115VAC power to the YDC after it was off for less than 2.5 seconds, causes the YDC to verify the correct configuration of the airplane program pins. After this test is complete, the actuator solenoid is energized and the yaw damper system is fully operational.

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22-12-01 Page 8  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  May 01/00 

5.  Yaw Damper BITE Operation
A.  Yaw damper system ground maintenance BITE is accessed through the YDC BITE control panel. The BITE display consists of two eight character lines of data and is shown on figure 5. The BITE mode is engaged by pressing the ON/OFF button followed by the YES button within 10 seconds. BITE entry will not be allowed if the YES push button is not selected with 10 seconds after ON/OFF push button is selected.
B.  Entry into BITE is enabled when the autopilot warning lights are illuminated. BITE entry will be prevented if the current to the ground path from the 28 VDC autopilot warning light is less than 20 mA.
C.  Once engaged, BITE can be disengaged at any time by pressing the ON/OFF button followed by the YES button. BITE will disengage automatically if no keys are pressed within a 5 minute interval.
D.  Upon entry into BITE, the first option in the menu is displayed (Existing Faults). Four different modes of operation can be selected; Existing Faults, Fault History, Ground Tests, and Other Functions.
(1)  
When the EXISTING FAULTS option is selected, the YDC will display a list of all existing fault messages on the BITE Control Panel. As the operator scrolls through the list, the option of seeing more details is offered for each fault. At the end of the list is the option to reset all latches.

(2)  
If the operator selects more details for any particular fault, a further list of details shows the message number, flight deck effect (if any), the likely LRU or LRUs at fault, and whether the fault is latched or not. A further option is then available to view the fault number or numbers that gave rise to the fault message. Note that there may be more than one fault causing any particular fault message, but in this case only the single fault message is displayed.

(3)  
If the operator chooses to reset latches, all YDC monitor latches are reset. After a short delay the software assesses whether any faults are still present or have reoccurred. If there are any faults then the list of fault messages is displayed.

E.  Fault History
(1)  
When the FAULT HISTORY option is selected the YDC scans the records of continuous monitor and power-up test faults in the YDC non-volatile memory (NVM). A list is displayed of those power cycles during which one or more faults were recorded. A new power cycle occurs whenever 115 VAC power is removed for more than 2.5 seconds. The power cycles are numbered in descending order relative to the current cycle. The operator has the option of displaying the faults for the currently displayed power cycle. If selected, the faults are displayed in a similar manner to the Existing Faults option. The only differences are firstly that in place of the latched indication the operator is told whether the fault was a hard or intermittent fault, and if intermittent, how many times it repeated during that power cycle. Secondly, there is no option to reset latches (fault history can only be cleared in the shop)
　
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