(2)  
YDC fault history has storage capacity for 256 fault messages. A maximum of 64 faults can be stored during a single power cycle. Fault history will be stored for approximately 32,000 power cycles, provided the total number of fault messages does not exceed 256.

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Sep 01/00  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  22-12-01 Page 9 

F.  Ground Test
(1)
 Self Test

(a)  
The purpose of the self test is to run a test of the YDC hardware, primarily to detect latent faults and to confirm faults found by continuous monitor and power-up tests.

(b)  
Since there is the possibility of rudder movement in the presence of a failure, a warning is given. The yaw damper has to be engaged for this test to run, even though it is not necessary to have hydraulic power supplied.

(c)  
If the test fails, the YDC displays a list of the faults that caused the failure. 

(d)  
If any existing faults are present that would prevent the test from completing, they are displayed as inhibit conditions before starting the test. The operator has the option of removing the YDC at that point or trying to fix the problem if it is external to the YDC.

(2)  
Servo Test

(a)  
The purpose of the servo test is to verify correct operation of the yaw damper actuator, verify that the actuator LVDT is rigged within acceptable limits, and to determine the actuator travel limits for programming of the YDC servo loop monitor.

(b)  
First, a test is run to ensure that the yaw damper actuator will not move when the actuator solenoid is de-energized. The yaw damper actuator is then driven to its maximum travel in both directions. During this sweep the actuator servo command response monitor is set to the lowest possible authority to avoid trips if the actuator is lower in authority compared with the YDC previously stored actuator authority. At this point, the following can cause test failure and exit: command response monitor trip, other internal YDC monitor trips, actuator limits or rigging not correct, or actuator authority of 2 degrees/0.15 inches in an airplane configuration where ride quality control laws will command up to 3 degrees/0.225 inches. If none of these are applicable then another sweep is performed with the correct authority limits applied, thus testing the command response monitoring over its entire range.

(c)  
If the test fails at any point the YDC displays a list of the faults that caused the failure. These are displayed in a similar manner to the Existing Faults option. The only difference is that there is no option to reset latches.

(d)  
If any existing faults are present that would prevent the test from completing, they are displayed as inhibit conditions before starting the test. The operator has the option of removing the YDC at that point or trying to fix the problem if it is external to the YDC.

(3)  
Pressure Reducer Test

(a)  
The pressure reducer test is a functional test of the rudder pressure reducer system, including the pressure reducer solenoid, the pressure sensor and the A System Low Pressure warning lamp. The pressure reducer solenoid is taken through a no-reduction, reduction and no-reduction cycle, and the resultant pressure is checked at each stage. The Low Pressure warning is taken through an on-off cycle. Tests of internal YDC circuitry and monitors are also performed.

(b)  
If the test fails at any point, the YDC displays a list of the faults that caused the failure. These are displayed in a similar manner to the Existing Faults option. The only difference is that there is no option to reset latches.

(c)  
If any existing faults are present that would prevent the test from completing, they are displayed as inhibit conditions before starting the test. The operator has the option of removing the YDC at that point or trying to fix the problem if it is external to the YDC.

(4)  
Program Pin Test

(a)  
This test confirms that the two YDC program pins (D295, pins 17 and 53) are correct. The settings are stored to NVM at the end of the test. If the YDC is moved to a different type airplane, or the program pins fail to a different setting, then this test has to be run before that yaw damper will engage, since the YDC detects a difference between the program pins and the values stored in NVM.

(b)  
If the operator does not confirm the apparent settings of the program pins, the test fails and the YDC displays the fault.

(c)  
If any existing faults are present that would prevent the test from completing, they are displayed as inhibit conditions before starting the test. The operator has the option of removing the YDC at that point or trying to fix the problem if it is external to the YDC.
　
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