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MAINTENANCE MANUAL
3) The yaw damper provides damping via the rudder to eliminate dutch roll.
(c) Turn Coordination
1) When the airplane is rolled into a turn, a yawing moment is generated by changing lift vectors due to the changes in relative wind. This moment produces a forward component (reduced drag) on the down-going wing and a rearward component (increased drag) on the up-going wing. The result of the yaw moments produce yaw opposite to the intended turn direction. Rudder deflections to counter roll induced yaw are required to achieve a coordinated turn.
(2) Yaw Damper Operational Modes (Fig. 7)
(a)
The yaw damper modules provide self-tests, full flight regime yaw damping control, and turn coordination. To accomplish this, each module has eight opertional modes described in the following paragraphs.
(b)
Power-Up
1) The power-up mode is entered when power is supplied to the yaw damper modules (power applied to bus). The CSEU power supply modules must be operative and the bus circuit breakers must be closed. The mode can be entered when on the ground or in the air. If the airplane is on the ground, the system proceeds to the power-up test. If power-up is entered in the air, the system proceeds to the flight mode via a 5 second easy-on function. Upon power-up, the yaw damper modules initialize all interfaces, latches, command, and state variables to a safe state. The YDS monitors are reset; however, the fault ball indicators are latched and do not reset during power-up.
(c)
Power-Up Test
1) The power-up test is performed when the YDS is powered up on the ground. The test lasts approximately 20 seconds. The yaw damper INOP amber light is turned on and an EICAS message is displayed during the power-up test. If a fault is detected, the INOP light remains on. The following tests are conducted during the power-up test: a) The channel fault disengagement test is performed for
each channel (control and monitor). It monitors disengage relay status and reports a fault if the relay does not disengage.
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Yaw Damper Operational Modes
Figure 7
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b) AIRPLANES WITH YDM DASH NUMBERS -101 THRU -121; this sets the YDM faultball.
c) AIRPLANES WITH -122 YDM AND SUBSEQUENT YDM DASH NUMBERS; This causes a YDM fault message to be recorded.
d) The servo loop monitor test verifies the ability of the monitor to detect loop faults that cause auto-disengagement.
e) The command coincidence monitor test (for both channels) verifies YDM fault signal latching and automatic disengagement. The YDM fault signal is removed following successful completion of the test.
f) The test resets the YDM fault latch following the successful completion of the preceding tests. It also verifies that the yaw damper is not automatically disengaged and that no fault reset signals are active.
g) The test performs a Random Access Memory (RAM) read/write test.
2) AIRPLANES WITH YDM DASH NUMBERS -101 THRU -121; During the power-up test there is no rudder actuator movement. If a test failure is detected, the system enters the FAULT/INOP mode (faultball sets and the INOP light illuminates). If the test is successful, the system proceeds to the ground operational mode.
3) AIRPLANES WITH -122 YDM AND SUBSEQUENT YDM DASH NUMBERS; During the power-up test there is no rudder actuator movement. If a test failure is detected, the system enters the FAULT/INOP mode (a fault message is recorded and the INOP light illuminates). If the test is successful, the system proceeds to the ground operational mode. If the test is successful, the system proceeds to the ground operational mode.
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