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757 MAINTENANCE MANUAL
 7)  AIRPLANES WITH -122 AND SUBSEQUENT YDM DASH NUMBERS; The common mode sum voltage from the LVDT is also monitored. If it is less than 3 vrms for more then 2 seconds, an LVDT fault message will be shown.
INOP Indication
 1) The yaw damper control panel indicates INOP with the following conditions present: a) During the power-up test b) During the preflight test c) With disengagement of a yaw damper channel (yaw damper servo solenoid valve not actuated)
 2)  When yaw damper INOP conditions are valid, a caution annunciation (L or R YAW DAMPER) appears on the EICAS display unit (Ref 31-41-00).
 3) Disengagement of a yaw damper channel occurs under the following conditions: a) Manual disengagement from the control panel ON switch b) Auto-disengagement resulting from: any YDM fault, power-up test failure, preflight test actuator failure, preflight test LVDT failure, no IRU data valid, or a servo fault during normal operation. c) The interlock between modules is broken by one yaw damper module not being installed.
 NOTE: One YDM must be installed to convert the
____ opposite YDM open circuit to a ground for the INOP lamp.
GUN 102-999;
 Each yaw damper also sends a discrete signal (yaw damper
 engage) to the digital flight data requisition unit (DFDAU).
Maintenance Reset
 1)  AIRPLANES WITH YDM DASH NUMBERS -101 THRU -121; the yaw damper module front panel contains a RESET switch. Pressing the switch initiates the power-up test if the airplane is on the ground, and resets all faultballs.
 2)  AIRPLANES WITH -122 YDM AND SUBSEQUENT YDM DASH NUMBERS; The yaw damper module front panel contains a RESET switch. Pressing the switch initiates the power-up test if the airplane is on the ground, and pushing the RESET button resets all the yaw damper module monitors and control logic.
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757
MAINTENANCE MANUAL

 (j) Yaw Damper Module Auto-reset.
 1)  The YDM fault latch is responsive to transient fault recovery of YDM fault monitors. YDM fault monitors capable of transient fault recovery are as follows: a) Airplane type code b) Yaw damper command coincidence c) Activity monitor d) Monitor or control watchdog monitor fault
 2)  The YDM fault latch resets if all YDM monitors exhibit no failures for 10 seconds continuously and within 15 seconds of detecting a YDM fault. Autoreset is limited to a maximum of four resets per flight. There is no limit on autorest when the airplane is on ground.
 3)  The following faults are incapable to automatic recovery: a) Power-up reset YDM fault b) Preflight test YDM fault c) Background Read Only Memory (ROM) fault
 4)  AIRPLANES WITH YDM DASH NUMBERS -101 THRU -121; YDM faultball resets at application of channel engagement (i.e., solenoid engage goes true) when YDM faultball set conditions are not present.
 (4) Yaw Damper Control Law Schematic (Fig. 9)
 (a) Yaw Damping Command
 1)  Lateral acceleration from the IRUs is filtered and corrected for the effect of yaw rate and IRU displacement from the center of gravity. This value is gain scheduled by true airspeed to obtain the sideslip angle rate. The yaw rate required for roll coordination is computed from roll attitude and true airspeed. The actual yaw rate is subtracted from this value to give the total yaw rate due to roll that must be damped. Roll rate is multiplied by a body angle of attack factor to form the third damping command factor. The above factors are summed and gain scheduled by impact pressure to generate the yaw damping command (CMD).
 2)  Sideslip angle rate is synthesised by summing together yaw rate, lateral acceleration scheduled by true airspeed, roll attitude scheduled by true airspeed, and roll rate multiplied by body angle of attack. The lateral acceleration signal from the IRU's, is filtered and corrected for the effect of yaw rate and IRU displacement from center of gravity. Sideslip angle rate is gain scheduled by impact pressure to generate the yaw damping command.
 (b) Turn Coordination Command
 1)  The turn coordination command is derived from the roll attitude gain scheduled by true airspeed. This value is summed with the yaw damping CMD to form the output command.
　
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