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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400 MAINTENANCE MANUAL
c) Each computing channel of a yaw damper module performs self monitoring. The channel self monitoring consists of a background ROM check, activity monitor, and a watchdog monitor. The watchdog monitor checks for total Central Processing Unit (CPU) loss, operating speed, and erroneous ROM sum check values. If the watchdog monitor detects a failure, the YDM fault is set.
(d)
Authority Limiting
1) The modal suppression command in each yaw damper is limited to | 1 degree of rudder, whereas the total output command of the control law from each yaw damper module may never exceed | 4 degrees rudder. The authority is further limited as the number of valid ADC and IRU inputs decreases.
(e)
Easy-On/Easy-Off
1) The control law output command is routed through a ramp gain schedule stage. The gain is zero when on the ground and one when in-air. With an air/ground relay indication that the airplane has taken off, the gain linearly rises from zero to one in 5 seconds. With an air/ground relay indication that the airplane has landed, the gain linearly decreases from one to zero in 5 seconds. If an in-air power-up occurs, the fade-in begins immediately following the power up.
(f) Command Output 1) The control microprocessor output is converted to analog, smoothed, and routed to the rudder power control module for rudder control. The output command operates the yaw damper electrohydraulic servo valve (EHSV). The EHSV directs hydraulic pressure flow to the yaw damper actuator piston in proportion to the magnitude and sign of the command signal.
2) The EHSV receives hydraulic pressure when the engage solenoid is actuated by 28 volts dc from the control panel ON switch (via the engage relay in the yaw damper module). The engage relay is closed unless an auto-disengage signal is generated, or there is a loss of operating power to the yaw damper module.
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747-400 MAINTENANCE MANUAL
3) A Linear Variable Differential Transducer (LVDT) is physically attached to the actuator piston. The LVDT uses 400-Hz, 26-volt ac power for excitation. The LVDT output voltage reflects the position of the yaw damper actuator piston. This data is routed back to the yaw damper module to complete the servo loop.
4) The LVDT feedback signal and the command output signal are compared in the yaw damper module. A fault message is sent to the CMC if the LVDT signal does not track the command signal.
5) The common mode sum voltage from the LVDT is also monitored. If it is less than 3 vrms for more than 2 seconds, a LVDT fault message is sent to the CMC.
(g) INOP Indication 1) The passenger oxygen and yaw damper control panel indicates yaw damper INOP with the following conditions present: a) During the power-up test b) When yaw damper is selected off or is invalid c) With disengagement of a yaw damper channel (yaw damper
servo solenoid valve not actuated) d) 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.
2) 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, ground test actuator failure, no IRU data valid, or a servo loop monitor fault during normal operation.
(h) Yaw Damper Module Autoreset
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
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