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MAINTENANCE MANUAL
AIRPLANES WITH -122 YDM AND SUBSEQUENT YDM DASH NUMBERS;
Airplane Type Code
a) The airplane type code is a five wire ground combination with a sixth wire for parity.
b) The parity monitor checks parity and compares type code with the software issue code. This ensures that the correct software is used for the airplane type in which the yaw damper module is installed. If installed correctly, an APL TYPE VALID signal is issued. A detected failure disengages the yaw damper, illuminates the INOP light, provides an EICAS yaw damper message, and the YDM fault message is recorded.
Channel Code
a) The channel code input is a two wire ground/open combination. One pin is grounded for the left installation of the module and the other pin is grounded for right module installation. The monitor checks to ensure only one channel code line is grounded.
b) AIRPLANES WITH YDM DASH NUMBERS -101 THRU -121; The YDM faultball is set if a failure is detected.
c) AIRPLANES WITH -122 YDM AND SUBSEQUENT YDM DASH NUMBERS; A YDM fault message is recorded if a failure is detected.
Hydraulic Pressure Switch
a) The hydraulic pressure switch is a two wire input. One wire is normally grounded at a time, indicating the hydraulic condition (high or low hydraulic pressure state). The failure mode is set if the hydraulic pressure HIGH signal agrees with the hydraulic pressure LOW signal for two seconds.
b) AIRPLANES WITH YDM DASH NUMBERS -101 THRU -121; This sets the HYD PRESS faultball. The faultball is reset by the maintenance RESET switch.
c) AIRPLANES WITH -122 YDM AND SUBSEQUENT YDM DASH NUMBERS; HYD SWITCH fault is recorded.
d) AIRPLANES WITH YDM DASH NUMBERS -101 THRU -121; The comparison monitor provides a VALID HI HYDRAULIC PRESS discrete signal and a HYD PRESS reset when high pressure is restored for 6 seconds after a low pressure indication.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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757 MAINTENANCE MANUAL
Air/Ground Relays
a) Air/ground information is received on three status lines from the air/ground relays. The status lines indicate: right gear aft tilt switch level (airplane on ground - through master A/G relay), right gear aft tilt switch level (airplane on ground - direct to YDM), and right gear forward tilt switch level (airplane on ground - direct to YDM).
b) A triplex majority vote signal selector/monitor is used to determine A/G status. The monitor detects when one of three previously valid inputs is different from the selected value for more than 12 seconds. When the two remaining valid inputs disagree for more than 12 seconds following a first failure, a second failure is detected.
ADC
a) The ADC parameter values for impact pressure, indicated angle of attack, and true airspeed are received on the ARINC 429 bus. Input signal processing includes active monitoring, parity monitoring, sign status matrix monitoring, and value substitution. Excessive signal failure rates or lack of activity result in an ADC input fault.
b) A median value for each parameter is selected for use in the control law. Incoming parameter values are compared with the median value and are fault registered if the comparison is beyond the maximum limits.
IRU
a) The IRU parameter values for roll rate, roll attitude, yaw rate, lateral acceleration, and ground speed are received on the ARINC 429 bus. Input signal processing includes: activity monitoring, parity monitoring, sign status matrix monitoring, and value substitution. Excessive failure rates or lack of activity results in an IRU input fault.
b) A median value for each parameter is selected for use in the control law. Incoming parameter values are compared with the median value and are registered as faults if the comparison is beyond the maximum limits.
Power Supply Modules (PSMs)
a) The CSEU PSMs (No. 1 and 2 left) each furnish power up reset and PSM valid signals. All four signals must be true to initiate a power-up reset of the yaw damper module.
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