1) All three FCCs communicate with each other via the four ARINC 429 high speed interchannel data buses and the three analog discretes data lines. The buses operate at 100 Kilobits Per Sec (100KBPS). The FCCs share sensor signals on the interchannel digital data buses. This allows signal voting and fault detection through comparison. The three FCCs share the interchannel discrete data lines containing engage and autoland status. This allows each FCC to determine proper annunciations based on the status of all FCCs.
(b)
Digital Buses
1) All cross-channel data is transmitted as non-standard words. The ARINC word is reformated so that data in the most significant bit is right shifted one position. This makes bit 29 a SSM bit. The right shifting makes bit 28 the most significant bit. This causes the full scale input range to double since the maximum input range is .5. Therefore, test equipment for a standard 429 bus cannot be used without manual conversion and interpretation of the data.
2) Sensor inputs, surface commands, input discretes, and status are carried on the buses as follows: a) Aileron, elevator, and rudder surface positions in
Binary Numerical Representation (BNR) format. b) Four discrete words contain past value, autoland,
armed, and active mode status. c) Two general purpose words are BNR spares. d) Two packed input discretes words. One word contains
go-around and slats extended mode status. The second word reserved for strapping pins. e) Two test words are generated by each FCC. They verify the validity of the intercommunication paths. f) Nineteen Inertial Reference Unit (IRU) signals contain vertical and lateral navigation data in BNR format.
g) Eight Air Data Computer (ADC) signals contain altitude, angle of attack, mach, airspeed, and impact pressure in BNR format.
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LEFT FCC RIGHT FCC
Autopilot/Flight Director Interchannel Data Schematic
Figure 1
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MAINTENANCE MANUAL
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757 MAINTENANCE MANUAL
h) Three Instrument Landing System (ILS) signals contain selected runway heading, localizer, and glideslope deviation in BNR format.
i) Three Flight Management Computer (FMC) signals contain vertical and lateral navigation in BNR format.
j) Flap position, stabilizer (stab) position, and radio height data in BNR format.
k) Heading and altitude reference data in BNR format for power-up synchronization.
(c) Discretes
1) Pitch and roll servo engage discretes are generated by center, left, and right FCCs. The FCCs use this as part of the information to establish autopilot disconnect warnings.
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757 MAINTENANCE MANUAL
YAW DAMPER SYSTEM - DESCRIPTION AND OPERATION
_____________________________________________
1. General (Fig. 1)_______
A. An inherent characteristic of all swept back wing airplanes is a yawing motion that induces sideslip and roll. The Yaw Damper System (YDS) uses the following inputs to dampen undesirable sideslip and rolling motions:
(1)
Air Data Computer
(2)
Inertial Reference Unit
(3)
AIRPLANES WITH -122 YDM AND SUBSEQUENT YDM DASH NUMBERS; Modal Suppression Accelerometer
B. The YDS commands drive yaw damper servos which control the rudder mechanical linkage and power control actuators. Rudder displacement commands are proportional to yaw rate. Turn coordination is also provided by the YDS. Roll attitude inputs are used to compute turn coordination commands.
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