757 MAINTENANCE MANUAL
(b)
If the FCC airspeed is invalid, the IAS/MACH window displays 200 knots. The display is blanked when the VNAV mode becomes active and when the speed control switch in the knob is pressed while speed (or Mach) is displayed. If the switch is pushed when either A/T SPD, FLCH, or F/D takeoff mode is active, the display does not change.
(c)
The airspeed and Mach number reference values are stored in the MCP and are transmitted in digital form to the TMC, the FCCs, and the mach/airspeed indicators to drive the airspeed reference cursors.
B. Flight Control Computer
(1)
The FCC is an 8 MCU component with a maximum weight of 28.1 lbs. It is cooled by blow-through air and has a single shell electrical connector per ARINC 600. Two removable side covers permit access to all circuit cards and modules.
(2)
Autopilot outputs of the FCC include arm, engage, and disengage logic to the local A/P servos; system engagement and autoland status annunciations; and aural and visual warnings of system disconnect.
(3)
The FCC generates steering signals for the F/D command bars. The command bars are displayed on the EADI when the FCC is in the F/D mode. The FCC to Electronic Flight Instrument System (EFIS) symbol generator selection is accomplished externally by the instrument source select switch.
(4)
The FCC provides automatic stabilizer trim commands to the
Stabilizer Trim/Elevator Asymmetry Limit Module (SAM).
C. Autopilot Disengage and Go-Around Switches
(1)
Autopilot Disengage Switches
(a)
The A/P disengage switches are single action, multiple pole, pushbutton switches.
(b)
Pressing either of the A/P disengage switches disconnects any engaged FCC. The autopilot disengage switch must be pressed a second time to reset the autopilot disengage warning.
(2)
Go-Around (GA) Switches
(a) Each palm actuated GA switch contains a set of contacts for each FCC. The switch provides discrete logic used in the GA mode.
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A
757
MAINTENANCE MANUAL
3. Operation_________
A. Functional Description
(1) Autopilot/Flight Director System Power
(a)
General
1) Power for the Autopilot/Flight Director System is provided by the engine or APU driven generators via the left and right AC buses. The left and right transformer rectifier units (TRUs) provide DC power to the left and right DC buses respectively. During normal operation, airplane cruise or takeoff, the center AC and DC buses are powered by the left buses. During autoland operation, the center AC and DC buses are powered by the static inverter and hot battery bus. Center bus switching is provided by the FCCs internal isolation request and system relay logic. The switching provides each FCC with an independent power source during autoland operation.
(b)
Autopilot/Flight Director System Power Distribution (Fig. 4) 1) AC Power
a) AIRPLANES WITHOUT CONTROL WHEEL STEERING; the 115v ac supplied to the FCCs is used by the FCC internal power supply. The FCCs output 26v ac to operate neutral shift transducers and Linear Variable Differential Transducers (LVDT)s in the flight control servos (ALCSs, APCSs, & ARGSs). The MCP receives 5v ac power for background lighting from the GLARESHIELD panel lights.
b) AIRPLANES WITH CONTROL WHEEL STEERING; the 115v ac supplied to the FCCs is used by the FCC internal power supply. The FCCs output 26v ac to operate neutral shift and control force transducers and Linear Variable Differential Transducers (LVDT)s in the flight control servos (ALCSs, APCSs, & ARGSs). The MCP receives 5v ac power for background lighting from the GLARESHIELD panel lights.
2) DC Power
a) Each FCC receives 28v dc from the standby bus thru the AUTOFLIGHT WARN circuit breaker for caution and warning monitor circuits. The FCCs receive 28v dc servo power for servo arming and engaging. The MCP receives 28v dc thru the MODE CONT PNL circuit breakers for power. A variable 12 or 28v dc is provided to the MCP from the master dim & test for annunciator illumination.
(c) Autopilot/Flight Director System AC Power 1) FCC AC Power
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