(3)
Standby Power
(a) The main battery and APU battery are the standby power source. If all generators should become inoperative, the standby system automatically supplies ac and dc power to essential functions.
(4)
Ground Power
(a) The APU generators provide the most convenient power source for ground operation. In addition, external 115/200 volt, 400 Hz, 3 phase ac power may be connected to the airplane through two receptacles on the right-hand side above the nose gear wheel well.
B. AFDS Power Distribution (Fig. 2)
(1)
Each FCC is powered from a separate ac and dc bus. The dc battery bus provides a second source of annunciation and warning power to each FCC.
(2)
FCC AC Power
(a)
Each FCC has internal power supply which receives 115v ac primary power and generates dc voltages required for the electronic circuits.
(b)
The 115v ac is also reduced to 26v ac servo excitation power and supplied to the following transducers: 1) Left and right central lateral control package and lateral
autopilot servo surface position and servo position linear variable differential transducers (LVDT's) 2) Left, center, and right elevator autopilot servo surface position and servo position LVDT's 3) Left, center, and right rollout power control package surface position and servo position LVDT's 4) Speed brake handle position rotary variable differential transducers (RVDT's)
(3) FCC DC Power
(a)
Each FCC receives 28v dc standby power from the battery bus as an independent power source for caution alert and warning monitor circuits.
(b)
28v dc servo power from dc buses are supplied to each FCC to power the aileron, elevator, and rudder arm and engage solenoids.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
P7 OVERHEAD CIRCUIT 28V DC MASTER DIM &
BREAKER PANEL TEST
B7080 AFCS MCP
Autopilot/Flight Director System Power Distribution
Figure 2
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400
MAINTENANCE MANUAL
(4)
AFCS MCP AC Power
(a) The MCP receives 5v ac power for background lighting from the glareshield panel lights dimmer control unit.
(5)
AFCS MCP DC Power
(a)
The AFCS MCP receives two sources of 28v dc power for logic power of the MCP electronics.
(b)
Two sources of dc power is provided to the MCP for master dim and master test of the indicator lights.
(c)
28v dc FCC servo power are also sent through the MCP to provide positive disengagement of the autopilot servos.
C. AFDS Autoland Bus Isolation (Fig. 3)
(1)
During the approach mode, three independent ac and dc power sources are required to accomplish a fail-operational Category III autoland operation.
(2)
The main engine-driven generators No. 1, 2, and 3 are the normal sources for the three independent sources. If one engine-driven generator is inoperative, the main engine-driven generator No. 4 may be substitute for that source.
(3)
All FCCs send the bus isolate request logic to the bus control unit 1 (BCU-1) during the approach arm mode. The BCU-1 uses BTBs on generators 1,2, and 3 to isolate the electrical buses to the FCCs. Isolation occurs within 4 seconds. Bus control unit 2 (BCU-2) monitors the BCU-1 to see that power is isolated for each autopilot channel. The BCU-2 then sends the bus isolated discrete to the FCCs and informs the FCCs that the buses are isolated.
(4)
The BCU-1 monitors the status of the generators, transformer rectifiers (TRs), generator circuit breakers (GCBs), bus tie breakers (BTBs), DC isolation relays (DCIRs), and the split system breaker (SSB).
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