(2)
For the removal/installation procedure for the source select modules, refer to AMM 31-61-07/401.
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747-400 MAINTENANCE MANUAL
HEADING REFERENCE SWITCH - REMOVAL/INSTALLATION
_______________________________________________
TASK 34-21-05-004-001
1. Heading Reference Switch Removal/Installation_____________________________________________
A. General
(1)
The heading reference switch is part of the EICAS control panel/FMC switch module. The EICAS control panel/FMC switch module is installed on the pilots' center instrument panel P2-4.
(2)
The removal/installation procedure for the EICAS control panel/FMC switch module is in AMM 31-61-04/401.
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2.
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747-400 MAINTENANCE MANUAL
STANDBY COMPASS - DESCRIPTION AND OPERATION
___________________________________________General
_______
A. The standby compass is used by the pilots as a quick reference and as an auxiliary compass. The standby compass is a self-contained unit and is installed forward of the forward overhead circuit breaker panel (P7).
Standby Compass (Fig. 1)
_______________
A. The standby compass is a highly sensitive instrument that is installed in a liquid filled case. The liquid serves to dampen rapid movement and oscillations of the compass.
B. The standby compass uses a circular card having two parallel and horizontal magnets attached to indicate magnetic direction. The card is free to rotate and tilt within the liquid filled case. The magnetic heading of the airplane is read from the card assembly against the fixed lubber line.
C. The standby compass front panel has N-S and E-W compensator adjusters for alignment of the compass card
D. The card assembly is illuminated by a bulb located at the top of the
compass. A lamp access cover permits easy replacement of the light bulb.Operation
_________
A. Functional Description
(1)
The magnets attached to the compass card align themselves with the earth's magnetic field. This rotates the compass card which is calibrated to indicate the airplanes magnetic heading.
(2)
The compensators adjust the local magnetic field to compensate for deviations produced by airplane components and currents in the airplane wiring. When small errors cannot be removed by the compensators, they are noted on a compass correction card mounted near the compass. The correction card lists the heading for the pilots to steer to maintain a desired heading.
B. Control
(1)
The only control for the standby compass is the circuit breakers for panel lighting of the indicator.
(2)
Close the following circuit breakers:
(a) P6 Main Power Distribution Panel
1) 6E21 FLT DK LIGHTS PANEL GLARE SHIELD
2) 6L1 FLT DECK LIGHT STBY INST
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