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COMPASS SYSTEM - MAINTENANCE PRACTICES
1. General
A. This procedure describes the manner in which the airplane compass systems are "swung." There are two main systems, the captain's and first officer's, plus a standby compass. A method of compensation is also included for airplanes fitted with remote compensator units.
NOTE: Disregard flux valve compensator adjustments on airplanes without remote magnetic compensator.
B. The procedure is written so that the swing for the standby compass system is described separately from that for the main systems. However, all systems may be swung simultaneously, if so desired.
C. The process of "ground swinging" an airplane compass consists of accurately aligning the airplane with at least four cardinal headings in turn, and observing the deviation between known heading and compass heading, in each case. The results are then tabulated, and after all necessary adjustments have been made to reduce the deviations to specified limits, another circuit is made for the purpose of compiling the compass correction card. This card remains with the compass.
D. Deviation errors fall into two categories, "index error" and "single cycle error," each of which is summarized below.
(1)
An index error exists when the direction error remains constant both in direction and value at each check point.
(2)
A single cycle error exists when the direction error is equal, but opposite in sign, at opposite check points on the compass.
E. The airplane's initial compass swing will normally require at least two complete circuits around the compass rose; once to take out the index error and the single cycle error, and once to make the compass correction chart. Passenger cabin seats and cockpit seats shall be installed prior to swinging.
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2. Swing Main Compass System
A. General
(1)
The tests described under Compass System - Adjustment/Test, should be carried out prior to this procedure, to ensure that a serviceable compass system is being swung.
(a)
Replacement of an RMI or CDI will not necessitate performing a compass swing if the unit has been properly tested at the component level.
(b)
Replacement of a flux valve does not require a compass swing on the compass rose. To verify that flux valve is operating properly, turn airplane 360 degrees and observe RMI’s. Check that one system closely follows the other.
(2)
Index errors may be eliminated or reduced by adjusting the differential synchro in the remote magnetic compensator (if installed).
(3)
Single cycle errors may be eliminated or reduced by adjusting the E-W and N-S potentiometers on the compensator (if installed).
B. Equipment and Materials
(1)
Compass rose having the following features.
(a)
A flat, obstruction free area, large enough to permit towing the airplane in a circle around a surveyors transit, and still leave 25 feet between its outer wing tip and the edge of the area (Fig. 201). The direction of the horizontal component of the earth’s magnetic field in the area should be constant within 0.2 degree throughout the test area.
(b)
A baseplate located in the center of the area to identify the position for the transit.
(c)
Control over an area extending 180 feet in all directions from the rose center, which may be used to exclude any type of traffic during a compass swing.
(2)
Two heading markers, whose magnetic bearing from the baseplate has been established
(3)
Tow tractor
(4)
Surveyors transit or equivalent (Accurate to 1 minute or less)
(5)
Compass Fixture Set, F80180-1 (Fig. 201)
(6)
Nonmagnetic tools for adjusting standby compass
(7)
One compass correction card (AN5823) for each compass
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H31442
567 Compass Swing Target Fixture
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
(8) 3-Phase, 115-volt, 400-cps external power supply (5 KVA capacity)
C. Prepare for Compass Swing
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