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AFPAM11-216 1 MARCH 2001 89
3.5.3. Variation. Variation has been measured throughout the world and the values found have been
plotted on charts. Isogonic lines are printed on most charts used in aerial navigation so that, if the
aircraft's approximate position is known, the amount of variation can be determined by visual
interpolation between the printed lines. At high altitudes, these values can be considered quite realistic.
Conversely, at low altitudes, these magnetic values are less reliable because of local anomalies.
Variation changes slowly over a period of years and the yearly amount of such change is printed on most
charts. Variation is also subject to small diurnal (daily) changes which may generally be neglected in air
navigation.
3.5.4. Deviation. Because deviation depends upon the distribution of magnetic forces in the aircraft
itself, it must be obtained individually for each magnetic compass on each aircraft. The process of
determining deviation, known as compass swinging, should be discussed in the technical order for each
compass.
3.5.4.1. Deviation changes with heading (Figure 3.6). If the net result of all magnetic forces of the
aircraft (those forces excluding the earth's field) is represented by a dot located just behind the wings of
the aircraft. If the aircraft is headed toward MN, the dot attracts one pole of the magnetic compass (in
this case, the South Pole) but, on this heading, does not change its direction. The only effect is to amplify
the directive force of the earth's field. If the aircraft heads toward magnetic east, the dot is now west of
the compass, and attracts the South Pole of the compass, causing easterly deviation. The illustration in
Figure 3.6 also shows that the deviation is zero on a south heading, and westerly when the aircraft is
heading west.
Figure 3.6. Deviation Changes With Heading.
3.5.4.2. Deviation can be reduced (but not eliminated) in some direct-indicating magnetic compasses by
adjusting the small compensating magnets in the compass case. Remaining deviation is referred to as
residual deviation and can be determined by comparison with true values. This residual deviation is
90 AFPAM11-216 1 MARCH 2001
recorded on a compass correction card showing actual deviation on various headings or the compass
headings as illustrated in Figure 3.7. From the compass correction card illustrated in Figure 3.7, the
navigator knows that to fly a magnetic heading (MH) of 270o, the pilot must steer a compass heading
(CH) of 268o.
Figure 3.7. Compass Correction Card.
3.5.5. Errors in Flight. Unfortunately, deviation is not the only error of a magnetic compass. Additional
errors are introduced by the motion of the aircraft itself. These errors have minimal effect on the use of
magnetic compasses and come into play normally during turns or changes in speed. They are mentioned
only to make you aware of the limitations of the basic compass. Although a basic magnetic compass has
some shortcomings, it is simple and reliable. The compass is very useful to both the pilot and navigator
and is carried on all aircraft as an auxiliary compass. Because compass systems are dependent upon the
electrical system of the aircraft, a loss of power means a loss of the compass system. For this reason, an
occasional check on the standby compass provides a excellent backup to the main systems.
3.5.6. Remote-Indicating Gyro-Stabilized Magnetic Compass System. A chief disadvantage of the
simple magnetic compass is its susceptibility to deviation. In remote-indicating gyro-stabilized compass
systems, this difficulty is overcome by locating the compass direction-sensing device outside magnetic
fields created by electrical circuits in the aircraft. This is done by installing the direction-sensing device
in a remote part of the aircraft, such as the outer extremity of a wing or vertical stabilizer. Indicators of
the compass system can then be located throughout the aircraft without regard to magnetic disturbances.
3.5.6.1. Several kinds of compass systems are used in Air Force aircraft. All include the following five
basic components: (1) remote compass transmitter, (2) directional gyro (DG), (3) amplifier, (4) heading
indicators, and (5) slaving control. Though the names of these components vary among systems, the
principle of operation is identical for each. Thus the N-1 compass system shown in Figure 3.8 can be
considered typical of all such systems.
AFPAM11-216 1 MARCH 2001 91
Figure 3.8. N-1 Compass System Components.
3.5.6.2. The N-1 compass system is designed for airborne use at all latitudes. It can be used either as a
magnetic-slaved compass or as a DG. In addition, the N-1 generates an electric signal which is used as
an azimuth reference by the autopilot, the radar system, the navigation and bombing computers, and
　
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