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Section 3C— Compasses
3.4. Basics. A compass may be defined as an instrument that indicates direction over the earth's surface
with reference to a known datum. Various types of compasses have been developed, each of which is
distinguished by the particular datum used as the reference from which direction is measured. Two basic
types of compasses are in current use—the magnetic and gyrocompass.
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3.4.1. The magnetic compass uses the lines of force of the earth's magnetic field as a primary reference.
Even though the earth's field is usually distorted by the pressure of other local magnetic fields, it is the
most widely used directional reference.
3.4.2. The gyrocompass uses as its datum an arbitrary fixed point in space determined by the initial
alignment of the gyroscope axis. Compasses of this type are widely used today and may eventually
replace the magnetic compass entirely.
3.5. Magnetic Compass. The magnetic compass indicates direction in the horizontal plane with
reference to the horizontal component of the earth's magnetic field. This field is made up of the earth's
field in combination with other magnetic fields in the vicinity of the compass. These secondary fields are
caused by the presence of ferromagnetic objects, etc. Magnetic compasses may be divided into two
classes: (1) the direct-indicating magnetic compass in which the measurement of direction is made by a
direct observation of the position of a pivoted magnetic needle; and (2) the remote-indicating gyrostabilized
magnetic compass. Magnetic direction is sensed by an element located at positions where local
magnetic fields are at a minimum, such as the vertical stabilizer and wing tips. The direction is then
transmitted electrically to repeater indicators on the instrument panels.
Figure 3.2. Magnetic Compass.
3.5.1. Direct-Indicating Magnetic Compass. Basically, the magnetic compass is a magnetized rod
pivoted at its middle, with several features incorporated to improve its performance. One type of directindicating
magnetic compass, the B-16 compass (often called the whiskey compass), is illustrated in
Figure 3.2. It is used as a standby compass in case of failure of the electrical system that operates the
remote compasses. It is a reliable compass and will give good navigational results if used carefully.
3.5.2. Magnetic Variation and Compass Errors. The earth's magnetic poles are joined by irregular
curves called magnetic meridians. The angle between the magnetic meridian and the geographic
meridian is called the magnetic variation. Variation is listed on the charts as east or west. When variation
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is east, magnetic north (MN) is east of true north (TN). Similarly, when variation is west, MN is west of
TN (Figure 3.3). Lines connecting points having the same magnetic variation are called isogonic lines, as
shown in Figure 3.4. Compensate for magnetic variation to convert a compass direction to true direction.
Figure 3.3. Variation Is Angle Between True North and Magnetic North.
Figure 3.4. Isogonic Lines Show Same Magnetic Variation.
3.5.2.1. Compass error is caused by nearby magnetic influences, such as magnetic material in the
structure of the aircraft and its electrical systems. These magnetic forces deflect a compass needle from
its normal alignment. The amount of such deflection is called deviation which, like variation, is labeled
east or west as the north-seeking end of the compass is deflected east or west of MN, respectively.
88 AFPAM11-216 1 MARCH 2001
3.5.2.2. The correction for variation and deviation is usually expressed as a plus or minus value and is
computed as a correction to true heading (TH). If variation or deviation is east, the sign of the correction
is minus; if west, the sign is plus. A rule of thumb for this correction is easily remembered as east is least
and west is best.
3.5.2.3. Aircraft headings are expressed as true headings or magnetic headings. If the heading is
measured in relation to geographical north, it is a TH. If the heading is in reference to MN, it is a
magnetic heading (MH); if it is in reference to the compass lubber line, it is a compass heading (CH).
CH corrected for variation and deviation is TH. MH corrected for variation is TH.
3.5.2.4. This relationship is best expressed by reference to the navigator's log, where the various
headings and corrections are listed as TH, variation (var), MH, deviation (dev), and CH. Thus, if an
aircraft is flying in an area where the variation is 10o E and the compass has a deviation of 3o E, the
relationship would be expressed as follows, assuming a CH of 125o (Figure 3.5):
TH var MH dev CH
138 - 10 = 128 - 3 = 125
Figure 3.5. To Find True Heading, Work Backwards.
　
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