A compensator assembly mounted on the top or bottom of the
compass allows an aviation maintenance technician (AMT)
to create a magnetic field inside the compass housing that
cancels the influence of local outside magnetic fields. This is
done to correct for deviation error. The compensator assembly
has two shafts whose ends have screwdriver slots accessible
from the front of the compass. Each shaft rotates one or two
small compensating magnets. The end of one shaft is marked
E-W, and its magnets affect the compass when the aircraft is
pointed east or west. The other shaft is marked N-S and its
magnets affect the compass when the aircraft is pointed north
or south.

Magnetic Compass Induced Errors
The magnetic compass is the simplest instrument in the
panel, but it is subject to a number of errors that must be
considered.

Variation
The Earth rotates about its geographic axis; maps and charts
are drawn using meridians of longitude that pass through tile
geographic poles. Directions measured from the geographic
poles are called true directions. The north magnetic pole to
which the magnetic compass points is not collocated with
the geographic North Pole, but is some 1,300 miles away;
directions measured from the magnetic poles are called
magnetic directions. in aerial navigation, the difference
between true and magnetic directions is called variation. This
same angular difference in surveying and land navigation is
called declination.

Figure 3-17 shows the isogonic lines that identify the number
of degrees of variation in their area. The line that passes near
Chicago is called the agonic line. Anywhere along this line
the two poles are aligned, and theme is no variation. East of
this line, the magnetic pole is to the west of the geographic
pole and a correction must be applied to a compass indication
to get a true direction.

Figures 3-17, Isogonic lines are lines of equal variation.

Flying in the Washington, D.C. area, for example, the variation
is 10° west. If the pilot wants to fly a true course of south (180°),
the variation must: be added to this resulting in a magnetic course
to fly of 190°. Flying in the Los Angeles, CA area, the variation
is 14° east. To fly a true course of 180° there, the pilot would
have to subtract the variation and fly a magnetic course of 166°.
The variation error does not change with the heading of the
aircraft; it is the same anywhere along the isogonic line.

Deviation
The magnets in a compass align with any magnetic field.
Local magnetic fields in an aircraft caused by electrical current
flowing in the structure, in nearby wiring or any magnetized
part of the structure, conflict with the Earth's magnetic field
and cause a compass error called deviation.

Deviation, unlike variation, is different on each heading, but it is
not affected by the geographic location. Variation error cannot
be reduced or changed, but deviation error can be minimized
when a pilot or AMT performs the maintenance task known
as "swinging the compass."

Most airports have a compass rose, which is a series of lines
marked out on a taxiway or ramp at some location where there
is no magnetic interference. Lines, oriented to magnetic north,
are painted every 30°, as shown in Figure 3-18.

Figure 3-18. Utilization of a Compass Rose Aids Compensation
fin' Deviation Errors.

The pilot or AMT aligns the aircraft on each magnetic
heading and adjusts the compensating magnets to minimize
the difference between the compass indication and the actual
magnetic heading of the aircraft. Any error that cannot be
removed is recorded on a compass correction card, like the one
in Figure 3-19, and placed in a cardholder near the compass.
If the pilot wants to fly a magnetic heading of 120° and the
aircraft is operating with the radios on, the pilot should fly a
compass heading of 123°.