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The magnetic compass works on the principle of
magnetism. The glider pilot must have a basic understanding
of the principles of operation of the magnetic
compass. A simple bar magnet has two centers of magnetism,
which are called poles. Lines of magnetic force
flow out from each pole in all directions, eventually
bending around and returning to the other pole. The area
through which these lines of force flow is called the
field of the magnet. For the purpose of this discussion,
the poles are designated "north" and "south." If two bar
magnets are placed near each other, the north pole of
one will attract the south pole of the other. There is evidence
that there is a magnetic field surrounding the
Earth, and this theory is applied in the design of the
magnetic compass. It acts very much as though there
were a huge bar magnet running along the axis of the
Earth, which ends several hundred miles below the surface.
[Figure 4-15]
The lines of force have a vertical component (or pull),
which is zero at the Equator, but builds to 100 percent of
the total force at the poles. If magnetic needles, such as
the glider magnetic compass bars, are held along these
lines of force, the vertical component causes one end of
the needle to dip or deflect downward. The amount of dip
increases as the needles are moved closer and closer to the
poles. It is this deflection or dip that causes some of the
larger compass errors.
MAGNETIC VARIATION
Although the magnetic field of the Earth lies roughly
north and south, the Earth’s magnetic poles do not
coincide with its geographic poles, which are used in the
construction of aeronautical charts. Consequently, at
most places on the Earth’s surface, the direction-sensitive
steel needles, which seek the Earth’s magnetic field,
will not point to True North but to Magnetic North.
Furthermore, local magnetic fields from mineral
deposits and other conditions may distort the Earth’s
magnetic field and cause an additional error in the position
of the
compass’ north-seeking magnetized needles with reference
to True North. The angular difference between True
North and the direction indicated by the magnetic compass—
excluding deviation error—is variation. Variation
is different for different points on the Earth’s surface and
is shown on the aeronautical charts as broken lines connecting
points of equal variation. These lines are isogonic
lines. The line where the magnetic variation is zero is an
agonic line. [Figure 4-16]
MAGNETIC DEVIATION
A compass is very rarely influenced solely by the
Earth’s magnetic lines of force. Magnetic disturbances
from magnetic fields produced by metals and electrical
accessories in a glider disturb the compass needles and
produce an additional error known as deviation.
Figure 4-16. Earth’s magnetic field.
4-13
If a glider changes heading, the compass’ directionsensitive
magnetized needles will continue to point in
about the same direction while the glider turns with
relation to it. As the glider turns, metallic and electrical
equipment in the glider change their position relative
to the steel needles; hence, their influence on the compass
needle changes and deviation changes. The deviation
depends, in part, on the heading of the glider.
Although compensating magnets on the compass are
adjusted to reduce this deviation on most headings, it
is impossible to eliminate this error entirely on all
headings. A deviation card is installed in the cockpit in
view of the pilot, enabling the pilot to maintain the
desired magnetic headings. [Figure 4-17]
COMPASS ERRORS
Since the compass card is suspended in fluid, the magnetic
compass is sensitive to in-flight turbulence. In
light turbulence, you may be able to use the compass
by averaging the readings. For example, if the compass
swings between 40º and 70º, you can estimate the
approximate magnetic heading of 55º. In severe turbulence,
however, the magnetic compass may be so
disturbed that is unusable for navigation.
Since the magnetic compass is the only direction-seeking
instrument in most gliders, the pilot must be able to turn
the glider to a magnetic compass heading and maintain
this heading. It will help to remember the following characteristics
of the magnetic compass, which are caused by
magnetic dip. These characteristics are only applicable in
the Northern Hemisphere. In the Southern Hemisphere
the opposite is true.
ACCELERATION ERROR
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Glider Flying Handbook(40)
