limited by their design because they are allowed to pivot freely in liquid.
As you can imagine, any aircraft movement—be it a climb, bank,
descent, or jolt due to turbulence—can upset the fluid and ball.
True north is
the North Pole. Most maps (including aviation maps) are produced in relation
to true north. Magnetic north, however,
is the northern pole of the earth’s magnetic, not geographic, poles.
Its position is currently in Canada in the Hudson Bay region. Its position
also changes slightly every year due to magnetic field changes. The difference
between the true and magnetic poles is called variation.
Depending on your position on the earth’s surface, you must
add or subtract a set number of degrees of variation to convert your map
positioning, based on true north, to your flight positioning, which is
based on magnetic north.
A compass error
called deviation is caused by different
metals; electrical fields produced by other aircraft instruments and accessories;
and disturbances generated by deposits or metals or power plants.
Most compasses are placed above the instrument panel in the windshield
or screen to reduce the effects of deviation, but this phenomenon can
never be entirely eliminated. The airplane manufacturer or a certified
mechanic will place a compass correction card in the airplane underneath
the compass to note how many degrees of deviation must be considered for
deviation a major consideration in flight planning? Often,
yes. Sometimes the differences caused by deviation may only be one or
two degrees on a particular heading, but sometimes they may be as much
as six or seven degrees. A quick look at the compass correction card will
tell you how much change must be made and prevent you from incorrectly
determining headings for navigation.
Another error is called magnetic
dip. Magnetic dip is caused by the downward pull of the magnetic
poles and is greatest near the poles themselves. A weight is often placed
on the compass of the aircraft on the equatorial side to help negate this
effect. Compass navigation near the polar regions, however, is nearly
impossible due to the errors caused by this effect.
As an airplane
accelerates or decelerates on an easterly or westerly heading, acceleration
error is created. When an airplane accelerates easterly or westerly, the
weight on the compass in the magnet causes it to indicate a turn towards
the north. When an airplane decelerates on an easterly or westerly heading,
the opposite occurs, and the compass will indicate an incorrect turn toward
the south. A simple memory aid to remember this error is the acronym ANDS,
‘Accelerate North and Decelerate South.’
The final compass
error we will discuss is turning error.
Turning error is caused when turning on a heading of east or west to the
north or south. In a turn toward the south, the compass heading ‘leads’
the actual heading of the aircraft. In a turn toward the north, the compass
heading ‘lags’ behind the aircraft’s actual heading.
When using the aircraft compass as the primary directional control for
the aircraft, it is necessary to consider the lead or lag error and predict
it. The memory aid used to remember this error’s effect is called
UNOS, ‘Undershoot North and Overshoot South.’
the magnetic compass has so many errors, then why do we use it?
The magnetic compass is used in conjunction with other instruments in
straight and level, unaccelerated flight. It is used as the information
reference to the heading indicator, which in modern aviation is the primary
directional indicator on board the aircraft. During your flight training,
you will learn how to use the magnetic compass correctly, and you will
be given the opportunity to fly with it as one of your only instruments
as well so that you can learn to use it in case of other instrument failure.