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Pilot's Handbook of Aeronautical Knowledge
Flight Instruments
Compass Systems

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Flying in the Washington, D.C., area, for example, the variation
is 10° west. If a pilot wants to .y a true course of south (180°),
the variation must be added to this, resulting in a magnetic course
of 190° to fly. Flying in the Los Angeles, California, 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.

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 an 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 ramp or maintenance runup area where there
is no magnetic interference. Lines, oriented to magnetic north,
are painted every 30°, as shown in Figure 7-33.

compass rose
Figure 7-33. Utilization of a compass rose aids compensation for
deviation errors.

The 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 7-34, and placed in a cardholder near the compass. The
pilot can taxi the aircraft to the compass rose and maneuver
the aircraft to the headings prescribed by the AMT, and if
authorized to do so, the AMT can also taxi and maneuver the
aircraft; however, only the AMT can adjust the compass or
complete the compass correction card. 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°.

compass correction card
Figure 7-34. A compass correction card shows the deviation
correction for any heading.

The corrections for variation and deviation must be applied
in the correct sequence and is shown below, starting from
the true course desired.

Step 1: Determine the Magnetic Course
True Course (180°) ± Variation (+10°) = Magnetic Course (190°)
The magnetic course (190°) is steered if there is no deviation
error to be applied. The compass card must now be considered
for the compass course of 190°.
Step 2: Determine the Compass Course
Magnetic Course (190°, from step 1) ± Deviation (–2°, from
correction card) = Compass Course (188°)
NOTE: Intermediate magnetic courses between those listed
on the compass card need to be interpreted. Therefore, to
steer a true course of 180°, the pilot would follow a compass
course of 188°.
To find the true course that is being flown when the compass
course is known:
Compass Course ± Deviation = Magnetic Course ± Variation=
True Course

Dip Errors
The lines of magnetic flux are considered to leave the Earth
at the magnetic North Pole and enter at the magnetic South
Pole. At both locations the lines are perpendicular to the
Earth's surface. At the magnetic equator, which is halfway
between the poles, the lines are parallel with the surface. The
magnets in a compass align with this field and near the poles
they dip, or tilt, the float and card. This dip causes two very
noticeable errors: northerly turning error and acceleration error.