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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




An aircraft magnetic compass, such as the one in Figure 7-31,
has two small magnets attached to a metal float sealed inside a
bowl of clear compass fluid similar to kerosene. A graduated
scale, called a card, is wrapped around the float and viewed
through a glass window with a lubber line across it. The card
is marked with letters representing the cardinal directions,
north, east, south, and west, and a number for each 30°
between these letters. The final "0" is omitted from these
directions. For example, 3 = 30°, 6 = 60°, and 33 = 330°.
There are long and short graduation marks between the letters
and numbers, each long mark representing 10° and each short
mark representing 5°.

A magnetic compass.
Figure 7-31. A magnetic compass. The vertical line is called the
lubber line.

The float and card assembly has a hardened steel pivot in its
center that rides inside a special, spring-loaded, hard glass
jewel cup. The buoyancy of the .oat takes most of the weight
off the pivot, and the fluid damps the oscillation of the .oat
and card. This jewel-and-pivot type mounting allows the float
freedom to rotate and tilt up to approximately 18° angle of
bank. At steeper bank angles, the compass indications are
erratic and unpredictable.

The compass housing is entirely full of compass fluid To
prevent damage or leakage when the fluid expands and
contracts with temperature changes, the rear of the compass
case is sealed with a flexible diaphragm, or with a metal
bellows in some compasses.

The magnets align with the Earth's magnetic field and the
pilot reads the direction on the scale opposite the lubber line.
Note that in Figure 7-31, the pilot sees the compass card from
its backside. When the pilot is flying north as the compass
shows, east is to the pilot's right. On the card, "33", which
represents 330° (west of north), is to the right of north. The
reason for this apparent backward graduation is that the card
remains stationary, and the compass housing and the pilot turn
around it, always viewing the card from its backside.

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

The Earth rotates about its geographic axis; maps and charts
are drawn using meridians of longitude that pass through the
geographic poles. Directions measured from the geographic
poles are called true directions. The magnetic North 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 7-32 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 there is no variation. East
of this line, the magnetic North Pole is to the west of the
geographic North Pole and a correction must be applied to a
compass indication to get a true direction.

Isogonic lines
Figure 7-32. Isogonic lines are lines of equal variation.