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Instrument Flying Handbook
Flight Instruments
The Basic Aviation Magnetic Compass

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Instrument Flying


Table of Contents

Chapter 1. Human Factors
Chapter 2. Aerodynamic Factors
Chapter 3. Flight Instruments
Chapter 4. Section I
Airplane Attitude Instrument
Using Analog Instrumentation
Chapter 4. Section II
Airplane Attitude Instrument
Using an Electronic Flight

Chapter 5. Section I
Airplane Basic
Flight Maneuvers
Using Analog Instrumentation
Chapter 5. Section II
Airplane Basic
Flight Maneuvers
Using an Electronic Flight

Chapter 6. Helicopter
Attitude Instrument Flying

Chapter 7. Navigation Systems
Chapter 8. The National
Airspace System

Chapter 9. The Air Traffic
Control System

Chapter 10. IFR Flight
Chapter 11. Emergency

Color Codes for an Airspeed Indicator.
Figure 3-15. Color Codes for an Airspeed Indicator.

The Basic Aviation Magnetic Compass

One of the oldest and simplest instruments for indicating
direction is the magnetic compass. it is also one of the basic
instruments required by 14 CFR part 91 for both VFR and
IFR flight.

Magnetic Compass Overview
A magnet is a piece of material, usually a metal containing
iron, which attracts and holds lines of magnetic flux.
Regardless of size, every magnet has two poles: a north
pole and a south pole. When one magnet is placed in the
field of another, the unlike poles attract each other and like
poles repel.

An aircraft magnetic compass, such as the one in Figure 3-16,
has two small magnets attached to a metal float sealed inside a
bowl of clear compass fluid similar to kerosene. A graduated

A Magnetic Compass.
Figure 3-16. A Magnetic Compass. The vertical line is called the
lubber line.

scale, called a card, and 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, with each long mark representing 10° and each
short mark representing 5°.

Magnetic Compass Construction

The float and card assembly has a hardened steel pivot in its
center that rides inside a special, spring-loaded, bard-glass
jewel cup. The buoyancy of the float takes most of the weight
off the pivot, and the fluid damps the oscillation of the float
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.

Magnetic Compass Theory of Operations

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 3-16, 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, but 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
remain stationary, and the compass housing and the pilot turn
around it, always viewing the card from its backside.