Since the magnetic compass works on the principle of magnetism, it is well for the pilot to have at least a basic understanding of magnetism. 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 3-12]

Figure 3-12.—Earth’s magnetic field.

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 airplane 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 which causes some of the larger compass errors.
 

Figure 3-13.—Magnetic compass.

The magnetic compass, which is the only direction-seeking instrument in the airplane, is simple in construction. It contains two steel magnetized needles fastened to a float around which is mounted a compass card. The needles are parallel, with their north-seeking ends pointed in the same direction. The compass card has letters for cardinal headings, and each 30° interval is represented by a number, the last zero of which is omitted. For example, 30° would appear as a 3 and 300° would appear as 30. Between these numbers, the card is graduated for each 5°. [Figure 3-13]  The float assembly is housed in a bowl filled with acid-free white kerosene. The purposes of the liquid are to dampen out excessive oscillations of the compass card and relieve by buoyancy part of the weight of the float from the bearings.

Jewel bearings are used to mount the float assembly on top of a pedestal. A line (called the lubber line) is mounted behind the glass of the instrument that can be used for a reference line when aligning the headings on the compass card.

Compass Errors

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. Variation will be discussed further in Chapter 8, Navigation.

Deviation

 Actually, 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 an aircraft disturb the compass needles and produce an additional error. The difference between the direction indicated by a magnetic compass not installed in an airplane, and one installed in an airplane, is deviation.

 If an aircraft changes heading, the compass’ direction-sensitive magnetized needles will continue to point in about the same direction while the aircraft turns with relation to it. As the aircraft turns, metallic and electrical equipment in the aircraft change their position relative to the steel needles; hence, their influence on the compass needle changes and deviation changes. Thus, deviation depends, in part, on the heading of the aircraft. 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. Therefore, a deviation card, installed in the cockpit in view of the pilot, enables the pilot to maintain the desired magnetic headings. Deviation will be discussed further in Chapter 8, Navigation.

Using the Magnetic Compass

 Since the magnetic compass is the only direction-seeking instrument in most airplanes, the pilot must be able to turn the airplane 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.

• If on a northerly heading and a turn is made toward east or west, the initial indication of the compass lags or indicates a turn in the opposite direction. This lag diminishes as the turn progresses toward east or west where there is no turn error.

• If on a southerly heading and a turn is made toward the east or west, the initial indication of the compass needle will indicate a greater amount of turn than is actually made. This lead also diminishes as the turn progresses toward east or west where there is no turn error.

• If a turn is made to a northerly heading from any direction, the compass indication when approaching north lags behind the turn. Therefore, the rollout of the turn is made before the desired heading is reached.

• If a turn is made to a southerly heading from any direction, the compass indication when approaching southerly headings leads behind the turn. Therefore, the rollout is made after the desired heading is passed. The amount of lead or lag is maximum on the north-south headings and depends upon the angle of bank used and geographic position of the airplane with regard to latitude.

• When on an east or west heading, no error is apparent while entering a turn to north or south; however, an increase in airspeed or acceleration will cause the compass to indicate a turn toward north; a decrease in airspeed or acceleration will cause the compass to indicate a turn toward south.

• If on a north or south heading, no error will be apparent because of acceleration or deceleration.
 The magnetic compass should be read only when the aircraft is flying straight and level at a constant speed. This will help reduce errors to a minimum.

 If the pilot thoroughly understands the errors and characteristics of the magnetic compass, this instrument can become the most reliable means of determining headings.