Magnetic Compass

The following information is provided for your information. During your CAP glider flight academy experience you will be required to fly on specific headings, therefore having some idea of how the magnetic compass works will be helpful.

The magnetic compass is made up of a liquid camber filled with a specific type of fluid that dampens a magnetic float with a compass card attached.

Magnetic Attraction
A magnet is a piece of metal that has the property of attracting another metal. The force of attraction is greatest at the poles or points near the end of the magnet; the least attraction is in the area halfway between the two poles. Lines of force flow from each of these poles in all directions, bending around and flowing toward each other's poles to form a magnetic field. Such a magnetic field surrounds the earth, with lines of force oriented approximately to the north and south magnetic poles.

Compass Errors

Variation
Since the geographic and magnetic north poles are different, in order to fly a magnetic course referencing our maps, whose longitude lines point to the geographic North Pole; we must correct for the angular variation between magnetic and geographic. Lines of magnetic variation are printed on your sectional chart. These lines are called Isogonic Lines and are given a numerical value with an either east or west orientation. The line representing zero variation between magnetic and geographic north is called the Agonic Line. From the Agonic Line, Isogonic Lines depart either east or west. (See the illustration.)

To determine true heading, you must take into consideration the amount of variation near your flight path. When the variation is east, you must subtract the variation from your true heading as determined from your map (the map references geographic north). Example: If true heading is 60 degrees and you have a 10-degree east variation, the magnetic course would be 50 degrees. With a west variation you should add the variation. Example: A true heading of 50 degrees, and a variation of 10 degrees west would yield a magnetic course of 60 degrees.
A memory aid is "East is least and West is best." Doesn't fit the situation exactly perfect, but it has worked for me.

Deviation
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 aircraft and one installed in an aircraft 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. This error is dealt with by means of a deviation card. The card displays headings to steer if a certain magnetic heading is desired. Example: To fly a 220 degree course, steer 215 degrees.

Magnetic Dip
No ladies, it's not some attractive dork. The Isogonic Lines run parallel to the Earth's surface at the Equator and curve increasing downward closer to the magnetic poles.

Magnetic dip is the tendency of the compass needles to point down as well as to the magnetic pole. Dip is greatest near the poles and least near the Magnetic Equator. The compass card is designed to operate in the horizontal, therefore, any movement from the horizontal plane introduces dip error.

The needle of your magnetic compass will be parallel with Earth's surface at the Magnetic Equator, but will point increasing downward as it is moved closer to the Magnetic Pole.

Northerly turning error is due to the mounting of the compass. Since the card is balanced in fluid, when the aircraft turns, the card is also banked as a result of centrifugal force. While the card is banked, the vertical component of the Earth's magnetic field causes the north-seeking ends of the compass to dip to the low side of the turn. When making a turn from a northerly heading, the compass briefly gives an indication of a turn in the opposite direction. When making a turn from the south, it gives an indication of a turn in the correct direction but at a faster rate.

Acceleration error is also due to the dip of the Earth's magnetic field. Because of the way the compass card is mounted, the aft end of the compass card is tilted upward when accelerating, and downward when decelerating during airspeed changes. This error is most pronounced on an east / west heading. When accelerating on an east or west heading, the error indicates a turn to the north. When decelerating on an east or west heading the error is toward the south.

As a memory aid use: Accelerates north and decelerates south.

Oscillation Error
Is caused by turbulence or rough control movements and results in erratic movement of the compass card.

Using the Magnetic Compass
To turn an aircraft to a magnetic heading and maintain that heading understanding the following characteristics of the magnetic compass, caused by magnetic dip, will help.

1. 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. The lag diminishes as the turn progresses toward the east or west where there is no turning error.

2. If on a southerly heading and a turn is made toward 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.

3. If a turn is made to a northerly heading from any direction, the compass indication when approaching northerly heading leads or is ahead of the turn. Therefore, the roll out 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 lags behind the turn. Therefore, the roll out is made after the desired heading is passed. The amount of lead or lag is maximum on north / south headings and depends upon the angle of bank used and the latitude of the aircraft.

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

If on a north or south heading, there will be no error due to acceleration or deceleration.

To keep errors at a minimum read the magnetic compass when the aircraft is flying straight at a constant speed.

Lets review compass errors:

Turning Error
A turn from the north lags or indicates a turn in the opposite direction. So to roll out on the correct heading one must roll out of the turn, past the correct heading.

A turn from the south leads. So to roll out on the correct heading one must roll out of the turn, before reaching the correct heading.

REMEMBER: THE SOUTH LEADS AND THE NORTH LAGS and

THERE IS NO ACCELERATION / DECELERATION ERROR ON A NORTH OR SOUTH HEADING.

Acceleration and Deceleration Error
When on an east or west heading, any increase in airspeed (Acceleration) will cause the magnetic compass to indicate a false turn toward the north, and any decrease in airspeed (deceleration) will cause the magnetic compass to indicate a false turn toward the south.

REMEMBER: ACCELERATE NORTH & DECELERATE SOUTH and

THERE IS NO TURNING ERROR ON A EAST OR WEST HEADING.

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