The Magnetic Compass

Do you know what a Boeing 747 has in common with a Boy Scout? Both use magnetic compasses to determine their direction of travel. The common compass is a self-contained, reliable instrument as long as one understands its limitations and the properties of magnetism.

The Earth rotates around an axis formed by the geographic, or true, north and south poles. Magnetic poles also exist. Because Earth has a natural magnetic field, the magnetic needle in a compass aligns itself with the lines of magnetic force or flux. This alignment causes the compass to point to Earth's magnetic north pole, which is located in Canada.

Because navigation charts are oriented to true north, the pilot must account for this difference by converting true direction to magnetic direction. This is done by applying a correction called variation to the angular difference between the two. Displayed on navigation charts are isogonic lines that connect points having equal variation and agonic lines that define points where the variation is zero. The farther west one moves from the agonic line, the greater the easterly variation. This is because the compass is pointing more and more to the east of true north. To correct for this, the amount of easterly variation must be subtracted from the reading on the chart. When east of the agonic, the opposite is true.

Due to the molten nature of the Earth's core, the magnetic pole is free to slowly drift. Not only do these poles drift, but they have also switched polarity several times over the course of Earth's history. You needn't be concerned that this switch will occur while you are flying and cause the pilot difficulty in navigating. The switch occurs only about every 100,000 years. But when it does occur, it's catastrophic. As the magnetic field diminishes, the Earth loses the protection of its magnetic shield. The bombardment of charged solar particles wreaks havoc with plant and animal life, causing pronounced genetic mutations. Enough to make El Nio seem mild!

Because a magnetic field is always created around an electric current, pilots must also account for the magnetic effect of the electric equipment located on the flight deck. The magnetic compass is calibrated for this effect and maintenance technicians periodically check this calibration. Any error, or deviation, is accounted for by using a deviation table located on the compass.

Although the lines of magnetic force are parallel to the Earth's surface at the magnetic equator, the lines of magnetic flux point increasingly downward as the latitude approaches the magnetic poles. This magnetic dip affects the indication of the compass while the plane is banking in a turn. It also affects the indication of the compass when acceleration force tips the compass correction card. Pilots are trained to correct for this magnetic dip error.

The humble compass, usually located above all the others on the instrument panel, is indispensable. Without it, all of the sophisticated electronic navigating devices would be useless.

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