Magnetic Compass

An aircraft magnetic compass 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 magnets align with the Earth’s magnetic field and the pilot reads the direction on the scale opposite the lubber line.

Useful Definitions

• Variation: The difference between true and magnetic directions is called variation.
• Deviation: The magnets in a compass align with any magnetic field. Some causes for magnetic fields in aircraft include flowing electrical currents. These aircraft magnetic fields create a compass error called deviation.

Turning Errors

A turn towards the north causes the compass to momentarily over-read. Because of this lead of the compass card, a northerly turn should be stopped prior to arrival at the desired heading. A turn towards the south causes the compass to momentarily lag. Because of this lag, a southerly turn should be after arrival at the desired heading. “North Before, South After”  is a useful way to remember the turning error.  Also keep in mind that the magnitude of the error increases the closer you are to either pole.

Acceleration/Deceleration Errors

When accelerating on either an easterly or westerly heading, the compass incorrectly displays a turn towards the north. When decelerating on an easterly or westerly heading, the compass indicates a turn towards the south. A mnemonic for the effect of acceleration error is the word “ANDS” (Acceleration – North/Deceleration -South). Note that the error is not apparent on north/south headings and most prominent on east/west headings.