Magnetic Compass

The magnetic compass is the most basic and most reliable heading reference instrument in an aircraft. It requires no electrical power, no vacuum system, and no external inputs — it simply aligns with the earth's magnetic field to indicate magnetic heading. For this reason, it is required as a standby instrument even on aircraft equipped with sophisticated electronic navigation systems.

The magnetic compass has known errors that pilots must account for:

• Variation: The angular difference between true north and magnetic north, which varies by location
• Deviation: Errors caused by the aircraft's own magnetic fields (engines, electrical systems, metal structure). Deviation is measured and posted on a compass correction card
• Dip errors: Caused by the vertical component of the earth's magnetic field, resulting in acceleration/deceleration error (ANDS — Accelerate North, Decelerate South) and turning error (UNOS — Undershoot North, Overshoot South)