Definition
Direction-finding instruments that use a magnetized element aligned with Earth's magnetic field to indicate the aircraft's heading relative to magnetic north. In autopilot systems, magnetic compass information (typically through a slaved gyro or flux valve) provides the heading reference the autopilot uses to hold or track a selected course.
Plain English
Instruments that show which way the aircraft is pointing by sensing Earth's magnetism. The autopilot can use this heading information to keep the aircraft flying in the direction the pilot has selected.
Context Anchor
Seen in cockpit instrument discussions, autopilot discussions, and basic aircraft equipment descriptions where the airplane needs a direction reference.
Derivation
From Latin 'magneticus' (relating to the magnet, originally from Magnesia, a region in Greece where naturally magnetic stones were found) and Latin 'compassus' (a circle, or stepping around). Together they describe an instrument that uses magnetism to step around the circle of directions.
Why Pilots Care
They supply the basic heading reference that autopilots use to maintain course and that pilots cross-check against other instruments.
Intuition Check
Do not assume a magnetic compass shows the exact direction to the North Pole on a map. In aviation, it shows direction relative to Earth’s magnetic field, so pilots must account for the difference between magnetic direction and true geographic direction when needed.
Example Sentence 1
Before engaging the autopilot in heading mode, the pilot cross-checked the magnetic compass against the heading indicator to confirm they agreed.
Example Sentence 2
Pilots compare magnetic compasses with the directional gyro before engaging heading mode.