Definition
A self-contained navigation instrument that uses a magnetized element, suspended in a fluid-filled housing, to align with Earth's magnetic field and indicate the aircraft's heading relative to magnetic north.
Plain English
An instrument in the cockpit that shows which direction the airplane is pointing, using a small magnet that lines up with the Earth's magnetic field.
Context Anchor
Seen during the visual preflight inspection and used in flight as a basic direction reference or backup to other direction instruments.
Derivation
From Latin 'compassus' meaning 'a stepping together' or 'a circle' — the same root as 'pace' or 'measure around.' Combined with 'magnetic' (from Magnesia, a region in ancient Greece where naturally magnetic stones were found), the name reflects an instrument that uses magnetism to measure direction around a circle.
Why Pilots Care
It remains the primary, independent source of heading when electrical power, GPS, or other instruments are lost.
Intuition Check
Do not assume the magnetic compass points to true north. It points toward magnetic north, and its reading can be affected by nearby metal, electrical equipment, and airplane movement.
Example Sentence 1
During preflight, the pilot checked that the magnetic compass was full of fluid, free of bubbles, and swinging freely.
Example Sentence 2
After takeoff the pilot cross-checked the magnetic compass against the heading indicator to verify the aircraft was on the assigned course.