Definition
A self-contained, liquid-filled instrument that uses a freely pivoting magnetized element to align with Earth's magnetic field, providing the pilot with a direct indication of magnetic heading. It requires no electrical power and serves as the primary backup heading reference in the aircraft. The instrument consists of a float assembly with magnets attached, a graduated card showing headings in degrees, a lubber line for reading the heading, and a fluid-filled case that dampens oscillations.
Plain English
A simple compass mounted in the cockpit that shows which way the airplane is pointed relative to magnetic north. It floats in liquid, runs without electricity, and is the most basic heading instrument in the aircraft.
Context Anchor
Seen on the instrument panel and in instrument flying discussions about heading, navigation, and compass errors.
Derivation
Compass' comes from the Latin 'com-' (together) and 'passus' (step or pace), originally meaning to measure or step around — fitting for an instrument that lets you find direction in any circle around you. 'Magnetic' refers to its reliance on Earth's magnetic field rather than a gyroscope or electronic sensor.
Why Pilots Care
It supplies the only independent heading reference when electrical systems or other instruments fail.
Analogy
Think of it as a pocket compass mounted in the airplane and calibrated to ignore the plane's own metal parts.
Intuition Check
Do not assume the magnetic compass points to true north or always gives a perfectly steady reading. In aviation, it points with Earth’s magnetic field and must be understood with its normal compass errors.
Example Sentence 1
After the heading indicator tumbled, the pilot used the basic aviation magnetic compass to maintain a steady course back to the airport.
Example Sentence 2
During the instrument scan the basic aviation magnetic compass remained steady while the heading indicator was reset.