Definition
The rotating disk inside a magnetic compass that displays heading information. The card is attached to a float assembly with small magnets fixed to it, and the whole assembly pivots freely in a fluid-filled bowl. Because the magnets align with the Earth's magnetic field, the card stays oriented to magnetic north while the aircraft turns around it. The pilot reads the current heading at the lubber line on the front of the case.
Plain English
The round, numbered dial you see inside a magnetic compass. It floats in liquid and stays pointed at magnetic north on its own, so the heading you're flying shows up at the reference mark on the front.
Context Anchor
Seen when reading or checking the aircraft’s magnetic compass, especially as a backup to other heading instruments.
Derivation
The word 'card' here comes from the old maritime 'compass card' -- the printed disk of headings that mariners read on shipboard compasses. Aviation inherited the same name and the same idea: a circular card marked with degrees and cardinal points (N, E, S, W) that the pilot reads against a fixed reference line.
Why Pilots Care
Supplies the only direct, non-electrical heading reference and is required for compass swings, heading indicator alignment, and emergency navigation.
Intuition Check
Do not think of “card” as a separate paper card or checklist item. Here it means the marked direction display inside the magnetic compass.
Example Sentence 1
After the vacuum pump failed, the pilot used the magnetic compass card as the primary heading reference for the rest of the flight.
Example Sentence 2
During the compass swing the technician noted the magnetic compass card readings at each cardinal heading.