Definition
The rotating component inside a magnetic compass consisting of a hollow metal float fitted with two small bar magnets and a graduated heading card marked with degrees and cardinal points. The float and card assembly pivots on a jeweled bearing and is suspended in a fluid-filled housing, allowing the magnets to align with Earth's magnetic field while the heading card displays the aircraft's magnetic heading.
Plain English
The moving part inside the compass that turns to show the aircraft's heading. It carries small magnets that line up with Earth's magnetic field and a numbered card that you read through the compass window.
Context Anchor
Seen when studying how a traditional aircraft magnetic compass is built and why its display moves as the aircraft changes direction.
Derivation
The word 'float' is used because the assembly is buoyed in a damping fluid that supports its weight and keeps it level. 'Card' refers to the circular dial printed with heading numbers, similar in concept to the printed face of an old mariner's compass.
Why Pilots Care
Keeps the directional dial level and free to move so the pilot receives accurate heading information without sticking or tilting errors.
Intuition Check
Do not picture a separate paper card floating loose in the instrument. The “card” is the marked direction display, and it is part of a balanced floating unit inside the compass.
Example Sentence 1
During preflight, the pilot checked that the float and card assembly moved freely and that the fluid in the compass housing was clear and free of bubbles.
Example Sentence 2
If the float and card assembly sticks, the pilot may see an incorrect heading during turns.