Definition 1 of 2
Definition
The rotating circular dial inside a heading indicator, HSI, or magnetic compass that displays direction in degrees from 0 to 360. On an HSI, the compass card rotates so that the aircraft's current heading appears at the top under the lubber line, while the rest of the card shows headings in all other directions around the aircraft.
Plain English
The round, numbered dial inside a direction-showing instrument. It turns as the aircraft turns, so the number at the top is always the direction the nose is pointing.
Context Anchor
Seen on the HSI display, where the rotating compass card surrounds the course and heading information.
Derivation
Compass comes from the Latin compassare, meaning 'to measure out' or 'pace off,' which became the name for the instrument that measures direction. Card here uses the older sense of a flat printed disc or face — the same sense as the face of a clock. So a compass card is simply the printed face of the compass that shows the directions.
Why Pilots Care
It gives an immediate visual reference for the aircraft's current magnetic heading, which is fundamental for navigation, course tracking, and orientation.
Analogy
Think of it like the numbered face of a clock, except it shows directions from 001 to 360 instead of hours from 1 to 12.
Intuition Check
Do not think of card as a paper card here. In this instrument, the compass card is the circular direction scale, usually part of the display itself.
Example Sentence 1
As the aircraft turned right, the compass card rotated under the lubber line until 270 appeared at the top.
Example Sentence 2
During the turn the compass card rotated smoothly until the desired heading aligned with the lubber line.