Definition
A small placard mounted near the magnetic compass that shows the heading the pilot should fly (the compass reading) to actually track each cardinal and intercardinal direction, after accounting for deviation errors caused by the aircraft's own electrical and metallic components.
Plain English
A small chart next to the compass that tells you what the compass will read when you are actually heading north, 30 degrees, 60 degrees, east, and so on. It corrects for the fact that the airplane's own metal and electronics pull the compass slightly off true magnetic north.
Context Anchor
Seen during preflight inspection and when using the magnetic compass as a heading reference in flight.
Derivation
"Deviation" is the compass error the card corrects for, caused by the airplane itself. The card is called a "correction card" because it tells the pilot what correction to apply. The numbers on it come from a procedure called "swinging the compass," done on the ground by a technician.
Why Pilots Care
Provides the exact corrections needed to compensate for the aircraft’s own magnetic interference, preventing navigation errors on every heading.
Intuition Check
The card does not automatically fix the compass. It tells the pilot what compass indication to use after known aircraft-caused error is accounted for.
Example Sentence 1
During preflight, the pilot confirmed the compass correction card was present and legible near the magnetic compass.
Example Sentence 2
On a heading of 180 the card showed a two-degree correction, so the pilot flew 182 on the compass to track the desired course.