Definition
An error in the magnetic compass caused by magnetic fields produced inside the aircraft itself — from electrical wiring, radios, engine components, and other ferrous metal — which pull the compass needle away from magnetic north. Deviation is unique to each aircraft and varies with heading, so it is measured during a compass swing and recorded on a compass correction card mounted near the instrument.
Plain English
The aircraft's own metal and electrical equipment tug on the compass and make it read slightly wrong. The error changes depending on which way you're pointing, so a small card near the compass tells you what to fly to actually get the heading you want.
Context Anchor
Seen in magnetic compass use, compass checks, and when comparing the compass reading with the heading you intend to fly.
Derivation
Deviation comes from the Latin deviare, meaning 'to turn aside from the way.' Here the compass needle is being turned aside from magnetic north by the aircraft's own magnetic influences.
Why Pilots Care
Accurate heading information requires applying corrections from the deviation card to avoid navigation errors.
Analogy
It is like holding a small magnet near a camping compass. The compass still moves and looks normal, but the nearby magnet pulls the needle slightly off the true direction.
Intuition Check
Compass deviation does not mean the aircraft is drifting off course. Here, it means the compass reading itself is being pulled off by magnetic effects inside the aircraft.
Example Sentence 1
After turning on the pitot heat and landing light, the pilot noted that compass deviation increased slightly on a westerly heading.
Example Sentence 2
After installing new radios, the mechanic swung the compass to measure and record updated deviation values.