Definition
Errors in magnetic compass readings caused by magnetic fields generated within the aircraft itself — from electrical wiring, radios, engine components, and ferrous metal parts — which pull the compass needle away from magnetic north. Because each aircraft has its own unique magnetic signature, deviation errors are measured during a compass swing and recorded on a compass correction card mounted near the instrument, showing the heading the pilot must fly to achieve a desired magnetic heading.
Plain English
Small mistakes in what the compass shows because metal and electrical equipment inside the aircraft tug on the compass needle. A correction card next to the compass tells the pilot how much to adjust for it.
Context Anchor
Seen when using the magnetic compass as a heading reference, including timed turns, where the pilot may need to turn by time instead of trusting the compass alone.
Derivation
Deviation comes from the Latin 'deviare,' meaning 'to turn aside from the way.' Here, the compass needle is being turned aside from where it should point — not by anything outside the aircraft, but by magnetic influences inside it.
Why Pilots Care
Uncorrected deviation produces heading errors that accumulate during turns or instrument navigation and can lead to disorientation or missed waypoints.
Intuition Check
Do not read “deviation” here as simply drifting away from your planned path. In this context, it means an error in the compass reading caused by magnetic influences inside the aircraft.
Example Sentence 1
Before starting the timed turn, the pilot checked the compass correction card and noted that a heading of 090° required flying 092° to account for deviation.
Example Sentence 2
Before the instrument flight, the mechanic swung the compass to reduce deviation errors at all cardinal headings.