Definition
The error in a magnetic compass reading caused by magnetic fields produced within the aircraft itself, such as those from electrical wiring, radios, engine components, and ferrous metal structure. Deviation is unique to each individual aircraft and varies with the heading flown. It is recorded on a compass correction card mounted near the compass, showing the heading to steer to achieve a desired magnetic heading.
Plain English
The aircraft's own electrical and metal parts pull the compass needle slightly off, so the compass doesn't always show true magnetic north exactly. A small card near the compass tells the pilot how much to adjust for it on different headings.
Context Anchor
Seen when using the magnetic compass in the cockpit and reading the compass correction card.
Derivation
Deviation comes from the Latin deviare, meaning 'to turn aside from the way.' The compass needle is being pulled aside from the magnetic heading by influences inside the aircraft.
Why Pilots Care
Pilots must apply the corrections listed on the aircraft's compass deviation card to fly accurate magnetic headings and avoid navigation errors.
Analogy
It is like placing a small magnet near a handheld compass. The compass still works, but the nearby magnet pulls the needle slightly off.
Intuition Check
Do not read deviation as just any difference or error. In this context, magnetic deviation specifically means compass error caused by magnetism in the aircraft itself, not by the Earth’s magnetic field.
Example Sentence 1
After tuning the radios and turning on the pitot heat, the pilot glanced at the compass correction card to check the deviation for the new heading.
Example Sentence 2
After the avionics upgrade the mechanic performed a compass swing to record the new magnetic deviation values for each heading.