Definition
An error in a magnetic compass reading caused by magnetic fields generated within the aircraft itself, such as those from electrical equipment, radios, and ferrous metal components. Because these onboard influences pull the compass needle away from magnetic north, the resulting error is unique to each aircraft and varies with heading. It is compensated for during a procedure called a compass swing, and any residual error is recorded on a deviation card mounted near the compass.
Plain English
The aircraft's own metal and electrical systems tug on the compass, making it point slightly off from magnetic north. That small built-in error is called deviation, and a card in the cockpit tells the pilot how much to adjust for it on different headings.
Context Anchor
Seen when checking, adjusting, or using the aircraft’s magnetic compass, especially after maintenance or equipment changes.
Derivation
From the Latin 'deviare', meaning 'to turn aside from the way' (de- 'off' + via 'road'). The compass needle is being turned aside from where it should be pointing by the aircraft's own magnetic influences.
Why Pilots Care
Uncorrected deviation error produces heading mistakes that accumulate during navigation and can lead to significant course deviations, especially on long flights or in instrument conditions.
Intuition Check
Deviation error is not a pilot straying from a route. Here, deviation means the compass reading is being pulled off by the aircraft’s own magnetism.
Example Sentence 1
After installing the new radio stack, the technician performed a compass swing to measure the deviation error and updated the deviation card.
Example Sentence 2
Before takeoff the pilot checked the deviation card to apply the correction for deviation error at the planned heading.