Definition
An angular difference between true north and magnetic north in which magnetic north lies to the east of true north. On navigation charts, east variation is shown by isogonic lines labeled with a degree value and the letter E (for example, 5°E). To convert a true course to a magnetic course in an area of east variation, the variation is subtracted; to convert magnetic to true, it is added.
Plain English
In some parts of the world, the compass doesn't point exactly to true north — it points a little to the east of it. That sideways error is called east variation, and it has to be accounted for when planning a heading.
Context Anchor
Seen in navigation planning when using aeronautical charts and converting a true course to a magnetic course.
Derivation
Variation' comes from the Latin variare, meaning 'to change or differ.' Here it describes how much the magnetic compass differs from true north. 'East' simply tells you which side of true north the magnetic pole sits from your location.
Why Pilots Care
Failing to apply east variation produces a heading error that grows with larger variation values and can lead to significant navigation deviation.
Grounding Statement
At a place with 10° east variation, compass north points 10° to the right of true north.
Intuition Check
East variation does not mean the airplane is drifting east or that the route goes east. It means magnetic north is east of true north at that location.
Example Sentence 1
Flying in the Pacific Northwest, the pilot noted 15°E variation on the chart and subtracted it from her true course to get the magnetic course.
Example Sentence 2
The sectional depicts east variation with a plus sign next to the isogonic line value.