Definition
West variation is the angular difference between true north and magnetic north at a given location, measured when magnetic north lies to the west of true north. It is applied to a true heading or true course by adding the variation value to obtain the corresponding magnetic heading or magnetic course.
Plain English
At some places on Earth, a magnetic compass points a few degrees west of true north instead of straight at it. That sideways error is called west variation, and pilots add it to a true heading to get the correct magnetic heading.
Context Anchor
Seen in navigation planning when comparing chart directions, which are based on true north, with compass directions, which are based on magnetic north.
Derivation
‘Variation’ comes from the Latin variare, meaning ‘to change or differ.’ It describes how a magnetic compass differs from true north depending on where you are on Earth. ‘West’ simply tells you which side of true north the compass is pointing toward.
Why Pilots Care
Applying west variation incorrectly produces heading errors that increase with distance and can lead to airspace or terrain conflicts.
Intuition Check
West variation does not mean the airplane is flying west. It means magnetic north is west of true north at that location.
Example Sentence 1
The sectional showed 10° west variation, so the pilot added 10° to the true course of 090° to get a magnetic course of 100°.
Example Sentence 2
On the sectional chart the pilot notes 8 degrees west variation and applies it when converting the planned true course to a compass heading.