Definition
The yearly rate at which magnetic variation at a given location changes due to the gradual movement of Earth's magnetic poles. Annual drift values are published on aeronautical charts and are used to determine when magnetic variation data, runway magnetic headings, and instrument procedures must be updated.
Plain English
How much the magnetic compass direction at a place shifts each year because Earth's magnetic poles slowly move. Charts show this yearly amount so that headings can be corrected before they become inaccurate.
Context Anchor
Seen in discussions of magnetic variation on charts, instrument procedures, airports, and navigation data, especially when older information must be compared with current values.
Derivation
‘Drift’ here comes from the ordinary sense of something slowly moving from its position over time. ‘Annual’ simply means per year. Together the phrase describes the yearly amount of slow positional change in magnetic variation — not aircraft drift in flight.
Why Pilots Care
Keeps navigation calculations accurate as Earth's magnetic field slowly shifts over time.
Grounding Statement
Annual drift values describe the slow year-by-year movement of the magnetic reference used for compass-based directions.
Intuition Check
Do not read drift here as wind pushing an airplane sideways. In this context, drift means the slow yearly change in magnetic variation.
Example Sentence 1
The chart legend listed the annual drift values so the crew could estimate how much the magnetic variation had shifted since the last update.
Example Sentence 2
Updated charts include new annual drift values so that magnetic courses remain correct months after publication.