Definition
Magnetic variation values that are computed by an aircraft's onboard navigation system in real time, based on the aircraft's current geographic position and a stored mathematical model of the Earth's magnetic field. Because the model produces a value that updates as the aircraft moves and as time passes, the variation used by the system can differ from the fixed (station-declination) variation values printed on charts or coded into ground-based navaids and procedures.
Plain English
These are magnetic variation numbers the aircraft works out for itself, on the fly, using its own internal model of Earth's magnetic field. Because the aircraft is calculating its own number rather than using the one printed on the chart or set at the navaid, the two values can disagree slightly.
Context Anchor
Seen in instrument procedure and avionics discussions when charted magnetic courses are compared with courses calculated or displayed by GPS, FMS, or other navigation systems.
Derivation
Dynamic comes from Greek dynamikos, meaning 'powerful' or 'active' — used here in the sense of constantly changing, as opposed to static (fixed). The variation values are 'dynamic' because the system recalculates them as the aircraft's position and the date change, rather than using a single fixed number.
Why Pilots Care
Using current values prevents heading and course errors that would accumulate if outdated static chart values were applied instead.
Grounding Statement
As the airplane moves across the earth, the angle between true north and magnetic north changes, so some navigation systems update that correction as the position changes.
Intuition Check
Dynamic does not mean the magnetic variation is bouncing around unpredictably during flight. Here it means the value is calculated from position and date rather than taken from one fixed published number.
Example Sentence 1
The crew noticed a one-degree course offset on the RNAV overlay and recognized it as a difference between the FMS dynamic magnetic variation values and the station declination used to build the original VOR approach.
Example Sentence 2
Before flight the pilot confirms the database holds updated dynamic magnetic variation values for the destination area.