Definition
A computed, real-time model of the angular difference between true north and magnetic north at the aircraft's current position, calculated continuously by an onboard navigation system using a mathematical model of Earth's magnetic field. It updates as the aircraft moves and as time passes, rather than relying on a fixed value stored in a database.
Plain English
A live calculation of how far magnetic north is offset from true north, worked out by the aircraft's own systems for wherever the aircraft happens to be right now.
Context Anchor
Seen in instrument procedure and navigation database discussions, especially where RNAV or GPS equipment converts true courses into magnetic courses.
Derivation
Dynamic comes from the Greek dynamikos, meaning powerful or in motion. Here it signals that the value is changing and being recomputed continuously, rather than being fixed. This contrasts with a static, charted variation that stays the same until a chart is updated.
Why Pilots Care
Using an outdated fixed value instead of the dynamic one can introduce heading errors that grow over time or in regions where the magnetic field shifts noticeably.
Analogy
It is like a phone updating your local time zone as you travel, instead of making you use the time from your departure airport for the whole trip.
Grounding Statement
As the aircraft moves across the earth, the angle between true north and magnetic north can change, so the navigation system may calculate a new local value.
Intuition Check
Dynamic does not mean unstable or unreliable here. It means the magnetic variation is calculated for the current place or route point instead of being treated as one fixed number.
Example Sentence 1
The crew noticed the FMS-displayed course was a degree off the charted value because their system uses dynamic magnetic variation rather than the fixed value published on the approach plate.
Example Sentence 2
Before departure the crew verified that the navigation database included the latest dynamic magnetic variation model for the route.