Definition
Discrepancies that arise in RNAV procedure design and flight guidance because magnetic variation — the angular difference between true north and magnetic north — changes with location and over time. On path and terminator legs that reference a magnetic course, these discrepancies can cause the displayed course, the charted course, and the actual ground track to differ slightly, requiring the avionics database, the chart, and the aircraft system to apply variation values consistently.
Plain English
The compass doesn't point to true north — it points to magnetic north, and the difference between the two changes depending on where you are on Earth and slowly shifts over the years. When a flight procedure is built around magnetic headings, these shifting differences can cause small mismatches between what the chart says, what the avionics show, and where the airplane actually flies.
Context Anchor
Seen in instrument procedure design, RNAV database coding, and path-and-terminator leg discussions where courses and fixes must be flown using the correct north reference.
Derivation
Magnetic variation comes from Latin 'variatio' meaning 'a difference' or 'a change.' The term captures the idea that the angle between true north and magnetic north is not fixed — it varies by location and drifts over time as Earth's magnetic field changes.
Why Pilots Care
Unresolved magnetic variation issues can produce track errors on instrument approaches or departures, especially in areas with large or rapidly changing variation.
Grounding Statement
Magnetic north is not in the same place as true north, and it slowly moves, so navigation information that depends on magnetic north must use the right value for the place and date.
Intuition Check
Do not read “variation” here as a small random difference. In this context, magnetic variation is a specific angle between true north and magnetic north, and an “issue” means that angle is being applied differently or incorrectly.
Example Sentence 1
The crew briefed potential magnetic variation issues on the arrival because the chart's variation value was older than the most recent avionics database update.
Example Sentence 2
When flying across a large variation gradient, the navigation system must resolve magnetic variation issues to keep the aircraft on the published track.