Definition
A magnetic compass error in which the compass card lags behind, or shows a slower change than, the actual heading change of the aircraft during a turn. In the Northern Hemisphere, lagging error appears during turns through a southerly heading, where the compass momentarily indicates a turn faster than the aircraft is actually turning, requiring the pilot to roll out past the desired heading. The error is caused by the vertical component of Earth's magnetic field acting on the compass card as the aircraft banks.
Plain English
When you turn through south in the Northern Hemisphere, the magnetic compass shows the turn happening faster than it really is. To stop on the heading you want, you have to keep turning past it before rolling out.
Context Anchor
Seen in magnetic compass discussions, especially when learning why a compass behaves oddly during turns near north and south headings.
Derivation
From 'lag,' meaning to fall behind or move more slowly than something else. The name describes the compass card's behavior relative to the aircraft: even though the compass appears to race ahead in indication, the term traditionally describes how the compass lags in settling on the correct heading after the turn.
Why Pilots Care
Pilots must lead the rollout to avoid overshooting the desired heading.
Analogy
It is like a person walking behind you with a sign showing your direction. If they cannot keep up during a turn, their sign shows where you were a moment ago, not exactly where you are now.
Intuition Check
Lagging error does not mean the airplane is turning slowly. It means the compass indication is behind the airplane’s actual heading.
Example Sentence 1
While turning from a heading of 120 to 180 in the Northern Hemisphere, the pilot anticipated lagging error and rolled out past the indicated heading to settle on south.
Example Sentence 2
During the southerly turn the compass lagged, so the instructor reminded the student to watch the actual turn rate on the turn coordinator.