Definition
Errors in the magnetic compass indication that occur during turns to or from northerly or southerly headings, caused by the vertical component of the Earth's magnetic field acting on the tilted compass card. When turning through north in the Northern Hemisphere, the compass lags behind the actual heading; when turning through south, it leads the actual heading. The size of the error roughly equals the latitude.
Plain English
When you turn the airplane near a heading of north or south, the magnetic compass doesn't show the right heading. Near north it reacts slowly and shows a heading behind where you actually are. Near south it reacts too quickly and shows a heading ahead of where you actually are.
Context Anchor
Seen when using the magnetic compass during instrument flying, partial-panel flying, or any situation where the compass is being used to confirm heading during a turn.
Why Pilots Care
Uncorrected, these errors cause pilots to overshoot or undershoot headings during turns, leading to navigation deviation especially under instrument conditions.
Analogy
It is like a delayed pointer on a dial: near north, the airplane has already turned more than the compass seems to show, so waiting for the exact number can make you turn too far.
Grounding Statement
Picture turning toward north while the compass is slow to catch up; by the time the compass shows north, the airplane may already be past north.
Intuition Check
Do not assume “northerly turning errors” means any mistake made while turning north. Here it means a built-in magnetic compass indication error that happens near north during a turn.
Example Sentence 1
On a partial-panel approach, the pilot remembered to lead the rollout when turning through south, knowing the compass would indicate ahead of the actual heading.
Example Sentence 2
During the instrument approach, awareness of northerly turning errors kept the compass from misleading the pilot through the final turn.