Definition
Errors in the magnetic compass that cause it to momentarily indicate a turn in the wrong direction, or fail to indicate a turn at all, when the aircraft turns from a northerly or southerly heading. On a northerly heading, the compass initially shows a turn in the opposite direction (lag); on a southerly heading, it shows a turn faster than the actual turn (lead). These errors are caused by the vertical component of the Earth's magnetic field acting on the tilted compass card during a banked turn.
Plain English
When you start a turn from north or south, the compass briefly lies. Turning from north, it first swings the wrong way before catching up. Turning from south, it races ahead of your actual turn. These wrong readings are called lag (north) and lead (south).
Context Anchor
Seen when studying magnetic compass errors and when practicing turns using the magnetic compass.
Derivation
Lag means to fall behind; lead means to go ahead. The compass either falls behind the actual turn (north) or runs ahead of it (south).
Why Pilots Care
Understanding this behavior prevents over-correction and allows accurate heading control during turns without relying solely on other instruments.
Grounding Statement
Picture the compass as a direction pointer that can be pulled slightly off during a turn, so it may not point exactly with the airplane’s nose until the turn settles.
Intuition Check
“Leads” does not mean electrical wires or the metal lead here. “Lags or leads” means the compass reading is behind or ahead of the airplane’s actual heading during a turn.
Example Sentence 1
When turning from a northerly heading, the pilot anticipated the compass lag and rolled out before the compass showed the desired heading.
Example Sentence 2
In the Southern Hemisphere the same turn produces the opposite lead effect, so the pilot adjusted the rollout timing accordingly.