Definition
Errors in the magnetic compass that occur during turns through northerly and southerly headings. Lead is when the compass shows a turn before it has actually reached the new heading, which happens when turning through a southerly heading. Lag is when the compass shows a turn after the actual heading has been reached, which happens when turning through a northerly heading in the Northern Hemisphere. Both errors are caused by the vertical component of the Earth's magnetic field tilting the compass card as the aircraft banks.
Plain English
When you turn the airplane through north or south, the magnetic compass doesn't show the right heading at the right moment. Turning through south, the compass gets ahead of you (leads). Turning through north, the compass falls behind you (lags). You have to roll out early or late to end up on the heading you wanted.
Context Anchor
Seen when using the magnetic compass during turns, especially in instrument flying when the compass is being used to help confirm heading.
Derivation
From everyday English: to lead means to go ahead of something, to lag means to fall behind. The compass card is doing exactly that — running ahead of the aircraft's actual turn, or trailing behind it.
Why Pilots Care
Recognizing lead and lag prevents overshooting or undershooting headings when turning with the magnetic compass alone.
Intuition Check
Lead does not mean an instruction to start the turn, and lag does not mean engine delay. Here, lead and lag describe whether the magnetic compass indication is ahead of or behind the airplane’s real heading during a turn.
Example Sentence 1
When turning to a northerly heading, the pilot rolled out early to account for compass lag.
Example Sentence 2
Approaching a northerly heading in a turn, the pilot continues past the indicated heading to compensate for lag.