Definition
A magnetic compass error that occurs when turning from a southerly heading in the Northern Hemisphere, in which the compass indicates a turn at a faster rate than the aircraft is actually turning, causing the heading reading to lead the actual heading.
Plain English
When you turn the airplane while heading south (in the Northern Hemisphere), the compass spins faster than the airplane is actually turning. So the compass shows you further around the turn than you really are.
Context Anchor
Seen when learning magnetic compass limitations, especially the rule for turning to or from north and south headings.
Derivation
From 'lead' meaning 'to go in front of.' The compass reading runs ahead of the aircraft's actual heading, so the error is said to lead.
Why Pilots Care
Failing to correct for leading error produces heading overshoots that can cause navigation deviations or spatial disorientation.
Grounding Statement
During certain turns, the magnetic compass does not show the airplane’s direction instantly and accurately; it can temporarily get ahead of the real turn.
Intuition Check
Leading error does not mean the pilot should lead the turn. It means the compass indication itself is leading, or showing more turn than has actually happened.
Example Sentence 1
Turning from a heading of 180 to 270 at 40 degrees north latitude, the pilot accounted for the leading error by rolling out 20 degrees before the compass showed 270.
Example Sentence 2
On a southerly heading the compass leads the turn, so the pilot watches for the lead angle before rolling out.