Definition
The predictable inaccuracies displayed by a magnetic compass while an aircraft is turning, caused by the compass card's tendency to dip and lag or lead the actual heading because of magnetic dip and the aircraft's bank angle. In the Northern Hemisphere, turns to the north cause the compass to lag behind the true heading, and turns to the south cause it to lead ahead of the true heading. These errors are most pronounced near headings of north and south and disappear near east and west.
Plain English
When you turn the airplane, the magnetic compass doesn't show the right heading right away. Heading north it reads behind where you actually are, and heading south it reads ahead. Pilots have to roll out of the turn early or late to end up on the heading they wanted.
Context Anchor
Seen in instrument flying when a pilot uses the magnetic compass to check or set a heading during turns, especially if other heading instruments are unavailable or being cross-checked.
Why Pilots Care
Failing to anticipate these errors produces large heading deviations that can lead to loss of situational awareness or airspace violations.
Grounding Statement
Picture the compass as a small floating direction indicator: when the aircraft banks in a turn, that floating part can tilt and momentarily show a misleading direction.
Intuition Check
Do not assume the magnetic compass shows the correct heading at every moment in a turn. In this context, “error” means the compass indication is temporarily misleading, not that the compass is broken.
Example Sentence 1
On the checkride, the examiner covered the heading indicator and asked the pilot to roll out on a heading of north, requiring them to apply the standard correction for errors during compass turns.
Example Sentence 2
Knowing Errors During Compass Turns allowed the pilot to apply the proper lead or lag when rolling out on the desired heading.