Definition
Errors shown by the magnetic compass when an aircraft turns to or from a southerly heading in the Northern Hemisphere. When turning from a southerly heading, the compass leads the actual turn — it shows a turn in the correct direction but at a faster rate than the aircraft is actually turning. To compensate, the pilot rolls out of the turn after passing the desired heading by an amount roughly equal to the latitude minus a small allowance for the bank angle. The cause is magnetic dip: the vertical component of Earth's magnetic field pulls the compass card, and the resulting tilt produces predictable errors during turns through southerly headings.
Plain English
When you turn to or away from south while flying in the Northern Hemisphere, the magnetic compass spins faster than you are actually turning. So when rolling out on a southerly heading, you have to let the compass go past the heading you want before stopping the turn.
Context Anchor
Seen when using the magnetic compass for heading information during instrument flight, especially when practicing compass turns or when other heading instruments are not available.
Derivation
“Southerly” means relating to south. “Error” comes from an older idea of wandering away from the correct path; here it means the compass indication is off, not that the pilot made a mistake.
Why Pilots Care
Failing to anticipate the error produces heading deviations that can lead to loss of situational awareness or airspace violations in instrument conditions.
Grounding Statement
In a banked turn near a south heading, the compass indication can move ahead of the airplane’s real direction.
Intuition Check
Do not read “error” as a pilot mistake. Here it means a known compass behavior that must be allowed for during the turn.
Example Sentence 1
During partial-panel practice, the instructor reminded the student to account for southerly turning errors by rolling out about 30 degrees past 180 when turning to a heading of south.
Example Sentence 2
In the northern hemisphere, southerly turning error caused the compass to indicate a completed turn before the aircraft had actually reached the new heading.