Definition
The collective term for the predictable inaccuracies of a magnetic compass during flight, including variation, deviation, magnetic dip, acceleration/deceleration error, and northerly/southerly turning error. These errors arise because the compass is a free-swinging magnet aligning with the Earth's magnetic field, which is not parallel to the surface and is influenced by the aircraft's own magnetic and motion-related forces.
Plain English
The known ways a magnetic compass gives a wrong reading in flight. Pilots learn these errors so they can trust the compass when it's accurate and ignore it when it isn't.
Context Anchor
Seen when using the magnetic compass to start, control, or stop a turn, especially during instrument flight when outside visual references are limited.
Derivation
Compass comes from older words meaning to measure or go around. Error comes from a Latin word meaning to wander. That fits the aviation use: the compass indication can temporarily wander away from the direction the airplane is actually pointing.
Why Pilots Care
Failing to compensate for these errors leads to incorrect headings, disorientation, and unsafe flight paths.
Grounding Statement
During a turn, the airplane may be changing direction correctly while the magnetic compass is temporarily showing a direction that is ahead of or behind the real heading.
Intuition Check
Compass errors are not pilot mistakes. They are known, predictable limits in what the magnetic compass shows while the airplane is moving.
Example Sentence 1
After losing the heading indicator, the pilot mentally worked through the compass errors before rolling out of the turn early to account for northerly turning error.
Example Sentence 2
Acceleration errors cause the compass to indicate a false turn toward north when the airplane speeds up on an east or west heading.