Definition
An error displayed by the magnetic compass during changes in airspeed on an easterly or westerly heading. When the aircraft accelerates, the compass indicates a turn toward the north; when it decelerates, it indicates a turn toward the south. The error is caused by the compass card's center of gravity being offset from its pivot, combined with the vertical component of Earth's magnetic field acting on the card during the speed change.
Plain English
A false reading the magnetic compass shows when you speed up or slow down while flying east or west. Speeding up makes it look like you are turning toward the north; slowing down makes it look like you are turning toward the south, even though your heading has not changed.
Context Anchor
Seen in instrument flying when learning magnetic compass errors and when cross-checking the compass during changes in airspeed.
Derivation
Acceleration comes from Latin roots meaning “to make quicker.” Error comes from a root meaning “to wander” or “go off course.” Together, the term points to a compass indication that wanders away from the correct reading when the aircraft changes speed.
Why Pilots Care
It produces false heading indications that can lead to incorrect course corrections if the pilot does not anticipate the error.
Grounding Statement
Picture flying straight east and adding power: for a short time, the compass may swing as if the nose is moving north even though the airplane is still going straight.
Intuition Check
Acceleration error does not mean the pilot made an error while accelerating. It means the compass itself gives a temporary wrong indication because the aircraft’s speed is changing.
Example Sentence 1
While flying west and adding power to climb, the pilot noted the compass swing toward north and recognized it as acceleration error rather than an actual turn.
Example Sentence 2
The student learned to ignore the momentary swing caused by acceleration error before resuming the assigned heading.