Definition
Temporary heading indications shown by a magnetic compass when the aircraft accelerates or decelerates while flying on an easterly or westerly heading. On acceleration, the compass shows a turn toward the north; on deceleration, it shows a turn toward the south. The errors are caused by the tilt of the compass card as inertia acts on its weighted, pendulous mounting in the Earth's magnetic field, and they are most pronounced at mid-latitudes and absent at the magnetic equator.
Plain English
When you speed up or slow down on an east or west heading, the magnetic compass briefly lies to you about your heading. It does not mean the aircraft has actually turned.
Context Anchor
Seen in magnetic compass use, especially during compass turns, takeoff, level-off, or any time the aircraft changes speed while the pilot is checking the compass.
Derivation
From Latin 'accelerare' meaning 'to hasten or speed up.' The term names the source of the error directly: it appears during changes in speed, not during turns.
Why Pilots Care
Uncorrected acceleration errors produce heading excursions that can lead to disorientation or deviation from assigned courses in instrument conditions.
Grounding Statement
If the aircraft is flying straight east or west and the speed changes, the compass may move even though the aircraft has not turned.
Intuition Check
Acceleration errors are not errors in how the airplane accelerates. They are errors in what the magnetic compass shows while the airplane is speeding up or slowing down.
Example Sentence 1
Flying west at night on partial panel, the student saw the compass swing north as he added power and correctly identified it as an acceleration error rather than a turn.
Example Sentence 2
While decelerating on final approach to a southerly runway the pilot anticipated the compass swing away from north and adjusted the rollout point accordingly.