Definition
A temporary error in the magnetic compass indication caused by the inertia of the compass card during a rapid decrease in airspeed on an easterly or westerly heading. When decelerating, the compass card briefly tilts in a way that makes the compass indicate a turn toward the south, even though the aircraft is still flying straight.
Plain English
When you slow down quickly while heading east or west, the compass briefly shows a false swing toward the south. The aircraft hasn't actually turned -- the compass is just reacting to the change in speed.
Context Anchor
Seen in instrument flying when using the magnetic compass for compass turns or when checking heading changes during speed changes.
Derivation
From Latin 'decelerare' -- 'de-' meaning down or away, and 'celer' meaning swift. So deceleration literally means 'slowing down from swift.' The error is named for the moment it occurs: while the aircraft is decelerating.
Why Pilots Care
Failing to recognize these swings can cause unnecessary heading corrections and loss of situational awareness.
Grounding Statement
If you slow down while flying east or west in the Northern Hemisphere, expect the magnetic compass to swing falsely toward south.
Intuition Check
Deceleration error does not mean the pilot made a mistake while slowing down. It means the magnetic compass itself gives a false indication because the aircraft is slowing.
Example Sentence 1
After reducing power to begin the descent on a westerly heading, the student saw the compass swing south and remembered that deceleration errors can mimic a turn.
Example Sentence 2
After slowing the aircraft on a west heading the pilot waited for the deceleration error to settle before reading the compass.