Definition
A magnetic compass error that occurs when an aircraft decelerates while flying on an easterly or westerly heading, causing the compass to momentarily indicate a turn toward the south. The error is caused by the magnetic dip of the compass card reacting to the change in speed, and it is most pronounced on east/west headings, absent on north/south headings.
Plain English
When you slow down while heading east or west, the magnetic compass briefly shows a false turn toward the south, even though you haven't actually turned.
Context Anchor
Seen in instrument flying when learning magnetic compass limitations and when comparing the compass to other heading instruments.
Derivation
From Latin 'decelerare' (to slow down), formed from 'de-' (away from) and 'celer' (swift). The error occurs specifically during deceleration, hence the name.
Why Pilots Care
The false nose-down indication can prompt an unnecessary nose-up correction, resulting in altitude gain or entry into an unusual attitude.
Intuition Check
Deceleration error does not mean the airplane is making a bad slowdown. It means the compass can give a false direction indication while the airplane is slowing.
Example Sentence 1
After reducing power on a westerly heading, the student noticed the deceleration error showing a false southerly turn on the magnetic compass.
Example Sentence 2
The instructor demonstrated deceleration error by reducing power in level flight and showing how the attitude indicator briefly displayed a descent.