Deceleration error in a magnetic compass is characterized by which of the following behaviors?

• A. A magnetic compass error that occurs when the aircraft accelerates on easterly or westerly headings, causing the card to rotate toward North.
• B. A magnetic compass error that occurs when the aircraft decelerates while flying on northerly or southerly headings, causing the card to rotate toward East.
• C. A magnetic compass error that occurs when the aircraft decelerates while flying on an easterly or westerly heading, causing the compass card to rotate toward South.
• D. A magnetic compass error that occurs due to altitude changes.

Answer: (C)

Deceleration error on a magnetic compass comes from the inertia of the suspended, magnetized card and how it interacts with the Earth's magnetic field. When you slow down on an easterly or westerly heading, the card tends to lag and deflect toward the south. This is the deceleration side of the acceleration/deceleration error, which produces opposite deflections depending on whether you’re accelerating or decelerating and on which heading you’re flying. So, the reading described—decelerating on an easterly or westerly heading causing the compass card to rotate toward the south—is the characteristic behavior of this error. The other patterns described don’t match this same heading and deceleration interaction, and altitude changes don’t cause this compass error.