Deceleration error in a magnetic compass occurs on which headings and in which direction does the compass card rotate?

• A. On northerly headings, the compass rotates toward East.
• B. On southerly headings, the compass rotates toward West.
• C. On easterly or westerly headings, the compass rotates toward South.
• D. On easterly or westerly headings, the compass rotates toward North.

Answer: (C)

When a magnetic compass experiences deceleration, it’s the inertia of the suspended magnet and the damping fluid that causes the needle to lag behind the changing motion. On easterly or westerly headings, this lag results in the compass card deflecting toward the south as you slow down. The effect is tied to the orientation of the Earth's magnetic field relative to the aircraft’s track, so the same deceleration on those headings produces a southward deflection, while accelerating would produce the opposite direction. On northerly or southerly headings, the deflection pattern due to deceleration isn’t the same, so the characteristic southward shift on east/west headings specifically identifies this deceleration error.