What is a deceleration error in the context of magnetic compasses?

In the context of magnetic compasses, a deceleration error specifically refers to the phenomenon where the compass indicates a turn towards the South when an aircraft is decelerating. This occurs because the magnetic compass relies on the Earth's magnetic field for its readings and is sensitive to changes in the aircraft's dynamics, including speed and heading changes. When an aircraft decelerates, the compass may momentarily show a heading that does not accurately reflect the aircraft's true orientation.

As the aircraft slows down, the inertia can cause the compass card to rotate towards South due to the magnetic dip. This behavior can mislead the pilot, especially when making navigational decisions based on the compass reading immediately after a deceleration. Understanding this error is crucial for pilots using magnetic compasses, as it highlights the need to interpret compass indications carefully during speed transitions.

The other options describe different types of compass errors, such as errors related to acceleration or magnetic interference, but they do not accurately capture the essence of what a deceleration error is in this context.