What does a compass indicate when the aircraft decelerates while turning?

When an aircraft is turning, the compass indicates the direction based on the aircraft's heading relative to the Earth's magnetic field. However, if the aircraft is decelerating during a turn, the compass can exhibit certain errors, particularly the phenomenon known as magnetic dip and the resulting compass lead and lag.

In a turn, if the aircraft is decelerating, the compass will tend to lag behind the actual heading. When an aircraft is in a left turn (which is typically associated with a deceleration), the compass indicates a turn toward the south due to the lag. This occurs because the compass card is influenced by the inertial forces acting on the aircraft, which can cause the compass to show a position that leads or lags the true heading, especially in relation to the established magnetic declination and the changing dynamics of turn rates and velocities.

Therefore, as the aircraft decelerates while turning, the compass may inaccurately suggest a westward turn when, in fact, the aircraft is turning towards the south, making the indication shown in the answer correct.