What must be periodically realigned with the magnetic compass to receive accurate indications during flight from a heading indicator?

The heading indicator, often referred to as a directional gyro, is a crucial instrument for navigation in an aircraft. It provides pilots with a stable reference for their aircraft's heading. However, over time and with movement, especially in turbulent conditions, the gyro system can experience precession, which means it can gradually drift out of alignment with the actual magnetic heading.

To ensure the heading indicator provides accurate readings, it needs to be periodically realigned with the magnetic compass. The magnetic compass, while it has its own limitations due to magnetic deviation and variation, represents true heading based on the earth's magnetic field and serves as a reliable reference for correcting any drift that may have occurred in the gyro.

Realigning the heading indicator with the magnetic compass is essential for navigating accurately, especially during maneuvers where precise heading information is crucial, such as during turns or when entering a traffic pattern.

The other instruments listed do not relate to the need for realignment with the magnetic compass in the same way the gyro system does. The barometric altimeter measures altitude, the vertical speed indicator indicates the rate of climb or descent, and the turn coordinator is primarily used for indicating the rate of turn, rather than heading navigation.