What is the purpose of the heading indicator's reset every 15 minutes?

The purpose of the heading indicator's reset every 15 minutes is to compensate for drift. This drift is primarily due to the inherent limitations of the magnetic heading system. Even though the heading indicator is calibrated at the beginning of a flight, it can deviate over time due to factors such as the Earth's magnetic field variations and the gyroscopic precession inherent in magnetic compasses. Resetting the indicator at regular intervals helps ensure that pilots maintain an accurate heading reference, crucial for navigation during flight.

While other factors such as temperature variation and magnetic variation can influence navigation systems, they do not directly relate to the need for frequent resets of the heading indicator. Altitude accuracy is managed by altimeters, not the heading indicator, making it unrelated to the question. Therefore, focusing on the aspect of drift provides a clear understanding of why resetting the heading indicator regularly is essential for accurate navigation.