What is the function of a Course Deviation Indicator (CDI)?

The Course Deviation Indicator (CDI) is specifically designed to assist pilots by displaying the aircraft's lateral position concerning the desired course or track. When navigating, pilots use the CDI to determine whether they are on the intended flight path or if they need to make adjustments to stay on course. If the aircraft deviates from the desired track, the CDI will indicate this drift, allowing the pilot to take corrective action to return to the correct path.

This functionality is essential in instrument navigation, where visual references are often not available. By monitoring the CDI, pilots can ensure they remain on course, which is particularly crucial when flying in poor visibility conditions or when navigating towards waypoints or approaches. Thus, understanding how to use the CDI effectively is a fundamental skill for instrument-rated pilots.