What is the function of a course deviation indicator (CDI)?

The function of a course deviation indicator (CDI) is to indicate the aircraft's position relative to the desired track. This is a critical tool in aviation navigation, as it allows pilots to visually monitor their alignment with the planned flight path. The CDI works by providing information on whether the aircraft is on course or if it needs to adjust its heading to stay on track.

When the aircraft deviates left or right from the intended course, the CDI displays the direction and extent of the deviation. This guiding information is essential for maintaining an accurate flight path, especially during approaches and landings or when navigating through controlled airspace.

The other options do not pertain to the CDI's purpose. For instance, measuring fuel efficiency or displaying aircraft speed involves different instruments and systems specific to performance monitoring, while maintenance status is indicated through separate systems that monitor aircraft health and diagnostics. The CDI's sole focus is on navigation and positioning with respect to the desired course.