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

The primary purpose of a Course Deviation Indicator (CDI) is to show lateral position relative to a desired course. The CDI is an essential instrument in navigation, particularly when using systems like VOR (VHF Omnidirectional Range) or GPS. It displays the aircraft's position in relation to the intended flight path or course, allowing pilots to see whether they are on track, left or right of the course line. This information is crucial for maintaining accurate navigation, especially during approaches, en route, or when making course corrections.

Other options pertain to different instruments or systems: monitoring fuel levels is generally done by fuel gauges, altitude changes are measured by altimeters, and engine performance data is tracked by engine monitors. Thus, while those aspects are important for overall flight safety and performance, they do not relate to the specific function of the CDI in navigation. The CDI directly assists pilots in maintaining their intended course effectively.