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

The primary purpose of a Course Deviation Indicator (CDI) is to show the aircraft's lateral position relative to the intended path. This instrument is crucial for navigation, especially during instrument flight, as it helps pilots determine if they are on course or if they need to make adjustments to their heading. The CDI visually represents the aircraft's deviation from the desired flight path, enabling pilots to make timely course corrections to ensure they stay on track to their destination. This function is vital for maintaining the correct flight route and avoiding potential navigational errors.

In contrast, monitoring fuel levels during flight is essential for flight management but is not the function of a CDI. Similarly, indicating the correct altitude for approach is managed by different instruments, such as altitude indicators or altimeters, rather than the CDI. Adjusting cabin pressure during ascent is a function of cabin pressurization systems and has no relation to the navigation provided by a CDI. Thus, the design and function of the CDI directly align with helping pilots navigate effectively by displaying their lateral position concerning the intended flight path.