Describe the function of a course deviation indicator (CDI).

The function of a course deviation indicator (CDI) is to show the aircraft's lateral deviation from the desired flight route. This instrument is crucial for navigating accurately, especially when flying along predefined airways or toward navigation aids.

When pilots set a CDI for a specific course, it provides visual guidance on whether the aircraft is on track or if adjustments are needed. If the indicator shows that the aircraft is left or right of the intended course, the pilot can take corrective actions to return to the desired path. This lateral guidance helps maintain situational awareness and ensures a successful navigation experience, which is particularly vital in instrument flight conditions where visibility is limited.

The other functions listed, such as displaying real-time speed, indicating fuel level, or measuring altitude, pertain to different instruments in the cockpit and do not represent the primary role of the CDI in navigation. Each of those functions is served by their respective instruments designed specifically for speed, fuel, or altitude measurement.