What is the primary function of an altimeter?

The primary function of an altimeter is to gauge the altitude of an aircraft above sea level. This instrument is crucial for pilots, as it allows them to maintain safe flying heights and avoid obstacles such as mountains and buildings. Altimeters operate using various methods, including barometric pressure measurements; as the aircraft climbs or descends, the change in atmospheric pressure is translated into altitude readings. Accurate altitude measurements are essential for safe air travel, particularly during takeoffs, landings, and when navigating through controlled airspaces. Understanding an aircraft's altitude helps in maintaining proper separation from other aircraft and terrain.

In contrast, measuring speed, controlling navigation, or indicating fuel levels pertain to different instruments and functions critical to the overall operation of an aircraft but do not define what an altimeter does.