What is the function of an altimeter in an aircraft?

The function of an altimeter in an aircraft is to determine the altitude above sea level. Altimeters operate based on atmospheric pressure, which decreases as altitude increases. When an aircraft climbs to higher altitudes, the air pressure decreases, and the altimeter converts this pressure change into a corresponding altitude reading. This is crucial for maintaining safe flight operations, particularly in controlled airspace and while conducting approaches and landings, where precise altitude information is necessary to avoid terrain and obstacles.

The other choices relate to different functionalities that are served by different instruments in the cockpit. For example, measuring airspeed involves a pilot's airspeed indicator, while fuel levels are typically monitored through fuel gauges. Navigation information is provided by various navigation systems and instruments, which guide the pilot in determining their position and trajectory during flight. Understanding the specific role of each instrument is vital for effective aircraft operation.