What is the primary function of an altimeter in an aircraft?

The primary function of an altimeter in an aircraft is to determine the aircraft's altitude above sea level. An altimeter measures the atmospheric pressure outside the aircraft and converts that measurement into an altitude reading. This is crucial for pilots as knowing the altitude helps with navigation, maintaining safe distances from terrain, and ensuring compliance with air traffic control instructions.

In aviation, altitude is typically referenced to sea level, which is why the altimeter is calibrated to standard atmospheric pressure. When flying, the altimeter provides important information that allows pilots to understand their vertical position relative to the ground and other obstacles. This data is vital for safe flight operations, especially during takeoff, landing, and in crowded airspace.

The other choices do not accurately describe the function of an altimeter; an altimeter does not measure speed, calculate fuel efficiency, or monitor engine performance. Each of these other functions is served by different instruments and systems within the aircraft.