What is the function of the altimeter in an aircraft?

The altimeter's primary function is to measure the aircraft's altitude above sea level. It does this by sensing changes in atmospheric pressure as the aircraft ascends or descends. The altimeter is calibrated to interpret these pressure changes and convert them into altitude readings based on the standard atmospheric model. This is crucial for safe flight operations as it helps pilots maintain the correct altitude, avoid obstacles, and comply with air traffic control requirements.

Other options do not pertain to the altimeter's role. For instance, speed measurement is the function of the airspeed indicator, while heading is indicated by the heading indicator or magnetic compass. Monitoring engine performance is the responsibility of engine instruments rather than the altimeter, making the understanding of each instrument's purpose vital for effective flight operations.