What does the altimeter measure?

The altimeter is specifically designed to measure the altitude of the aircraft above sea level, which is crucial for safe navigation and compliance with air traffic control regulations. It operates by sensing changes in atmospheric pressure; as an aircraft ascends, the atmospheric pressure decreases, and the altimeter reflects this change by indicating a higher altitude. This measurement is critical for pilots in ensuring they maintain proper separation from terrain and other aircraft, and for adhering to flight levels during their journey.

In contrast, other options relate to different aspects of aviation instrumentation. The speed of the aircraft is measured by aairspeed indicator, engine performance is monitored with engine instruments, and the direction the aircraft is facing is indicated by a heading indicator. Each of these instruments serves distinct functions that complement the overall operation of an aircraft, but they do not measure altitude like the altimeter does.