Which instrument measures the aircraft's altitude?

The altimeter is the instrument that measures the aircraft's altitude, as it determines the aircraft's height above a reference point, typically sea level. It does this by measuring changes in air pressure; the atmospheric pressure decreases with altitude, thus allowing the altimeter to provide a reading of the aircraft's altitude in feet or meters.

Understanding the function of the altimeter is essential for pilots, as maintaining the correct altitude is critical for safety and navigation. The altimeter is calibrated to the local pressure setting at sea level, allowing for accurate altitude readings based on variations in atmospheric pressure.

In contrast, the airspeed indicator measures the speed of the aircraft relative to the surrounding air, the vertical speed indicator indicates the rate of climb or descent, and the heading indicator shows the aircraft's direction of flight. None of these instruments provide altitude information, which distinguishes the altimeter as the correct choice for measuring altitude.