Which instrument indicates the aircraft’s altitude?

The altimeter is the instrument specifically designed to indicate the aircraft's altitude. It measures the atmospheric pressure outside the aircraft and translates that measurement into an altitude reading. This is based on the principle that air pressure decreases with an increase in altitude. As the aircraft ascends or descends, the pressure changes, and the altimeter reflects this change to provide a direct reading of altitude.

In contrast, the airspeed indicator displays how fast the aircraft is moving through the air, while the rate of climb indicator shows the rate at which the aircraft is gaining or losing altitude, usually in feet per minute. The vertical speed indicator, on the other hand, quantifies the rate of ascent or descent, again measured in feet per minute, but does not provide a precise altitude reading. Each of these instruments serves an important role in flight, but the altimeter is the one specifically responsible for altitude measurement.