What is the primary instrument for indicating the aircraft's altitude?

The primary instrument for indicating the aircraft's altitude is the altimeter. The altimeter measures the aircraft's height above sea level by comparing the atmospheric pressure outside the aircraft to a calibrated standard pressure. As the aircraft ascends or descends, the external air pressure changes, and the altimeter adjusts accordingly, providing a visual indication of altitude. This instrument is crucial for flight safety, especially during landing and navigating at different flight levels.

In contrast, the vertical speed indicator measures the rate of climb or descent, providing information on how quickly the aircraft is gaining or losing altitude, but it does not indicate the actual altitude itself. The airspeed indicator shows how fast the aircraft is moving through the air, while the heading indicator provides information about the direction the aircraft is facing in relation to magnetic north. Each of these instruments serves a specific purpose in flying, but the altimeter is the dedicated tool for altitude measurement.