Which instrument primarily indicates the aircraft's altitude?

The instrument that primarily indicates the aircraft's altitude is the altimeter. This device measures the atmospheric pressure outside the aircraft and translates that pressure reading into an altitude reading. As the aircraft ascends or descends, the changes in atmospheric pressure are reflected in the altimeter, allowing pilots to determine their altitude above sea level.

In the context of aviation, the altimeter is crucial for maintaining proper flight levels, especially in controlled airspace where altitude separation is vital for safety. It provides pilots with essential information for navigation and ensuring the aircraft remains at a safe altitude in relation to terrain and other air traffic.

While the vertical speed indicator provides information about the rate of climb or descent, it does not directly indicate altitude. The airspeed indicator shows how fast the aircraft is moving through the air, and the attitude indicator displays the aircraft's orientation relative to the horizon. These instruments serve different purposes and are integral to flight operation but do not indicate altitude like the altimeter does.