What device is most commonly used to show how fast an aircraft is traveling through the air?

The airspeed indicator is the device most commonly used to show how fast an aircraft is traveling through the air. This instrument measures the aircraft's speed relative to the surrounding air, providing critical information for pilots to ensure safe and efficient flight operations. The airspeed is expressed in knots or miles per hour, depending on the aircraft and its operating environment.

Understanding the airspeed is vital for various aspects of flying, such as calculating lift, optimizing fuel consumption, managing stall speeds, and ensuring compliance with airspeed restrictions during takeoff and landing. It gives pilots the necessary information to perform maneuvers safely and effectively.

In contrast, while the altimeter measures the aircraft's altitude, the tachometer provides engine speed in revolutions per minute, and the vertical velocity indicator indicates the rate of climb or descent. Each of these instruments serves a different purpose in flight but does not measure the aircraft's speed through the air.