What does the airspeed indicator measure?

The airspeed indicator is specifically designed to measure the indicated airspeed (IAS) of an aircraft. Indicated airspeed is the reading displayed on the airspeed indicator and reflects the aircraft's speed relative to the surrounding air. It is an important parameter for pilots, as it helps them understand the performance of the aircraft, including stall speeds, maneuvering speeds, and climb rates.

Indicated airspeed is derived from the difference between static pressure and dynamic pressure—where dynamic pressure is generated by the airflow over the aircraft's surfaces. This measurement is vital for safe flying since it directly correlates with the aerodynamic behavior of the aircraft in various phases of flight.

While true airspeed (the actual speed of the aircraft relative to the airmass) and ground speed (the speed of the aircraft relative to the ground) are important for navigation and performance calculations, these are not directly measured by the airspeed indicator. Instead, they require corrections for altitude, temperature, and wind conditions, which the indicated airspeed does not account for. Vertical speed is also not relevant here, as it pertains to the rate of climb or descent rather than speed through the air.