What does an airspeed indicator measure?

The airspeed indicator specifically measures indicated airspeed, which is the speed of the aircraft relative to the surrounding air. This reading is crucial for pilots as it helps determine how the aircraft will perform in various phases of flight, such as takeoff, cruising, and landing. Indicated airspeed is vital for ensuring that the aircraft is operating within safe and effective speed limits, which affect lift generated by the wings and overall flight performance.

Other measurements mentioned, such as altitude above sea level, ground speed, and vertical speed during a climb, are provided by different instruments. Altitude is indicated by an altimeter, ground speed is calculated using GPS or other navigation systems, and vertical speed during a climb is monitored with a vertical speed indicator. Therefore, the airspeed indicator's primary function is to provide the crucial information of indicated airspeed, making it the correct answer.