What does the airspeed indicator show?

The airspeed indicator displays the current speed of the aircraft relative to the surrounding air. This measurement is vital for several reasons: it helps pilots understand their performance in relation to aerodynamic principles, gauge stall speeds, and maintain proper flight control. When flying, the aircraft generates lift based on the air flowing over its wings, which directly relates to its speed through the air, not the ground. Consequently, the airspeed indicator is crucial for evaluating how efficiently an aircraft can perform maneuvers, as airspeed affects lift and drag.

Other options provided focus on different aspects of flight: the speed relative to the ground pertains to groundspeed, which can be influenced by wind; wind speed affecting the aircraft is an external factor that affects performance but is not reflected in the airspeed indicator; and the VSI measures vertical speed rather than airspeed. Each of these conveys important information for piloting but does not define the primary function of the airspeed indicator, which is specifically about the aircraft's speed through the air.