What is the purpose of the airspeed indicator?

The primary purpose of the airspeed indicator is to display the aircraft's speed relative to the surrounding air. This measurement, known as indicated airspeed (IAS), is critical for several reasons. It helps pilots ensure that they are flying within the aircraft's operational limits, as different speeds correspond to various phases of flight and performance characteristics, such as stall speed and maximum speed. Additionally, knowing the aircraft's speed relative to the air is essential for maintaining control during maneuvers, takeoff, and landing.

Airspeed affects aerodynamic forces acting on the aircraft, such as lift and drag, making it a vital piece of information for safe and efficient flying. Understanding airspeed relative to the surrounding air also assists in performance calculations and helps in determining safe aircraft handling in varying weather conditions.

The other choices don't relate directly to the purpose of the airspeed indicator. Altitude above sea level is measured by the altimeter, the rate of climb or descent is portrayed by the vertical speed indicator, and wind speed is not indicated by any direct instrument but rather inferred from ground speed and airspeed in relation to the aircraft's heading and the wind condition.