What is the purpose of the airspeed indicator?

The airspeed indicator serves a critical function in aviation by showing the speed of the aircraft relative to the air through which it is moving. This information is crucial for pilots as it helps them understand how the aircraft performs in different phases of flight, including takeoff, landing, and cruising. The indicated airspeed is vital for ensuring the aircraft is operating within safe and efficient speed ranges, which can affect control, lift, and drag.

Comparatively, the other options highlight different instruments or measurements that do not pertain to the airspeed indicator's function. For instance, measuring altitude is the role of the altimeter, while the measurement of distance traveled is typically indicated by a navigation system or an odometer, not the airspeed indicator. Additionally, detecting changes in wind speed relates more to weather systems or anemometers, rather than airspeed measurement. Thus, the correct focus on relative airspeed is key to understanding the airspeed indicator's primary purpose.