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→Air speed vs. ground speed
An '''air speed indicator (ASI)''' is used to measure the '''airspeed''' of an aeroplane. The airspeed measured from an ASI is known as "'''indicated airspeed'''".
==== Air speed Airspeed vs. ground speed ====
Airspeed is the aeroplane's speed relative to the surrounding air (contrasted with '''ground speed''', which is the speed relative to the ground). The reason air speed airspeed can differ from ground speed is because the air itself can move, known as wind. If you fly at an airspeed of 40kts directly into a 40kts headwind, your ground speed will be zero, i.e. looking from the ground you will not be moving. This is because you move forward relative to the surrounding air, but the air is itself moving backwards relative to the ground, so the two effects cancel each other.
==== Function ====
