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→Air speed indicator
=== Air speed indicator ===
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 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 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 ====
The ASI is a pressure instrument, it works by measuring the total pressure and static pressure simultaneously. If you observe an ASI you will see it is plumbed both onto the pitot tube and the static port. An ASI is able to subtract these two readings, therefore, the dynamic pressure \(p_D\) can be obtained.
By using the formula introduced above, we obtain:
\[ V=\sqrt{\frac{2p_D}{\rho}} \]
If the density of the air is known. In an ASI, the density of air is assumed to be a constant, and this constant is implemented mechanically into the architecture.
==== Errors: TAS & IAS ====
