The Airspeed Indicator (ASI) simple measures the speed of the aircraft. As the aircrafts speed increases or decreases the indication will change, similar to the speedometer in your car. The ASI is designed to move air from the pitot tube into the diaphragm, while static pressure is introduced into the airspeed case.
There are different types of airspeed associated with the airspeed indicator.
Indicated airspeed (IAS) - The airspeed of the aircraft that is directly read by the pilot from the ASI.
Calibrated airspeed (CAS) - Is indicated airspeed (IAS) corrected for any instrument errors.
True airspeed (TAS) - Is when Calibrated airspeed is corrected for altitude and nonstandard temperature. This is due to the air density decreasing at higher altitudes. To maintain the same pressure differences between the pitot pressure and static pressure, aircraft needs to fly faster at higher altitudes.
Ground speed (GS) - Is the actual speed of an aircraft over the ground. It is True airspeed adjusted for wind corrections.
What to look for
The ASI should be checked prior to takeoff and read zero. Be aware, the ASI may show an indication higher than zero, if there is a strong wind blowing directly into the pitot tube. On the takeoff roll visually check to see the airspeed is increasing.
Errors
If the pitot system and or the static system becomes blocked by either moister, ice, dirt, bugs, etc. it will affect the airspeed indicator as well as the altimeter and the vertical speed indicator. Some aircraft are equipped with an alternate static source in the flight deck area. By opening the alternate static source, static pressure is drawn from the flight deck into the system.
Remember:
If either the pitot tube or static port appears to be blocked or something is lodged inside i.e., bug, never try to clear it yourself, have a certified mechanic check it.
References:
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