Understanding Your Airspeed
In its purest form, an air speed indicator is simply a differential pressure gauge. This means that it is displaying the difference between two different pressure sources that are being simultaneously applied. In the case of airspeed, the two pressures are Pitot Pressure and Static Pressure.
Pitot Pressure is the pressure that is generated when air is forced into the aircraft Pitot tube because of the forward motion of the aircraft. The Pitot tube is a slender tube that is typically mounted near the front of the aircraft. The opening of the tube is facing forward so that air is forced into it. This tube is connected directly to the input of the air speed indicator.
Static pressure is the standard air pressure at the current altitude. Static pressure will vary due to altitude changes and due to changes in weather.
Standard Static pressure at sea level altitude on a “standard” day is typically 29.92 In Hg (inches of mercury) as measured on a class “A” barometer. Static pressure on an aircraft is measured at the static port. The static port is a small hole usually located on the side of the aircraft. This pressure is applied to all instruments in the aircraft static system, of which the air speed indicator is one.