The airspeed indicator seems like a straightforward enough instrument at first glance. Theres only one needle, and it points to numbers on the gauge, and that's how fast youre
… going. It seems pretty easy, and it is! But there is much more information on the instrument than you might notice at first. For a start almost every aircraft's airspeed indicator is equipped with an adjustable outer ring which you can set for the current outside air temperature. This has the advantage of giving you "true airspeed" (corrected for temperature) as opposed to "indicated airspeed" Also, you may notice a series of colored arcs alongside the numbers on the dial. These arcs indicate ranges of air speeds where some maneuvers or devices are permitted or forbidden. For example, the green arc usually indicates the range of speeds where flaps can safely be used (with a white sub-arc showing where landing gear may be extended) extending the flaps or the landing gear at too high a speed may damage the mechanisms or the airframe. The bottom of these arcs would then be the aircrafts stall speed. The yellow arcs usually indicate maximum or cruising speeds (depending on turbulence factors) and a small red arc indicates the "never exceed" airspeed which is the aircrafts maximum design airspeed. Jet aircraft may have a separate gauge for displaying mach speed (which is dependent on altitude) this gauge is called a machmeter. (MORE) 
