An airspeed indicator is an instrument that measures the speed of an aircraft relative to the surrounding air. It displays this by measuring the differential between static pressure, or the still air, and that of the air moving into a pitot tube, ram pressure. As speed increases a greater difference is detected and displayed on the airspeed indicator inside the cockpit. Airspeed indicators will commonly display airspeed in miles per hour (MPH) and nautical miles per hour (KNOTS). They also display important speeds for the aircraft:
The green arc is the normal operating range, the top of the green arc being the maximum structural cruising speed. The slower end of the green arc is the power off stall speed in clean configuration (VS1, Flaps up, and gear up is applicable).
The white arc is the flap operating range, the bottom of this white arc is VS0, which is the power off stall speed in landing configuration (flaps and gear down).
The yellow arc is the caution range. You should only fly at these speeds in very calm air. It runs from the top of the green arc to the red line.
The red line indicates the never exceed speed, VNE. You can fly this fast or faster if you don’t like having wings attached to the fuselage.
On most twin engine aircraft there is a blue radial on the airspeed indicator that shows the best rate of climb with one engine inoperative.
Maneuvering speed (VA) is not shown on the airspeed indicator, but this is the speed at which it is safe to stall the aircraft. It is, however, listed in the aircraft flight manual. A good rule of thumb is to take 1.7 times the normal stall speed.