An airspeed indicator is an instrument in the cockpit of an airplane or helicopter that indicates to the pilot how fast the aircraft is traveling relative to the air outside. Airspeed indicator calibration is usually measured in knots (nautical miles per hour), which are units of speed equivalent to 1.852 kilometers per hour (about 1.151 miles per hour). Airspeed indicators are extremely important instruments that are used at all stages of flight, including taking off, climbing, cruising, descending, and landing. Different types of aircraft have different rules about what speed the pilot should aim for during each stage of flight, and the aircraft airspeed indicator helps the pilot to fly safely according to these rules.
Aircraft airspeed indicators typically measure speed indirectly, by using air pressure measurements. Speed is calculated by measuring the difference between static air pressure and impact air pressure. Static air pressure describes the normal pressure of the air at the height at which the aircraft is traveling, and is usually measured on an aircraft via a static port, located on the outside of the aircraft in a location where the air is relatively undisturbed. Impact pressure, also called pitot pressure, is measured via a tube, called a pitot tube. This tube is typically located on the front edge of the wing, or on the nose, where the force of the air generated by the movement of the aircraft can be measured.
An airspeed indicator forms part of the pitot-static system of the aircraft in which it is located. The pitot-static system functions by means of a number of devices that are sensitive to pressure. The system will typically includes a pitot tube, a static port, the aircraft’s airspeed indicator, and a number of other instruments, including an altimeter, which the pilot uses to determine how high above sea level he or she is flying.
It is important to understand that an airspeed indicator does not display the speed at which the aircraft is moving relative to the ground. It only measures the airplane's speed relative the air outside the plane. This is significant, because an aircraft's ground speed is irrelevant its performance in the air. Airspeed is not only affected by the aircraft's movement, but also by the speed of the wind. Consequently, while the airspeed at which a specific aircraft takes off will usually be the same, the ground speed will very greatly, depending on the direction and speed of the wind on a given day.