Three avionics instruments are the airspeed indicator (left), the altimeter (center),and the vertical speed indicator. The airspeed indicator is the aircraft's "speedometer." The altimeter measures pressure and displays this as height above sea level. The vertical speed indicator displays the rate of change in altitude. [larger image]Avionics refers to the electronic instruments on a fixed-wing or rotary aircraft or spacecraft that perform a variety of functions. These include navigation instruments; control instruments that aid in steering and controlling the craft; power indicators such as tachometers, torquemeters, and exhaust pressure ratio indicators; and performance indicators such as altimeters, vertical velocity indicators, and velocity gauges. The word is derived from the expression "aviation electronics" and also refers to the design and production of these instruments.