Aircraft Speedometers - Surprisingly Simple

Airspeed is the measurement of the speed a plane is traveling on its own, without help from tailwind or other factors. Rather than being measured by an intricate system of electrical parts, airspeed is found using a type of differential pressure gauge called a pitot tube. The tube has an open end which, when mounted on the wing, faces toward the flow of air or water. The airspeed indicator works by measuring the difference between a fixed sensor away from the air stream and a sensor, the pitot tube, in the air stream. While the aircraft is still, the pressure in each tube is equal and the airspeed indicator displays zero.

During flight, the flow of air into the pitot tube causes a pressure differential between the two sensors. This pressure differential is what causes the indicator to move. Air pressure then pushes against the diaphragm that moves a connected mechanical pointer on the speedometer. Each indicator is adjusted to compensate for airwinds to provide an accurate airspeed. In addition to this, most aircraft have electronics to account for altitude and air temperature while calculating an accurate air speed measurement. In the event that the pitot tube becomes blocked by insects, dirt, or other in-flight debris, air cannot enter the system. If this is the case, the system will drop to ambient pressure and the speedometer will read zero.

Because the maximum speed of jet aircraft is measured in knots and Mach, pilots need a speedometer and a Machmeter. A Machmeter measures the ratio of airspeed to the speed of sound called a Mach number. It appears on the Machmeter as a decimal number. The speed of sound is a common standard of airspeed measurement, and is expressed as Mach 1.

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