The airspeed indicator is an indispensable cockpit instrument. It’s one of the few required by the regulations to be operational on all types of flights, which demonstrates its significant role in keeping us safely flying. A quick glance at aviation’s speedometer can tell us much about the current state of the aircraft.
The airspeed indicator is the only flight instrument that uses the pitot tube. It takes the air pressure that enters the front of the pitot tube (more speed means more pressure) and directs it to the back of the instrument. There, it meets a closed disc or diaphragm that sits inside the back case. That case is sealed, except for a small hole that is directly connected to the static port. As a result, the pressure that enters the pitot tube is compared to the static air pressure.
The differential pressure is the key to the instrument’s operation. A gear or series of gears is attached to the diaphragm, which is then connected to a needle that indicates on the face of the instrument. What comes out on the calibrated face is the speed of the airplane through the air.
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