When a pilot is conducting a flight, it is paramount that they have ample data on various flight conditions ranging from altitude to heading. Airspeed in particular is very beneficial for safety, efficiency, and flight planning, and such values are measured through the use of the pitot-static system. Also supporting altitude and vertical speed indicators, the pitot-static systems garners readings of outside air pressure to create measurements that are computed by air data computers and transponders. To help you better understand how airspeed values are measured, we will provide a brief overview of the pitot-static system and how it creates readings for the airspeed indicator.
The pitot-static system is a collection of pressure-sensitive instruments, and it typically consists of a pitot tube, static port, and various cockpit instruments. The pitot tube is designed to collect pitot pressure, that of which is a measurement of ram air pressure (the pressure created as the aircraft moves through the air). To obtain accurate readings, the pitot tube is most often placed on a wing or the front of the fuselage where it is in contact with the relative wind. Meanwhile, the static port obtains static pressure, and it differs from the pitot tube in its placement in a relatively undisturbed area. Aircraft may vary in the amount of static ports that they feature, and they are generally a flush-mounted hole.
Unlike various cockpit instruments that may solely rely on either the pitot tube or static port, the airspeed indicator is the only one that uses both. During flight, static pressure will be directed into the instrument casing, while ram air from the pitot tube is sent to a diaphragm within the casing. As the pitot pressure causes the diaphragm to expand, the static pressure will act against it as it surrounds the device. Meanwhile, the diaphragm is connected to the instrument pointer with a mechanical linkage, and the expansion and contraction of the diaphragm will cause the pointer to adjust. When the aircraft is stationary on the ground, the indicator will generally show a reading of zero, this value increasing as the aircraft speeds up.
As the airspeed indicator relies on the entire pitot-static system to obtain readings, it is extremely important that neither the pitot tube or static port face blockage. If the pitot tube is blocked, the airspeed indicator will show a zero reading. Meanwhile, a blockage in the static port will make it so that airspeed indications are inaccurate. If both happen to be blocked, the pointer will remain at zero.
It is important to note that the reading taken from the airspeed indicator is known as the indicated airspeed (IAS), and this means that no corrections have been made to account for air density, instrumentation errors, and more. When correcting installation and instrument errors, the calibrated airspeed (CAS) is found. Meanwhile, altitude and nonstandard temperature corrections net the true airspeed value (TAS), that of which is used for flight planning.
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