How an Airspeed Indicator Works

As an aircraft travels through the atmosphere, it is important for its pilot to be able to gauge its speed and estimated time of arrival, in addition to other factors, for the benefit and safety of passengers and crew alike. Among the many monitoring systems present aboard aircraft, an airspeed indicator(ASI) takes advantage of the plane’s pitot-static system to provide the pilot with a real-time measurement of the speed of an aircraft. To learn how ASIs function, be sure to read on ahead.

The ASI (airspeed indicator) is a pressure instrument that works by accurately measuring both pitot pressure (total pressure) and static pressure. Its operation is simple; by subtracting the static pressure from the total pressure, one can obtain the dynamic pressure which is a function of velocity in accordance with Bernoulli’s equation. As such, the resulting velocity is a function of air density, but the pitot-static system does not give a real-time reading of this measure. Instead, the instrument is calibrated to sea-level density on a standard atmospheric day. This means that air density can only be properly measured by the ASI when it is operating in average weather conditions. Because there is frequent variation in atmospheric conditions, the indicated airspeed on the ASI is often not the same as the true airspeed.

Traditional mechanical ASIs have a body consisting of a sealed case. Their static pressure line is ported here which helps to maintain ambient atmospheric pressure. Connected to the pitot-pressure line is a capsule inside the instrument housing, that of which is kept at a set pressure. Since the pitot probe measures total pressure, which is a combination of dynamic and static pressure, the differential between the capsule and the case will give you the dynamic pressure. As dynamic pressure changes, the capsule accommodates this by expanding and contracting while also being mechanically linked to the instrument face. This will ultimately display a shifting dynamic pressure in the form of an airspeed value on the dial. Moreover, it is necessary to calibrate ASIs at average sea level conditions to fix the relationship between the dynamic pressure and resulting velocity.

Modern digital airspeed indicators also work using the same basic principles as their mechanical counterparts. Airspeed is measured digitally by using transducers that convert the resulting pressures into digital signals, often in the form of voltages. To determine dynamic pressure, these digital signals are subtracted before the start of a calibration exercise in which a relationship is defined between the resulting voltage output and airspeed. Digital ASIs still output their initial reading as an indicated airspeed, but they may also show the true airspeed by using a mathematical model that measures ambient air density, a function of temperature and altitude.

Those searching for high-quality airspeed indicators and other aircraft components are encouraged to get in contact with our team at Aviation Parts Circuit, where we are dedicated to our customers. You will be pleased to find that we source our products from trusted global manufacturers featured on our Approved Vendor List (AVL), in addition to subjecting countless parts to varying levels of quality assurance to guarantee their fit, form, and function. Moreover, those facing time constraints can rest easy knowing that we are able to expedite domestic and international shipping due to our expansive supply-chain network. Feel free to give us a call or email to learn more about the competitive pricing and rapid lead-times we offer on all of the items available in our inventory.


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