Airspeed Indicators : What To Know

An Airspeed Indicator is an instrument that measures the speed of an aircraft relative to the surrounding air through the difference between the pressure of still air (static pressure) and the moving air compressed by the aircraft’s forward motion (ram pressure). As speed increases, so too does the difference in these two pressures. This blog will explain how the airspeed indicator works as well as its greater role in an aircraft instrumentation system.

Airspeed indicators employ a Pitot tube to measure the air pressure differential. A Pitot tube is a U-shaped apparatus with two openings. One opening is perpendicular to the flow of air past the aircraft and the other faces directly into the flow. A bend in the tube is filled with mercury or a similar fluid, forming parallel columns balanced by the air pressure on each side. When static and ram pressure are the same, the two columns have the same height. As ram pressure increases, the mercury on that side of the tube is pushed back, causing the columns to become unbalanced. The difference of the columns can then be calibrated to indicate the speed. This value is known as indicated airspeed and can be represented in knots, miles per hour, and other measurements. For example, faster jet aircraft employ indicators that measure airspeed relative to the speed of sound, in a unit called Machmeters.

In two-pilot aircraft, both pilots have an ASI, each fed from an independent Pitot-static system. Should one system fail, both ASIs can be fed from the same Pitot tube. Most commercial aircraft also have a third ASI as a backup. On older aircraft, airspeed is indicated to the pilot via a graduated scale over which a pointer moves. Modern aircraft indicate airspeed on a speed tape which forms part of the electronic flight instrument system display. Calibration of the airspeed is critical to its operation, but because it is calibrated at a standard temperature and pressure, its readings are inaccurate at different temperatures and altitudes. An uncorrected indicated airspeed can still be used to estimate an aircraft’s tendency to stall.

There are two types of airspeed: indicated airspeed and true airspeed. The speed listed on the ASI is the indicated airspeed, and represents the speed of the aircraft relative to the body of air through which it is flying. True airspeed, however, is the airspeed corrected for altitude and temperature. Of the two, Indicated airspeed is more important to the pilot from an aerodynamics point of view. This is because true airspeed varies with altitude and temperature, while indicated airspeed does not. Therefore, critical speeds such as stalling speed and structural limit speeds remain constant regardless of ambient conditions.

Indicated airspeed depends on the density of the air, which changes with ambient temperature and pressure. As such, indicated airspeed is only the measure of the speed over the ground at sea level in conditions such as 1013.2 hPa (hectopascal pressure units, used to measure air pressure) and temperatures higher than 15 degrees celsius. In all conditions but these, the indicated airspeed will differ from the true airspeed. For instance, at an elevation of 30,000 feet, the true airspeed will be over 50% higher than the indicated airspeed. This is why calculations must be made to note the difference between indicated and true airspeed. There are instruments that record true airspeed, though they are still very uncommon in commercial aircraft.

When True Airspeed exceeds 300 knots, another factor must be accounted for. At this speed, the air entering the Pitot head begins compressing, causing the airspeed indicator to overread. The amount by which the indicator overreads is dependent on a combination of the indicated airspeed and ambient conditions. This error can be corrected for if desired, and other errors within the Pitot system can be reduced greatly by careful situating of the Pitot tube and static vents, as well as by accurate calibration.

A third type of speed, known as ground speed, also exists. Ground speed is the horizontal speed of an aircraft relative to the ground. An aircraft moving vertically has a ground speed of zero. Ground speed is calculated by the sum of the aircraft’s true airspeed and the current wind speed and direction. It is not commonly used by pilots and operators, though in commercial aircraft, the speed displayed to passengers through the aircraft entertainment system is often given in ground speed.



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February 25, 2020

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