Airspeed provides critical information to the aircrew about aircraft safety conditions during flight. Aircraft are expected to maintain level flight without exceeding certain speeds or flying under the standard limit. If the aircraft is going too slow, the aircraft may stall. Alternatively, if the aircraft is going too fast, the pilot risks structural failure. As such, an aircraft airspeed indicator is an integral part of the aviation industry. In this blog, we will cover the inner-workings of an aircraft airspeed indicator and its importance.
An airspeed indicator (ASI) is a differential pressure gauge that measures the dynamic pressure of the air through which an aircraft is flying. Typically expressed in knots (kn), this unit is the standard promoted by the International Civil Aviation Organization (ICAO). Moreover, an ASI’s display consists of a single needle that sweeps across a dial, starting with 0 at the 12 o’clock position. In order for an ASI to measure dynamic pressure, it must measure static air as well. Through the use of a pitot tube and static ports located on various parts of the aircraft, this pressure data can be acquired.
As an aircraft moves through the air, an airstream flows into the pitot tube, increasing the dynamic pressure. This movement is generally recognized as the ram air effect. Prior to taking off, an aircraft sits on the runway and its ram pressure is measured as a static element. However, once the aircraft is in motion, the ASI reflects the difference between the static pressure measured by the static ports on the fuselage and the ram pressure collected from the pitot tube on the wing or nose. The air from the pitot tube is delivered into a diaphragm inside the ASI that is surrounded by lower pressure air from the static port. Mechanical linkages between the diaphragm and ASI translate this movement into data displayed by radial motion of the ASI needle.
ASIs belong to a system of instruments recognized as the aviation “six pack” that include an attitude indicator, altimeter, turn coordinator, heading coordinator, vertical speed indicator, and of course, an airspeed indicator. The airspeed indicator, however, is unique in that its display features brightly colored arcs on the dial to indicate various airspeed ranges. A white arc indicates the range wherein the flaps on an aircraft should operate. Generally, this arc indicates the limits for landings and departures. A green arc shows the range for normal operating speed, and the upper limit of this range is the maximum cruising speed for an aircraft. A yellow arc should only be reached during optimal flying conditions and is recognized as a caution range. Lastly, a red arc is a range that should never be reached as this may lead to structural damage to the aircraft.
There are two types of ASIs popularly used for aircraft. The first is a steam gauge indicator which is the most commonly used and has been described above. The second type is a glass panel, or Electronic Flight Display (EFD), which is a digital display that compartmentalizes all the functions of the steam gauge onto one compact screen. The EFD displays airspeed in a vertical box called the airspeed tape which is situated to the left of the attitude indicator. Its compact design only allows for part of the numbers to be displayed, but has an arrow that indicates the current airspeed of the aircraft. Moreover, as previously discussed, the colors and their indications remain the same for both airspeed indicators.
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