Understanding Airspeed Types and Role of Airspeed Indicator (ASI)

In order for pilots to carry out safe and efficient flight operations, they must be able to monitor and track various forms of information on flight conditions and system functionality. Among the types of data that is tracked and used during flight, airspeed in particular is very crucial as it allows pilots to make informed decisions in relation to navigation, heading adjustment, and flight planning.

In this blog, we will discuss the common types of airspeed involved in aviation operations, providing insight into the specific instrumentation and resources that are used to offer reliable measurements and calculations.

One of the most basic elements of achieving airspeed measurement is an aircraft instrument known as the airspeed indicator (ASI). As a part of the pitot-static system, the ASI compares the difference of pressure between the relatively undisturbed air captured by the static port and the ram air directed into the pitot tube with diaphragms and mechanical linkages to calculate an aircraft’s speed relative to the surrounding air.

If the cockpit features mechanical gauges, airspeed readings will be directly displayed with a needle and dial. Meanwhile, modern aircraft will often provide instrument measurements on electronic displays and screens.

Straightforward in operation and reliable, airspeed indicators will obtain what is known as indicated airspeed (IAS). This serves as a pilot’s main form of reference during operations, where real-time measurements will be relied on for maintaining safe speeds to avoid stalls, prevent damage during flap extension, and generally maneuver within optimal limits.

Despite the heavy use of this information, indicated airspeed is not corrected for any issues that may occur as a result of incorrect installation, calibration mistakes, air density, and more.

To correct discrepancies stemming from the instrument or its positioning, pilots utilize airspeed calibration corrections. This results in an adjusted value known as calibrated airspeed (CAS), which is a more accurate representation of an aircraft’s momentum through the atmosphere during standard conditions. While this can be very useful for performance testing, it still does not account for many atmospheric variations that pilots will regularly encounter during flight.

By further refining calibrated airspeed to consider the effects of air compressibility at higher speeds and altitudes, a measurement known as equivalent airspeed (EAS) is obtained. EAS is fairly useful for high-performance aircraft that operate at altitudes where air density variations become more pronounced, as such data ensures that structural speed limitations can be adhered to with ease while maintaining optimal aerodynamic efficiency.

Flight planning is a critical aspect of operations and navigation, and it highly depends on what is known as true airspeed (TAS). As an even further corrected value, TAS is calculated by adjusting the CAS for variations in air density that are influenced by altitude and temperature changes.

While flight planning tools are commonplace for conducting corrections, modern aircraft are also outfitted with advanced onboard computers that can provide very accurate data that accounts for altitude, outside air temperature, pressure, and more.

As the last type of airspeed used in aviation, ground speed (GS) is also important for various forms of planning. This measurement simply indicates how quickly an aircraft is moving in relation to the surface of earth, being achieved by adjusting TAS values for wind effects.

Tailwinds that push an aircraft forward will increase ground speed and reduce travel times, while head wings have the opposite effect. As this has a direct impact on fuel consumption and duration of flight, having GS measurements is necessary for reaching the highest levels of efficiency for navigation and planning.

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