The speed of ground vehicles like cars is easy to measure, as they use only one type of speed. However, once you take flight, factors such as air density and wind come into play and must be accounted for. The quantifying of these factors is what necessitated the creation of four different measurements of airspeed. When someone mentions airspeed, they are referring to one of the following: indicated airspeed, true airspeed, ground speed, and calibrated airspeed. This blog will explain each of the four types and what they measure.
Indicated airspeed (IAS) is the speed of an airplane as it moves through the air. It is displayed on the cockpit airspeed indicator and is based on pressure readings from the pitot-static system. The indicated airspeed is inversely correlated with the true airspeed based on altitude. This means if true airspeed remains constant, the gauge will register a lower indicated airspeed at higher altitudes and a higher indicated airspeed at lower altitudes. This is due to the changes in air pressure at different elevations. The indicated airspeed is displayed on the airspeed indicator gauge in knots and is used when denoting aircraft speed limits, speed changes, and ATC speed restrictions. The published v-speeds for an aircraft will also be listed in indicated airspeed.
Groundspeed is the speed at which an aircraft is actually moving over the ground. It takes the true airspeed and adjusts for wind. For example, a tailwind pushes the plane, meaning the groundspeed is higher than the true airspeed. Adversely, a headwind makes the groundspeed lower than the true airspeed. Groundspeed is used to make time and distance calculations.
True airspeed is the speed the aircraft moves in relation to the air mass it is flying through. It is calculated by taking the indicated airspeed and adjusting it for pressure and temperature variables. When flying into a headwind, the true airspeed will be higher than groundspeed because the aircraft and the air mass are moving in opposite directions, meaning they pass each other more rapidly, the pitot tube reads a higher pressure, and therefore displays a higher true airspeed. When flying with a tailwind, the inverse is true. True airspeed is also affected by altitude. At higher altitudes, where air is less dense, the aerodynamic drag on the plane decreases and the true airspeed increases. True airspeed increases 2% per thousand feet. True airspeed is measured in knots and used for performance metrics and flight planning.
Calibrated airspeed uses the indicated airspeed and then corrects for known instrumentation or position errors. For example, things like the angle of attack or flap setting could affect the airflow and interfere with the airspeed reading. The calibrated airspeed takes the aircraft-specific calibrated airspeed offset and applies it to the indicated airspeed reading. This offset is unique to each aircraft and will be defined by the manufacturer in the pilot operating handbook. Calibrated airspeed is typically only a few knots different from indicated airspeed. The most significant variations occur at slower airspeeds, lower altitudes, and in nose-up attitudes. It is also important to realize that your aircraft stall speed is based on indicated airspeed. This means, when flying at slow speeds, depending on variables, the aircraft could stall despite the calibrated airspeed being above stall speed.
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