It’s pretty easy to understand speed in a ground vehicle: it’s read right off of the speedometer and all the driver is required to do is follow the speed limit. However, this isn’t the case with aircraft— aerodynamics makes it a little more complicated. There are different types of airspeed and they are indicated airspeed (IAS), true airspeed (TAS), groundspeed (GS), calibrated airspeed (CAS), and equivalent airspeed (EAS).
There is a measurement device on the outside of an aircraft, called a pitot tube, that measures fluid flow velocity. This information is displayed on the IAS. A pilot can read the IAS right off of the airspeed indicator on the instrument panel in the cockpit.
The TAS is the speed of an aircraft relative to the air through which it is moving. Both altitude and temperature affect the TAS. Air density decreases with an increase in altitude because there is less air from above and pushing it down, and gravity is weaker. Air density also decreases as temperature increases, and vice versa. Because the molecules are further apart as a result of lower air density, the pitot tube receives less air molecules and has an inaccurate read; it will display a lower airspeed. TAS is generally 2% higher than IAS with every 1,000 ft gained in elevation. Pilot operating handbooks contain information on an individual aircraft’s true airspeed and fuel consumption at various altitudes, power settings, and temperatures. Some aircraft have an airspeed indicator equipped with a true airspeed ring. The pilot will input altitude and temperature information and will then be able to read the true airspeed on the indicator.
GS is the movement of an aircraft relative to the ground. This information is obtained by adding the tailwind from the TAS or subtracting the headwind from the TAS. Unlike the IAS or TAS, the GS does not determine when the aircraft will stall and does not influence aircraft performance. The wind speed may be obtained using navigation landmarks, radio-aided position location, inertial navigation system, or GPS. Ground speed radar can also be used to measure it directly.
CAS is the IAS corrected for instrumental and positional errors. At various airspeeds and different flap settings, the instruments may display an incorrect airspeed. This is more common at low airspeeds and high pitch attitudes. The CAS and TAS are the same at sea level when under International Standard Atmosphere (ISA) conditions; and if there is no wind, it is the same as the GS.
The EAS is the same as the TAS at sea level under ISA standards. The difference between the CAS and EAS is negligible at lower altitudes. At higher altitudes and speeds, the CAS needs to be corrected for the compressibility of air.
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