There are four types of airspeed, each of which provides pilots with critical information about the aircraft they are flying. To better understand what each airspeed means, this blog will outline all four in detail.
Serving as the most basic of the four, indicated airspeed (IAS) can be read right off the airspeed indicator and is usually a point-of-reference in the cockpit for speed changes. An airspeed indicator (AI), or airspeed gauge, is a flight instrument that indicates airspeed of an aircraft in kilometers per hour, knots, miles per hour, and/or meters per second. Generally, the airspeed indicator provides thresholds for safe airspeeds. For example, one should not exceed 250 knots below 10,000 feet MSL.
True Airspeed (TAS)
True airspeed (TAS) is defined as the speed of your aircraft relative to the air it is traversing through. When climbing, true airspeed is higher than your indicated airspeed. Meanwhile, pressure decreases with higher altitudes. For any given true airspeed, as you climb, fewer molecules enter the pitot tube; thus, IA will be less than TAS. Specifically, for every thousand feet above sea level, TAS is about 2% higher than the IAS. For instance, at 10,000 feet, TAS is about 20% faster than what is indicated on the AI. Aircraft with gas turbine engines can reach higher TAS at higher altitudes as these engines are more efficient at higher altitudes.
Groundspeed (GS) is the movement of an aircraft relative to the ground. More specifically, it is TAS corrected for wind. With a TAS of 100 knots and a tailwind of 20 knots, you would fly a GS of 120 knots.
Calibrated Airspeed (CAS)
Calibrated airspeed (CAS) is an indicated airspeed which has been corrected for instrument and positional errors. With certain airspeeds and flap settings, the installation and instrument errors may total several knots. Keep in mind that this error is typically greatest at low airspeeds, with high nose pitch altitudes. When flying at sea level under International Standard Atmosphere (ISA) conditions, CAS is the same as TAS. Additionally, if there is no wind, it is also the same as GS.
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