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I think I know the answer to this question, but I want to confirm. Airline pilots normally fly a CAS/Mach profile, which means that above the CAS/Mach transition altitude they fly by Mach, and below it they fly by CAS (calibrated airspeed). Why do they fly by CAS rather than TAS (true airspeed)? My guess is that they use CAS because that is what their airspeed instruments show naturally, whereas TAS requires computation to determine. Is that right? Or is it also because it helps to slow down in descent and speed up in climb (since a constant CAS in climb corresponds to an increasing TAS)? Thanks.

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    $\begingroup$ I think you mean Indicated Airspeed (IAS)—not Calibrated Airspeed (CAS). The airspeed that shows up on the indictor is Indicated. At very low airspeeds with gear and flaps extended it often varies from Calibrated Airspeed (CAS) and there are tables that show the corrections. aviation.stackexchange.com/questions/41030/… $\endgroup$ – JScarry Dec 17 '18 at 1:33
  • $\begingroup$ No, I meant CAS, not IAS. I believe CAS is just IAS calibrated for instrument error. CAS is what pilots usually fly to, not IAS. $\endgroup$ – Russ Dec 17 '18 at 6:19
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    $\begingroup$ Could you provide a source for that statement. As a general aviation pilot, I fly by what is on the airspeed indicator. If I want to know the calibrated airspeed, I have to look it up in the POH. At cruise speed, CAS and IAS are the same for the airplanes I fly. They only differ in the landing configuration. My understanding of the guys that fly the big jets is that they too fly by what is indicated e.g. 250 kts max below 10,000' etc. $\endgroup$ – JScarry Dec 17 '18 at 14:58
  • $\begingroup$ I am not a pilot, and I don't know much about GA, so I should have stipulated that my comment applies to commercial airliners. "CAS/Mach profile" is a commonly used term, but I have never heard of "IAS/Mach profile." I assume that's because airliners have sophisticated enough avionics to automatically apply the correction to IAS to compute CAS. $\endgroup$ – Russ Dec 17 '18 at 18:36
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The reason is somewhat deeper than just the indication. Up until Mach-related effects come into play, CAS is what the airplane 'feels' as speed, and consequently, is all the pilot needs to know. The TAS, or even better, ground speed, is needed primarily for navigation, but not for flying per se.

All the most important aerodynamic quantities (lift, drag, moments, etc.) are proportional to dynamic pressure:

$$q = \frac{\rho V^2}{2}$$

where V is TAS. So apart from TAS, they all depend on the ambient air density $\rho$. CAS is, essentially, TAS 'corrected' for the factor $\sqrt {\rho / 2}$, i.e. the effect of density. Instead of two parameters now you need one. But it doesn't mean CAS is calculated from TAS; rather the opposite: it is easier to naturally measure CAS (or q), and hence it is displayed (becoming IAS). To get TAS, you need to measure $\rho$, which is normally done via known pressure altitude (i.e. static pressure) and temperature.

If you climb at a constant CAS, your TAS will increase as the air becomes thinner ($\rho$ falls), but most importantly, the airplane handling qualities remain largely the same. But at some point, you'll hit the Mach limit and will have to obey it. Technically, the transition is governed by Mach itself rather than the altitude.

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  • $\begingroup$ Interesting. So constant CAS means essentially constant dynamic pressure (until the Mach number gets up to the point where compressibility effects come into play). Since a pitot tube measures dynamic pressure, that makes sense. Thanks! $\endgroup$ – Russ Dec 17 '18 at 2:22
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    $\begingroup$ Yes if you stuck your arm out the window at 200kt indicated at sea level and at 200kt indicated at 40000 ft, the force trying to bend your arm back would feel the same even though you are moving twice as fast. $\endgroup$ – John K Dec 17 '18 at 4:01
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Indicated Airspeed, IAS, is a measurement of the relative pressure caused by resistance of the air moving forward. It is what is directly measured by a pitot tube referenced against a static port on an aircraft.

Calibrated Airspeed, CAS, is the adjustment of IAS for instrument error. Not everything is perfect, and pitot/static systems have errors. Consider when engaged in high alpha (high angle of attack) flight, that there may be less "area" of the pitot tube facing into the relative wind, resulting in a lower IAS than would be detected by a perfect airspeec indicator. With computer systems it is easy to utilize flight parameters to create a display of CAS, rather than IAS, so CAS is used in some aircraft.

True Airspeed, is airspeed adjusted for atmospherics, so that the airspeed corresponds to the ground speed in a no-wind situation. This adjustment is primarily based upon altitude (absolute air pressure) and temperature. Secondary effects include humidity.

While TAS corresponds to a true speed in the air, it is not a strict measure of molecules of air going past lifting surfaces the way that CAS is, and therefore, is not useful in safety of flight issues, such as landing speed. CAS, being a calibrated (adjusted for instrument error) measurement, effectively provides the number of molecules passing by in the relative wind, and is therefore the measurement best suited to provide safety of flight information such as landing speeds.

Hot temps, and high altitude, will result in higher TAS for the same CAS. CAS will effectively predict aircraft handling characteristics, such as lift.

Simply stated, TAS is for navigation and flight performance, and IAS/CAS is for conducting flight operations where the effect on the airframe is being utilized.

Disclaimer: This discussion is directed at subsonic flight, and does not address the effects of air compressiblility.

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  • $\begingroup$ "TAS... it is not a measure of molecules of air going past" - in some ways it is: it determines the time it takes for the molecules to pass the wing. This also matters, in particular for aeroelastic effects (flutter) and everything determined by the Reynolds number. TAS is just not a measure of the impulse exchanged between the wing and air. $\endgroup$ – Zeus Dec 17 '18 at 23:49

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