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What are calibrated airspeed (CAS) and equivalent airspeed (EAS) used for? When would a pilot need to know these?

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    $\begingroup$ I can't believe this has not been asked/answered on here before, but I couldn't find anything on them. $\endgroup$ – TomMcW Sep 20 '16 at 17:33
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The different airspeeds:

  • IAS is indicated airpseed
  • CAS is calibrated airspeed
  • EAS is equivalent airspeed
  • TAS is true airspeed

CAS is IAS corrected for instrumentation and position errors. The errors are most pronounced in slow and high angle-of-attack flight.

IAS figures in aircraft manuals are actually converted from CAS. For ease of use.

How CAS is determined (Wikipedia link).

enter image description here

However, a modern plane with an air data computer (ADC) actually displays CAS, despite being labelled as IAS.

Below 200 knots and 10,000 feet, for all purposes, CAS equals EAS.

As the plane goes faster and higher and compresses the air, the air changes density, an effect known as compressibility that affects the CAS reading.

enter image description here

EAS is computed by the ADC on a modern fast plane, and together with the density altitude, the TAS is then known.

Ground speed and track (from GPS, navaids, and/or INS) together with the TAS allows the FMS to calculate and display the wind speed/direction.

Before ADC's, pilots used charts like those in the answer.

Summary:

In a modern plane, what the pilot actually uses is CAS, despite being labelled IAS. And EAS is used in computing the TAS by the ADC.

For engineers, TAS (actual velocity through the air) is used in lift equations, EAS is used in calculating dynamic pressures on the plane (TAS corrected for air density).


Source used and a must read: Aerospaceweb.org—Types of Airspeed

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    $\begingroup$ From a pilot's perspective, all we generally care about is IAS, TAS, and ground speed, and mainly just IAS and ground speed. IAS/Mach for aircraft control, ground speed for navigation. $\endgroup$ – J Walters Sep 20 '16 at 19:50
  • $\begingroup$ I'm not sure what you mean by modern aircraft, but I fly a fairly modern aircraft with an ADC, and the displayed airspeed is IAS. Granted the ADC is part of a retrofit—G1000 Flight Deck—so perhaps the difference is what the aircraft was certificated with. $\endgroup$ – J Walters Sep 20 '16 at 22:04
  • $\begingroup$ The ADC can correct for alpha, if there are AoA vanes, but without side-slip vanes it can't correct for beta, so it can't really show CAS. And I've only seen side-slip vanes on A350. $\endgroup$ – Jan Hudec Feb 11 '17 at 11:11
  • $\begingroup$ Wouldn't Indicated Airspeed simply be the indicated airspeed, regardless of correction for error? $\endgroup$ – Hugh Feb 13 '17 at 8:11
  • $\begingroup$ By definition, IAS is what is indicated on the airspeed indicator in the cockpit. In old planes that was exactly the same speed as measured by pitot tubes on the plane. That speed needs to corrected for position and calibration errors, and is then called CAS. In modern planes that correction is already done by the ADC before it is displayed on the airspeed indicator, so it is not surprising that IAS == CAS in modern planes. $\endgroup$ – fishinear Mar 6 '18 at 17:28
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Equivalent Airspeed is a measure of dynamic pressure, the quantity on which most aerodynamic properties of the aircraft depend. It is almost always equivalent airspeed that the pilot wants to know.

However the device to measure dynamic pressure, pitot tube, is subject to various errors, so the pilot can't have equivalent airspeed. The best they can get

  • with simple fixed pitot tube is the Indicated Airspeed, which is affected by changes in angle of attack and side-slip angle (positional error) and also other instrument errors.
  • with pitot tube attached to an angle of attack vane to keep it aligned with the airflow, and with corrections applied for instrument error, the Calibrated Airspeed, which is the best approximation of EAS that can be provided.

Whatever the instruments provide is the best that can be obtained in given aircraft. There is no way for the pilot to pull out a calculator or E6B and calculate something more precise, and they won't have time for that at the moments it matters anyway. The best the pilot can do is know what magnitude of error can be expected in various flight regimes (which should have been determined in flight testing and listed in the POH/FCOM/QRH) and take it into account.

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  • $\begingroup$ That doesn't really answer what I'm after. Maybe I need to edit. Are they of some practical use for the pilot? Under what circumstances might a pilot pull out his calculator and compute the EAS or CAS? Are they something normally only used by engineers? $\endgroup$ – TomMcW Sep 20 '16 at 18:49
  • $\begingroup$ @TomMcW, pilot does not have time to pull out a calculator (or E6B) and calculate anything. And it wouldn't help anyway—the instrument does not correct the error because it does not measure the factors that affect it. Light aircraft rarely have AoA vanes, almost no aircraft have side-slip vanes and the effects of temperature, humidity and such are not usually measured either. $\endgroup$ – Jan Hudec Sep 20 '16 at 19:14
  • $\begingroup$ Fantastic answer for explaining what they are. Could do with a simpler/clearer explanation of their usage. How do they relate to true air speed? $\endgroup$ – Notts90 supports Monica Sep 20 '16 at 20:27
  • $\begingroup$ @Robert, the question does not mention TAS, so to keep things simple I would prefer not to mention it in the answer either. Feel free to post another question about the relation. $\endgroup$ – Jan Hudec Sep 20 '16 at 20:33
  • $\begingroup$ @JanHudec just a thought it might clarify explain it better. TAS is something you hear used more often. $\endgroup$ – Notts90 supports Monica Sep 20 '16 at 20:36
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What pilots see in the cockpit of almost all modern aircraft is CAS. Those without an air data computer display IAS. These two will differ only minimally in the number they display in most circumstances.

Pilots use this indicator to try and stay above the minimum flight speed, and below the maximum flight speed specified in the Aircraft Flight Manual. These speed limits are based on aerodynamics - exceed the lower limit and the aircraft will not be able to stay in level flight. (This is an oversimplification!)

The SR-71 Blackbird is one of the very few aircraft that also had an EAS indicator. At very high altitudes this is a more accurate indication than a CAS indicator of where the aircraft speed is in relation to the minimum limit.

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