Everything about Calibrated Airspeed totally explained
Calibrated airspeed (CAS) is
indicated airspeed, corrected for instrument error and
position error. At high speeds and altitudes, calibrated airspeed is further corrected for compressibility errors and becomes
equivalent airspeed (EAS).
When flying at sea level under International Standard Atmosphere conditions (15°C, 1013 hPa, 0% humidity) calibrated airspeed is the same as equivalent airspeed and
true airspeed (TAS). If there's no wind it's also the same as
ground speed (GS). Under any other conditions, CAS differs from the aircraft's TAS and GS.
Calibrated airspeed in knots is usually abbreviated as
KCAS, while indicated airspeed is abbreviated as
KIAS.
Practical applications of CAS
CAS has two primary applications in aviation:
- for navigation, CAS is traditionally calculated as one of the steps between indicated airspeed and true airspeed;
- for aircraft control, CAS (and EAS) are the primary reference points, since they describe the dynamic pressure acting on aircraft surfaces regardless of density altitude, wind, and other conditions.
With the widespread use of
GPS and other advanced navigation systems in cockpits, the first application is rapidly decreasing in importance -- pilots are able to read groundspeed (and often true airspeed)
directly, without calculating calibrated airspeed as an intermediate step. The second application remains critical, however -- for example, at the same weight, an aircraft will rotate and climb at the same calibrated airspeed at any elevation, even though the true airspeed and groundspeed may differ significantly. These
V speeds are usually given as IAS rather than CAS, so that a pilot can read them directly from the
airspeed indicator.
Spreadsheet calculation
A simple airspeed indicator has only one capsule measuring impact pressure (pitot - static differential). CAS must therefore be defined as a function of impact pressure alone. The instrument doesn't "know" the absolute static pressure or the
static air temperature. Static pressure and temperature are therefore defined by convention as standard sea level values. It so happens that the
speed of sound is a direct function of temperature, so instead of a reference temperature, we can define a reference speed of sound. This makes the math easier.
In a spreadsheet CAS can be computed as:
) may be entered as 10333 mm
.
The definition is based on a model of the air as a compressible fluid. CAS therefore represents true airspeed (TAS) at all
subsonic speeds under the reference conditions, for example standard sea level pressure and temperature.
At higher altitudes CAS can be corrected for compressibility error to give
equivalent airspeed (EAS). In practice compressibility error is negligible below about 10,000 feet and 200 knots.
Further Information
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