Calibrated airspeed (CAS) is indicated airspeed, corrected for instrument error and position error. For high-speed aircraft, calibrated airspeed is further corrected for compressibility errors and becomes equivalent airspeed (EAS). Aircraft display an Indicated Airspeed (abbreviated IAS) on an instrument called an airspeed indicator. ... This article belongs in one or more categories. ... Equivalent airspeed (EAS) is the airspeed which represents the same dynamic pressure at altitude that would be generated by flying at the corresponding true airspeed (TAS) at sea level. ...

When flying at sea level under International Standard Atmosphere conditions (15°C, 1013 hPa, 0% humidity) calibrated airspeed is the same as true airspeed (TAS). If there is no wind it also the same as ground speed (GS). Under any other conditions, CAS will differ from the aircraft's TAS and GS. True airspeed is the actual speed of an aircraft through the airmass in which it flys. ... Ground speed is the speed of an aircraft compared to the ground without factoring in the wind or other weather conditions. ...

Calibrated airspeed in knots is usually abbreviated KCAS, while indicated airspeed is abbreviated 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 ground speed;
• 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. GPS satellite in orbit, image courtesy of NASA GPS redirects here. ... V speeds are speeds that define certain performance and limiting characteristics of an aircraft. ... Airspeed Indicator The airspeed indicator is an instrument used in an aircraft to display the crafts airspeed, typically in knots, to the pilot. ...

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 does not "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. The speed of sound c (from Latin celeritas, velocity) varies depending on the medium through which the sound waves pass. ...

In a spreadsheet CAS can be computed as:

where:

• q_C = impact pressure
• P_SL = standard pressure at sea level
• A_SL = standard speed of sound at sea level.

The formula works in any units, just select the appropriate values for psl and asl. For example psl = 1013.25 hPa, asl = 661.48 knots.

This can then be used to calibrate an airspeed indicator when pitot pressure (qc) is measured using a water manometer or accurate pressure gauge. If using a water manometer to measure millimeters of water the reference pressure (psl) may be entered as 10333 mm H2O. A manometer is a pressure measuring instrument, often also called pressure gauge. ...

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, i.e. standard sea level pressure and temperature. Subsonic has two possible meanings: A speed lower than the speed of sound is called subsonic. ...

At higher altitudes CAS must be corrected for compressibility error to give equivalent airspeed (EAS), and EAS must in turn be corrected for density to give TAS. In practice compressibility error is negligible below about 10,000 feet and 200 knots. Equivalent airspeed (EAS) is the airspeed which represents the same dynamic pressure at altitude that would be generated by flying at the corresponding true airspeed (TAS) at sea level. ...