# Calibrated Airspeed.

#### Skinnah

##### Well-Known Member
How do you calculate Calibrated Airspeed?
-Junior

By definition, "calibrated airspeed" is indicated airspeed, corrected for position and installation errors (i.e., errors that result from the position of the pitot and static system in the changing flow field as a result of varying angle of attack &amp; configuration; as well as error that result from the physical installation of the lines, etc.).

The POH/AFM of your aircraft will have a chart allowing you to determine the calibrated airspeed, given an indicated airspeed and configuration.

Once you have determined the calibrated airspeed, you may use that to calculate true airspeed...in faster aircraft, the calibrated airspeed is first used to calculate equivalent airspeed, then true airspeed. There is a mnemonic, "ICE-T" which stands for "indicated - calibrated - equivalent - true," which gives the order in which you proceed. Below about 180 knots, equivalent airspeed may be omitted, as the effect of ram temperature rise is negligible...so for the typical trainer or light twin, the order becomes "indicated - calibrated - true."

so if you know your TAS how do you determine your CAS?

There is no need to derive CAS from TAS. CAS is found in the POH/AFM. You need that to find the "real" TAS, although usually just IAS is used in flight.

Sounds like you may be stuck on basic definitions. Not unusual at all.

First it helps to have a simple understanding of how the ASI works. It doesn't really measure "speed" the way the speedometer on a car does. What it's really doing is comparing impact (or ram) air pressure coming through the pitot tube with static air pressure coming through the static port.

Indicated Airspeed (IAS). Some folks try to make this more complicated than it is. It's simply the speed that is shown on the airspeed indicator. Period.

Calibrated Airspeed (CAS). When you are in level flight at cruise airspeed, the pitot tube is aligned directly with the relative wind. But when you are, say, flying level slowly, the pitot tube is pointed up at an angle to the relative wind. It's not getting a direct flow, so the measurement is a bit off. This is what the book calls "position error".

Calibrated airspeed is indicated airspeed corrected for that error. As was already mentioned, there's a table in the aircraft manual that shows how to convert from one to another.

We don't worry too much about calibrated airspeed when flying. It's primarily used for calculating speeds that are based on stall speed; 1.3 Vso for landing, for example.

True Airspeed (TAS). This is the actual speed through the air. How come it's different? Remember that the ASI is just measuring the difference between impact and static pressure. Well, as you go up in altitude, the air gets thinner. In order to get the same amount of impact in the pitot tube, you have to be going faster. So, for the same indicated airspeed, true airspeed will be higher the higher you go.

Ground Speed (GS). Might as well throw this in. It's your speed over the ground. It's your TAS corrected for the wind. 100 KT TAS with a 20 KT headwind = 80 KT groundspeed.

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so if you know your TAS how do you determine your CAS?

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If you know your pressure altitude, TAS, and OAT, then use your E6B computer to find CAS on the outer ring. (Outer or inner? I forget.)

I don't know how useful it will be, tho.

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