Factors affecting CAS when you fly into warmer air?

[akafrank07]

"What happens to your calibrated airspeed (CAS) or indicated airspeed (IAS) and true airspeed (TAS) if you fly at a constant CAS (IAS) into a warmer area?

The CAS (IAS) remains constant because CAS (IAS) is unaffected by temperature, whereas the TAS will increase because of the decrease in air density associated with warmer temperatures.

This above is an answer from my text book, though i would of thought that CAS would be affected by warmer air (temperature) as the air is less dense meaning less air going through the pitot tube?

[mad_jock]

There isn't any mengingfull quantiy of air entering the tube all it does is measure the pressure.


the air speed is measured by comparing the pressure at the pitot with the pressure at the static.

When the density changes it changes for both sources so the difference in pressure stays the same which gives you the same IAS.

The TAS changes because it is dependent on the density.

[keith williams]

If you read the question carefully you should see that there can be only one answer with regard to what happens to the CAS.


What happens to your calibrated airspeed (CAS) or indicated airspeed (IAS) and true airspeed (TAS) if you fly at a constant CAS (IAS) into a warmer area?



If you fly at constant CAS the CAS will remain constant regardless of anything else that may happen.

The wording of the question also implies that CAS and IAS are to be considered to be the same thing. If we apply this simplifying assumption then our constant CAS will result in a constant IAS.


To determine what happens to the TAS we need to look how the ASI works.

The ASI produces a IAS indication by measuring the dynamic pressure.

Dynamic pressure = 1/2Rho Vsquared where Rho is air density and V is the TAS.

From this equation it can be seen that for any constant value of IAS, if the value of Rho increases then the value of the Vsquared must decrease by the same factor, so that the dynamic pressure remains constant. And if the value of Rho decreases then the values of Vsquared must increase by the same factor so that the dynamic pressure remains constant.

As ambient temperature increases, the air will expand, causing its density to decrease. This decreased air density will require a higher value of TAS to produce the dynamic pressure required to produce a given IAS indication. So maintaining IAS and flight level constant, an increase in ambient temperature results in a higher TAS because the air density decreases.