Elementary CAS vs TAS q

[ExcessData]

Hi all

Fairly simplistic q, I know, but was talking to a mate the other day about the well-known CAS vs TAS relationship, and we were trying to work out what the TAS will do, relative a constant CAS, as you descend 'below mean sea level' (ie. a hypothetical case).

Any input greatly appreciated.

Cheers, ED

[Edited for syntax]

[3142]

CAS and TAS are density related, so as you fly below sea level you will need to know what is the density of the water you are diving (flying) in. The water density is now a function of its temperature and the salt content of it.

[ExcessData]

Sorry 3142, bad wording on my part I think!

Assuming a canyon existed down to say 3000' BELOW MSL, and I were to fly in it (ie. still flying in air, but in air of density GREATER than that of MSL, rather than less as is the usual case for flight AMSL), what would the TAS do relative to a constant CAS were I to descend from sea level down to -3000ft, in air.

Cheers, ED

[oxford blue]

It would decrease - just like it does when the air gets colder - thus more dense. Try the temperature relationship out on your Nav Computer.

[Keith.Williams.]

The trick to understanding the CAS vs TAS relationship is to remember that an ASI simply measures dynamic pressure and gives an indication that is proportional to it. Every time it senses any given value of dynamic pressure it will give the same indication. If we ignore the differences between IAS, CAS and EAS, this means that an aircraft climbing at constant CAS is climbing at constant dynamic pressure.

Dynaimc pressure is 1/2Rho V squared Where Rho is air density and V is TAS. As we climb Rho decreases, so V squared must increase by the same fraction to keep the same dynamic pressure at any given CAS. It is this relationship between CAS, Dynamic pressure, Rho and TAS that causes TAS to increase as Rho decreases.

If Rho increase such as when we descend, the TAS at any given CAS must decrease so that the reduction in TAS squared compensates for the increase in Rho. ASIs are calibrated to give CAS = TAS at ISA msl. At higher pressure altitudes where Rho is reduced, the TAS is higher than CAS. And at lower pressure altitudes (below ISA msl) where Rho is increased, the TAS is lower than CAS.

[ExcessData]

Thanks Keith/Oxford - explanations exactly what I was looking for, and spot on.

Cheers, ED