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.
Last edited by Keith.Williams.; 19th Sep 2003 at 05:28.