CAS

                                         EAS               TAS /MN

                                                                                     

                  Altitude

 

                                                                                                            At constant CAS

                                                                                                            As altitude decreases

                                                            Speed                                       TAS & MN decrease

Or:

CAS is a measure of dynamic pressure = ˝ρV2

Where ρ = air density (increasing in descent) and V = TAS (decreasing to keep CAS constant)

LSS is constant in isothermal conditions (temperature constant)

MN = TAS/LSS = Decreasing/Constant = Decreasing