Temperature will be increasing as we descend in the troposphere.

                                            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 increasing as temperature increases.

MN = TAS/LSS = Decreasing/Increasing = Decreasing