Use your ERTM graphs. R =
rectified air speed = CAS
The question tells you that the temperature is constant, this is an isothermal
layer like the tropopause ? See example in notes page
2.26.
The principle is to draw the ERTM lines against a graph showing altitude
against increasing/decreasing values of speed.
The question states constant CAS, so draw this line vertical. See fig 3-2-22
page 2.25. You can see that both TAS and Mach No decrease as you descend.
mathematical solution 1/2rho V2 = CAS and is constant.
As you descend Rho (density ) will increase. To keep
the summation the same V must go down. V is TAS?
Mach number equal TAS/Local Speed Sound (LSS) example 330/660 =M 0.5
Now we have just said TAS is decreasing ! So was 330
now 300 ? LSS changes with temp, it does not change in
a isothermal layer so LSS still = 660
300/660 = M0. 45 ? Mach number decreases.