Rats! Genghis beat me to it!
You have an IAS limit because at high IAS the airflow loads on the aircraft will pull the wings off. You have a Mach limit because (for transport aircraft) high M will cause al sorts of control and stability problems, probably ending up with the wings coming off.
At low level you will hit your IAS limit first. At height you will hit your Mach limit first. At low level range speeds are based on IAS, at height on the drag rise induced by shockwaves
Somewhere halfway up in the climb it will seem sensible to you to fly the speed on Mach numbers and no longer on IAS.
So far as absolute speed limits are concerned there is a point about half way up where Vmo and Mmo are the same. This is the crossover point
Last edited by Dick Whittingham; 22nd May 2012 at 14:48.