Airplane Envelope Speed/Altitude Envelope
I think it is a good idea for anyone who wants to understand the operation of an airplane to get a hold of that model's speed/altitude envelope. From a physics perspective the most interesting version (in my opinion) uses altitude as the vertical axis and true airspeed as the horizontal axis. Lines of constant VCAS and constant Mach provide the detail. At lowest altitudes, the max speed limit is usually a line of constant VCAS or possibly constant equivalent airspeed, KEAS. At high altitudes, most commercial transports are Mach limited. The corner where the limit changes from speed to Mach when climbing is called the Vmo/Mmo corner. The altitude of that corner will vary from model to model, but usually lies with the range of 25K to 30K feet.
When you understand what physics has to say about emergency descent along the Mmo line and the Vmo line and how you have to significantly shallow your descent angle/rate as you pass through the Vmo/Mmo corner you will much better understand the subject of this discussion.