When is Vmo/Mmo applicable?

Question

I have read that for most planes Vmo (maximum operating speed) is the limitation below 25000ft and Mmo (maximum operating Mach number) is the limitation above 25000ft. Is it correct to say this and, if not, how would you explain this correctly?

Answer 1

The limitation is always the lower one of the two, meaning your actual IAS must be below Vmo and your actual Mach must be below Mmo.

Let us consider this case: you accelerate all the way to Vmo and climb at that fixed IAS. Two things are happening now:

1. The TAS is increasing because the air is getting less dense.
2. The speed of sound is decreasing because the air is getting colder (speed of sound in an ideal gas is proportional to the square root of the temperature).

Both of these effects result in an increasing Mach number (M = TAS / speed of sound). Eventually, the Mach number will reach Mmo and now you are limited by Mmo rather than Vmo. At what altitude this happens depends on the exact values of Vmo and Mmo and also on the actual environmental conditions (e.g. temperature).

As an example, let us consider a Boeing 737NG in ISA conditions with a Vmo of 340 kt and an Mmo of 0.82 (source). Then Vmo and Mmo will be equal at an altitude of 23711 ft at a TAS of 496 kt.