Speed and Mach? Why do we change exactly?

[WhySoTough]

I cannot seem to understand the concept fully.
Does this change happen itself? And why do pilots change to Mach? And is there a fixed altitude where they do that?
What if they don't?

Someone please explain the concept to me.
Thanks in advance.

[Genghis the Engineer]

Most higher performance aeroplanes have both an IAS and an IMN limit.

At low altitudes, invariably Vne > Mne.

If you climb at a fixed airspeed, the Mach number rises as density and temperature drop. Eventually the Mach number will hit Mne without the IAS ever having increased.

Above that speed, you have to start staying below a limiting Mach Number, which will be below Vne.

The altitude where you switch from worrying most about airspeed to worrying most about Mach number is that where Mne = Vne.

G

[Dick Whittingham]

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

[redsnail]

The Altitude isn't fixed, but it occurs roughly around the 27-28,000' point.

[Northbeach]

The airspeed indicator works by picking up the increased pressure as the airplane travels through the air and displays that information as an airspeed.

But the atmosphere is not a constant density, the higher one goes up in the atmosphere the less dense the air is (thinner). So the basic airspeed indicator cannot compensate for the significant differences in density at the higher altitudes. What worked wonderfully at sea level and at 1,000 meters, does not work so well at 10,000 meters.

We need a new reference point from which to measure our relative speed. The speed of sound works well in this regime. So we transition from IAS (indicated airspeed) to Mach numbers.

[TonyDavis]

All the above is about right. I will add that above Mne you will encounter mach buffet and then a high speed stall as the shockwave over the wing starts to destroy lift. The thicker part of the wing will go first. This is invaribly the closest part of the wing to the fuselage and on swept wing aicraft will cause the nose to drop, causing mach tuck.

The normal way to operate is to climb the aircraft at a fixed IAS, typicaly around 280 to 300 kts and then into a mach number typicaly .72 to .78. depending on type and weight.