Why does aircraft fly at Mach?

[john_tullamarine]

Can we play nicely, please ? No need for trying to get further up the wall than the other chap ...

[SouthernFlyer2008]

I think another way to explain it is this----if an aircraft flies too close to Mach 1 (the speed of sound) without special design features, adverse effects such as high drag, air-frame buffet and handling difficulties may be encountered. Therefore it is very important for a pilot to know the aircraft Mach number at high altitude where high True Air Speeds(TAS) result in high Mach Numbers.
A Mach meter is fitted to jet transport aircraft which allows the pilot to fly at a precise Mach number (rather than IAS), and avoid the adverse effects of flying too close to the speed of sound. This is why Jet transport aircraft at high altitude fly according to Mach number, rather than Indicated Airspeed (IAS), as the IAS and TAS at a particular Mach number constantly change due to the changing local environmental factors (temperature, density etc.)

[henry_crun]

First you need to understand that the aerodynamics of an airplane depend on the indicated airspeed (IAS).

The slowest you can go is a particular IAS. But the ground speed for that IAS gets faster at higher altitudes.

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Then you need to understand that a particular design will become overstressed and break up at a particular Mach number.

So the highest speed your airplane can fly is that particular Mach number. The ground speed for this Mach number decreases with altitude.

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So at lower speeds you are worried about whether the airplane will fly at all, and this depends on the IAS.

And at higher speeds you are worried about whether the airplane will break up so you have to watch the Mach number.

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At higher altitudes the margin between minimum IAS and maximum Mach becomes less so you have less choice of speed.

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Hope I haven't offended too many pilots by making it simple. I'm just a dull old Engineer.

TAS is assumed the same as ground speed for this simple explanation

[Owain Glyndwr]

Let's try another way.

To get best range one should fly to maximise ((L/D) times VTAS)
For climb and early cruise this means flying at the maximum allowable airspeed, that is close to Vmo.
Vmo is obtained by applying statutory margins to the flutter clearance speed which is itself dependent on IAS (not Mach). Vmo is therefore defined by IAS limits and to optimise performance one should fly to IAS. This is usually a constant value invariant with altitude

As altitude is increased at constant IAS the Mach number increases and eventually one gets to the point where Vmo coincides with the wing design Mach number

If one exceeds the wing design Mach one risks encountering buffet ad/or drag rise which will screw up the L/D part of the range parameter. In such conditions the maximum usable IAS (VTAS) is limited by Mach number.
One could of course fly to a prescribed IAS schedule, but it is much simpler to just fly at constant Mach.

I know one can then fiddle with the selected Mach number to optimise for long range or hig speed cruise, but this does not change the basic argument.