# Why do aircraft need Mach number? [duplicate]

Why don't aircraft operates with IAS only?

Is there any other usage of Mach number except monitoring of shock waves formation?

How Mach number is calculated?

Mind you, not all aircraft need one, especially slower piston ones.

As you get closer to higher speeds, where compressiblity becomes significant, and you are approaching the speed of sound, the air starts to behave differently aerodynamicaly.

Also: remember how the air is a little bit accelerated over the wing because of the shape of the wing? Well, if you fly close to mach 1 and accelerate the air even further, a little bit of that air is now moving supersonically over the wing. And you now have a shockwave on your wing.

The plane might start pitching down (wing momentum changes), wing stalls (yeap, from too much speed), aerodynamic center moves etc, flight controls might stop working. Google ‘Mcrit’ and ‘mach tuck’ for more, but there are a lot of bad things generally happening in that region.

What it all boils down to is that the wing now has a built-in limitation by the engineers:do not fly faster than (say) mach 0.9 or you will be in trouble.

Now that we have a speed limit that depends on the speed of sound, we really need an instrument to show the pilots when that limit is approaching.

Very short explanation: Formula to calculate Mach N.: M = TAS / LSS Where LSS = local speed of sound (remember that the local speed of sound is proportional to Temperature, lower T lower LSS. and it is a value expressed in Kelvin.)

• To be precise: speed of sound is proportional to the square root of absolute temperature. Also slightly dependent on humidity. For practical purposes in aviation it is taken to be a function of altitude. May 17, 2018 at 10:18

All the above is true, and just to add a little something: above a certain altitude, all aircraft operate with Mach number and report to the controllers with that, that’s why you’ll hear quite often the control say: on conversion please maintain Mach .78 for example meaning when you switch from IAS to Mach keep that speed (here .78)