# What’s Mach speed?

Can someone please explain to me what Mach is? All I know is it is to do with speed? And what “Mach” do I use during cruise?

Thanks

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The speed at which you move at the speed of sound

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Here you can understand it a bit more.

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Also, depending on the altitude you’re going to fly, the M speed changes.

Mach speed is active for all aircraft on the A/P in infinite flight once above 28,000 ft MSL

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Yes that’s it

I’m not entirely sure so take what I’m about to say with a grain of salt. But for me I usually cruise at FL340 M0.79-0.82

Hello! I see you’ve been posting a lot of questions here on the forum. That’s great but have you checked out the tutorials section? Lot’s of great information that’ll get you flying like a pro in no time!

A320 Family: M0.78-M0.80
A330 Family & A340 Family: M0.80-M0.82
A350 Family (upcoming in Infinite Flight :)): M0.85
A380: M0.85

B717-200: M0.78
B737 Family: M0.78-M0.80
B747 Family: M0.85-M0.86
B757 & B767 Family: M0.80-M0.83
B777 Family: M0.84
B787 Family: M0.85

CRJ Series: M0.75-M0.78

DC-10/MD-11: M0.82-M0.86

Your mach speed is the speed of the aircraft in relation to the LOCAL speed of sound.

As you altitude increases the air density decreases and the local speed of sound reduces.

At sea level the speed of sound (mach 1) is approximately 660kts or 760 mph.

At 35000 ft the local speed of sound is about 554kts.

However, TAS increases in relation to IAS as you climb. Therefore to allow a constant speed to be achieved we use Mach. Whilst your ASI might be showing 280kts, your TAS will be up around 450kts which will give you a Mach number of approx 0.82 (554 x 0.82)

Getting too close to Mach 1 in an airliner will lead to onset of supersonic buffet which is highly uncomfortable!

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Mach Speed is your speed in comparison to the speed of sound. For example, Mach .80 is 8/10 of the speed of sound.

This can also helps you…

Not quite! Wrong way around I’m sorry to say!

Mach .8 is 8/10ths the ‘local’ speed of sound.

;D

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I go between Mach 0.75 or Mach 0.8 on cruise.