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Knots or Mach?

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Hi all.Just reading an excellent book called 'Flying the Big Jets', the 777 edition. Best read I've had in ages, and the Better Half is quite impressed with what I know! (Quite surprised myself too).Anyway, The Book says that at a certain stage of the flight (can't remember the specifics) pilots stop using knots to manage their speed, but the Mach number.What The Book doesn't tell me is - why.Now I know what Mach is, so no problems there, but can someone tell me why the differentiation?What difference does it make if I'm cruising at 400kts or .75Mach? Curious to hear any answers. Correct ones would be a bonus!Allblack

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Depending on the aircraft, if the normal cruise is stated as m .75 then that is what you would use. As you go higher the air gets thinner and IAS is no longer appropriate to use simply because you don't want to have to convert to TAS which Mach is, in order to come up with a cruise speed.At FL 300 or so you will be indicating about 300 and mach .82 you would not be able to cruise at 400 IAS simply because that would be a TAS of close to Mach 1.0Too simple an explanation but the performance charts will dictate what you use. Be sure to have IAS selected in your options and always use IAS because that is what a RW a/c uses.

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