Mach Confusion

Hey Guys!
Just wondering if Mach is easier to use then knots. Thanks

When your at cruising it kind of is

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Airspeed in knots switches to Mach speed once you pass around FL280 (28,000 feet).

Knots is best for remaining in the pattern so you know what airspeed you are flying at in relation to other aircraft around you, but as @Gm2kmike20 mentioned, Mach is a lot easier to go by when at cruising.


Could’nt have said it better. :D


Mach speed is used at higher altitudes because there the speed of sound is reduced from where it would be at lower altitudes. An aircraft needs to know how fast it’s going relative to Mach speed because of Mach effects. The aircraft is rated at this speed (B737/B738 at M0.79). If it goes much faster, its structural integrity will be at risk, possibly causing the whole aircraft to break apart.


Adding on what has already been said: Knots are also used by ATC below FL280, to ensure separation.

Conclusion: Knots below FL280 and Mach above FL 280 (IRL you may get ATC instructions for speed during decent slightly above FL280 in knots as well, but in cruise it would be Mach too (e.g. Atlantic crossings).


Mach is used up higher when your TAS (true airspeed) is higher due to the less dense air. Since drag depends upon TAS and at higher Mach numbers drag exponentially increases and buffets the aircraft. This means that most aircraft cannot exceed a certain Mach number due to structural limits.

IAS (Indicated Airspeed)/Knots are used lower in the atmosphere when TAS is lower and therefore you don’t need to worry about Mach compressibility effects.

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