Speed unit in AP

Hi again IFC,

I am currently in flight, and I have a little problem. The speed unit in the autopilot menu is in MACHs, why not in knots? It changed suddenly a few hours ago.
How do I return to the knots?


It is always going to be mach at cruising altitude, you can’t return to knots


Ok thank you. I just readed another topic and it says it is in MACH over 28,000 feet


It changes to mach speed at FL280, mach is used for cruise speed irl so thats what you use in IF


Thank you!


For reference, here is the Mach-Knots conversion rate:

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indeed as when passing FL280, your aircraft becomes subsonic as it gets closer to the speed of sound
Diagram below from NASA
So at this time, the “knots” are no longer used

You can still see you IAS (Indicated Airspeed) in the “Knots” units as your ground speed


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Keep in mind, however, that this conversion is most likely done at sea level, and mach number changes with altitude. There’s this NASA converter which you can set the altitude, however it’s only available in miles per hour, so you’d likely need to convert mph to knots as well:



A bit of background for the switch from IAS to Mach:

IAS is the speed reference for air pressure over the wings, to sustain flight (and manage wind force stress hitting the fuselage, flaps and gear).

Mach is the speed reference to manage the upper limits of TAS.

TAS is limited by hazards of subsonic aircraft having airflow over the wing reach supersonic speeds (which happens before the aircraft hits Mach 1.0, because air flow over the airfoils increases to a speed above the relative wind speed, that is, the air over the airfoils travels faster than TAS, to make lift).

The speed reference switches to Mach at higher altitudes because the TAS increases more and more above IAS as the air gets thinner.

So, the switch over from IAS to Mach occurs when the air is thin enough that the upper limit of TAS becomes the most relevant factor. Mach is a measure of TAS (it’s TAS relative to the speed of sound).

So, in a sense, you’ve gone from saying: “I’ve gotta keep my IAS up”, to saying “I can’t exceed my TAS limits,” as the air gets thinner with altitude.

(of course, there are engine performance setting issues as well)


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