How is the speed of a plane calculated when it’s on its cruising altitude?
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Do you mean how is it measured?
A computer, appropriately called an Air Data Computer, takes in information from sensors outside the aircraft and calculates the plane’s true airspeed. It’s typically the difference between dynamic pressure (what you would feel against your hand if you stuck it out the window) and static pressure, which is what you’d feel standing still. This may seem trivial but keep in mind that at altitude the static pressure is much much less than down at sea level.
As far as how we describe our speed, there are a few ways. There’s indicated airspeed, which is what the “speedometer” shows. Then there’s true airspeed, which is how fast the plane is actually travelling through the air. And then there’s also ground speed. Which is how fast the plane is moving over the ground. This is simply the true airspeed plus or minus the tailwind/headwind.
If you’re asking why indicated airspeed and true airspeed are not the same it’s a rather long explanation. But it’s essentially because indicated airspeed is a function of dynamic pressure and at altitude the dynamic pressure is much less (air molecules are farther apart). But this speed is still very important to pilots because it’s a measurement of what the airplane is “feeling “.
The last airspeed we use is Mach number. This is just plane’s speed divided by the speed of sound. Airliners typically fly around Mach 0.75-0.85. Our Mach speed is important because as you start getting close to Mach 1 weird things start happening to the airflow over the wing.
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