How is the speed of a plane calculated when it’s on its cruising altitude?
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There are multiple speeds and each has its own calculation.
Here are the two most important and how they are calculated:
1) Indicated Airspeed – This is calculated using two external sensors: A pitot tube and a static air port. The first measures how much air is being rammed into it by the speed of the plane. The second measures static air pressure. The readings from the two are combined in an air data computer to produce indicated airspeed. This is the speed the pilot uses to fly the plane safely. The benefit of using this speed is that it represents roughly the same conditions at all altitudes. A plane going 100 knots indicated at sea level and going 100 knots indicated at 30,000 feet are experiencing roughly the same amount of airflow over the wing. But how can this be, since the air is so much thinner at 30,000 feet? Well it works, and the trick is that the plane is actually flying faster at 30,000 feet even though it is only indicating 100 knots. The important point here is that, in general, pilots fly indicated speeds.
2) Groundspeed – There are multiple ways to calculate this, but since everything has GPS nowadays, that’s how most of them do it. With GPS providing both regular position updates as well as an accurate time source, the flight computer can do basic distance/speed/time equations to calculate ground speed. Since most aircraft have groundspeed indicators in the cockpit, it is possible to “fly” a specific groundspeed, but this can be trickier than it sounds. If you’re traveling at 100 knots indicated with a 20 knot tailwind at sea level, your ground speed will be 120 knots. If you make a level 180 degrees turn without changing your indicated speed, your groundspeed will drop by 40 knots, because that tail wind is now a head wind. This is part of why pilots rarely try to fly specific groundspeeds.
Since so many of these questions are very airplane-biased, here is an interesting point for some helicopters: Since indicated airspeed depends on a certain amount of air being pushed into the pitot tube, most airspeed indicators are unreliable below certain airspeeds, typically 30-40 knots. Helicopter pilots flying below that speed will just not have an airspeed reference, which usually isn’t an issue for those conditions. However, there are some missions where a speed is important, so for those, the pilot will actually reference ground speed. An example of this is low altitude rescue swimmer deployment over water (10 feet and 10 knots).
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