What measures speed in an airplane?

Answer 1

In an aircraft you have two different speeds to concern yourself with: 1) Speed over ground 2) Speed through the air Speed over ground is important for navigation, as that's what's taking you where you want to be going. The plane can't really measure that directly, so it's done indirectly through navigation, and these days, through GPS-units. Speed through the air is important for the aircraft, since it relies on a certain airflow over its wings to remain stable. If you were to try flying in a hard headwind and a hard tailwind with he same speed over ground you'd end up in a world of trouble, as the tailwind setting might leave the plane pretty much motionless with reference to the air. Speed through air is basically measured by a (pitot) tube that's open to the front. The faster you fly the higher the pressure in the tube becomes, and this can be measured.