How do aircraft know at what altitude they are flying?
It is all based on the principle that air pressure decreases with increasing altitude. A standard aircraft altimeter consists of a collapsible evacuated metal box, linked by gears to a set of pointers which show the altitude in feet. As the pressure changes, the box changes its size, changing the reading on the dials. Another instrument is the vertical speed indicator. This works similarly, but has a small hole separating two parts of the device. If the aircraft is ascending, the pressure in the chamber connected to the outside will be less than the pressure in the chamber on the other side of the hole, and pressure difference will be displayed by an indicator on the face of the instrument.
