How do aircraft know at what altitude they are flying?

Question

When an airplane (or any sort of motorized aircraft, for that matter) is flying, they have a gage that tells them the altitude they are at, and if they are losing or gaining altitude. It also flashes an emergency light if they are losing altitude too fast. How does the plane's systems know?

Answer 1

Simple altimeter works on principle of atmospheric pressure.It decreases with altitude. Radar can also be used to measure height. Send a signal down and measure the return time. If the return time is more, it means the distance is more.Altimeter can be calibrated in terms of meters or feet.

Answer 2

Yes, an altimeter is used.  It is a barometer that is mounted in an aircraft and reads out in feet.  The altimeter must be calibrated as the aircraft flies so as to remain accurate.  Airports have ATIS (automatic terminal information service) which gives the current barometric pressures so you can calibrate the altimeter.  

Answer 3

with a device called the altimeter

it measures air pressure, and air pressure decreases as you go up higher....  

Answer 4

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.

Answer 5

About the answer that said that using the altimeter was old-fashioned, and planes should use just GPS -- no way!  Pilots still need to worry about a malfunction in the planes electrical system, or losing the GPS signal.  Altimeters can work without electricity.  

Answer 6

OK, altimeter and radar, as explained above, but today the most accurate measurement will be with GPS.  Global positioning by satellite can measure altitude as well as position.  

I don’t know, but I’d bet the latest aircraft use GPS.  

And the altimeter is pretty archaic now.  It alsways has to be adjusted for local changes in barometric pressure – I cannot see future planes using that over GPS – that would be stupid.

Answer 7

Air pressure, traditionally.  GPS would be more accurate, and probably used more often nowadays, with maybe an traditional altimeter for redundancy.

Answer 8

They used something called altimeter that measure the air pressure. Since air pressure decrease as the height increase, air pressure can be used to predict the altitude.