How do aircraft measure their altitude above the ground?

Update:24 Jun 2021

To understand how an aircraft altimeter works we need t […]

To understand how an aircraft altimeter works we need to lay down some pretty basic info:
Firstly, air molecules have mass (and therefore weight) -if they didn’t windmills wouldn’t work, and you couldn’t blow out the candles on your birthday cake.

Our atmosphere is about 100 km deep (from Earth’s surface to the edge of space). Air doesn’t weigh very much, but a column of it 100 km high weighs something -there is a certain pressure from all those molecules piled up on top of each other pushing down.

As you go up into the atmosphere (say climbing a mountain) there is less air above you pressing down, so the air pressure decreases. If you blew up a balloon at sea level, and took it with you as you climbed your mountain, the balloon would get larger as you climbed higher because there is less pressure on the outside, but the same amount on the inside.

A simple barometric altimeter works a bit like that balloon. There is a sealed chamber inside the altimeter (called an Aneroid Wafer), and around this there is an outer chamber connected to the outside of the plane by a thin tube. Where the tube meets the skin of the aircraft is called the Static Port

That tube lets the outside air pressure push against the inner sealed chamber. As the aircraft goes up, the outside pressure decreases and sealed chamber expands (just like our balloon). Through a complicated series of levers and gears the expansion of the chamber is turned into a moving pointer on an altimeter.

Now all this would be great if Earth’s atmosphere was exactly the same all of the time, and every airport was at sea level. Pressure changes in the atmosphere, and temperature also has an effect. The lines you see on a weather map show areas of equal pressure (isobars). On this map the pressure in Idaho is 1020 mb, while in the Florida panhandle it’s 996.pressure gauge bordens Suppliers

If that special sealed chamber inside the altimeter was really sealed then in Miami where there was lower pressure the altimeter would tell a pilot that his plane was a few hundred few in the air when it was parked on the tarmac. Meanwhile in Boise the altimeter would under-read, telling the pilot that she was below ground level.

To fix this every altimeter has a knob (bottom left corner of photo) that allows the pilot to slightly vary the pressure indication on the altimeter face. By changing this setting (called QNH), you can dial in the local sea-level pressure, and your altimeter will show you your height above mean sea level. Which is useful because dangerous things like mountains are measured above mean sea level. All major airports have a special radio channel called ATIS that constantly broadcasts information like which runway is being used, what sort of weather they are having, how windy it is, and what the local pressure setting is -the QNH.
Here’s and example:

If you’re flying from the middle of the Canadian Prairies, or central Siberia, or Chad or the Australian Outback (where there is no local sea level) you can adjust the knob on the altimeter so that it reads the elevation of the local airfield. Pilots carry special documents that tell them info about an airport, like how long the runways are, and what the airport’s height above mean sea level is. (in the example below, the box in the center right tells me that the local ground height (FIELD ELEV) is 150 feet above mean sea level) This is called QFE, and is similar to but not exactly the same as QNH

So a basic altimeter measures the difference between outside air pressure and a special reference pressure to work out how high the plane is flying.

A Radar Altimeter works by bouncing a radar signal off the ground and measuring how long the signal took to return. The software know how fast radar waves travel (the speed of light), and how long it took, so it halves the total distance to get the altitude above the ground. Radar Altimeters are useful in level flight above level terrain. But they only give you height above the ground -not above mean sea level (unless you’re flying over the sea). They also over-read if the aircraft is nose-up, nose-down or in a turn, and generally give little warning of what’s ahead. So if you’re flying into steeply mountainous locations and your radar altimeter is coupled with a GPWS (Ground Proximity Warning System) you may get a few seconds of “Terrain! Terrain! Pull up! Whoop whoop! Pull up!” before you impact…


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