Hi folks,
Air traffic controller here.
Aircraft (well, their operators mainly) want to fly at the most efficient altitude. For modern high-bypass turbofan engined-airliners, that’s usually 30k to 40k feet. It will depend on weight, so a fully loaded 777 at max take off weight flying for 15hrs might only be able to initially climb to, say, 32k ft but a few hours later can then climb again to 34 or 36k ft.
We all still use the humble altimeter to determine what we call level (‘level’ in altimetery being a generic term, ‘altitude’, ‘flight level’ and ‘height’ all have specific meaning). So the altimeter is really just a barometer with the needle reading off feet, rather than hectopascals or inches of Mercury. The flight crew will set the altimeter with a pressure datum against which the altimeter will compare outside air pressure.
If the altimeter is set to what’s called QFE, the altimeter will read 0ft if the aircraft is on the ground, and if in the air will give the HEIGHT above ground level where the QFE reading was taken.
If the altimeter is set to QNH, the altimeter will read 0ft at mean sea level, and if in the air will give ALTITUDE above sea level.
So if a aircraft took off from, say, Miami airport at sea level, the QNH and QFE are the same, so would both read 0ft on an altimeter. Arriving at Denver airport, let’s say 5000ft above sea level, an aircraft has a HEIGHT of 0ft, but an ALTITUDE of 5000ft.
Now, as mentioned above in the thread, air pressure varies as one flies through the air, and over time even if one stays in the same location.....each nation will promulgate what’s called a ‘Transition Altitude’, below which aircraft fly on QNH or QFE. Above this Transition Altitude, they all set their altimeters to an arbitrary setting of 1013.2hPa (29.92in). This ensures that all aircraft are working off the same datum. This pressure setting gives us ‘flight level’, which you might have heard referenced by flight crew over the PA on a flight.
Flight Level 320 is nominally 32k ft, if the pressure at sea level is 1013.2hPa. It could actually be 30k ft, or even 33k ft depending on actual air pressure. So even though the in flight map displayed to the passenger keeps saying a constant 32k ft, an aircraft could go from 31k to 33k ft above seal level over the course of a few hours as the air pressure changes.
GNSS-derived altitude is not used at higher levels, but is becoming more common in encoding flight paths nearer airports.
Radar altimeters only work for measuring heights of 2500ft or less.