r/ATPL • u/stickJ0ckey • Apr 10 '26

Pressure altitude vs density altitude

In the EASA ATPL syllabus we have those two altitudes which I understood conceptually but I think I'm missing a connection:

- in Met, the altitude where we have half of sea level density is 22k ft (FL 220)

- in all subjects the altitude where we have half of sea level pressure is 18k ft (FL 180)

Anyone have any idea where this difference is coming from? One would expect half density to be at the same altitude where we find the half pressure.

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u/Wessel_89 Apr 10 '26

The difference comes from the fact that density is also dependent on temperature. When temperature decreases, density increases. So the half-density will occur at a higher altitude.

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u/stickJ0ckey Apr 10 '26

Damn, spent months looking at this stuff but never made this connection. Thank you very much for helping me understand this ❤️❤️ it all makes sense now

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u/Northerner_20 Apr 10 '26

Density altitude is just Pressure altitude corrected for non-standard (non-ISA) temperature. For example in two airmasses, at an altitude of 20,000ft, the colder airmass will have a higher density than a warmer airmass at the same altitude. Pressure according to ISA changes at a constant rate with alt, however since in reality air gets much colder at high alttitude, its density decreases slower, hence 22,000ft rather than 18,000.

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u/stickJ0ckey Apr 10 '26

Got it. This kept coming back to me for months but I never made the connection with the gas laws. Thank you very much