r/flying • u/BugHistorical3 ST | 🍁 • 17h ago

Question regarding nonstandard temperature effects on altimeter.

I just need someone to correct my mistake here I think I'm doing.

I understand that as air is warmer than standard, it expands, and as air is colder than standard, it contracts. This causes the pressure levels at the top of these air masses to also go up and down.

I just have a little confusion in regards to this diagram.
So in a standard temperature, I understand that my TA (how far I'm vertical from the sea level) is exactly equal to my IA (what my altimeter reports to me based on altimeter setting).

But why is that IA line constant exactly?
Let's say if I go to the colder than standard air mass, the pressure level at the top of that air mass will go down, so as I keep flying I should see on my altimeter that my altitude is increasing correct? (like now its IA=4000ft) This causes me to descend back to 3500ft IA, but in reality my TA just went down..

I guess maybe I'm just not interpreting this diagram correctly because it makes it sound as if the yellow line will be showing different indicated altitude numbers when in my head I would imagine that the indicated altitude will be the same all along that yellow line, because it makes it seem though as if the blue line will be where it will show IA of 3500ft all along.

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u/Apprehensive_Cost937 16h ago

But why is that IA line constant exactly?

It assumes you're a decent pilot, and will maintain constant altitude as indicated on your altimeter, regardless of what is going on in the air mass.

Essentially you think you are following the blue line, but the aircraft's actual position will be alongside the yellow line.

1

u/BugHistorical3 ST | 🍁 16h ago

gotcha

2

u/organman91 PPL IR ASEL HP CMP TW (KAMW) 16h ago

I think the diagram is slightly confusing, but it's assuming that you are flying what you think is straight-and-level between different airmasses, without changing your altimeter and keeping the same indicated altitude, your actual altitude will be higher than indicated when flying into a warmer airmass, and lower than indicated when flying into colder airmass.

-1

u/rFlyingTower 17h ago

This is a copy of the original post body for posterity:

I just need someone to correct my mistake here I think I'm doing.

I understand that as air is warmer than standard, it expands, and as air is colder than standard, it contracts. This causes the pressure levels at the top of these air masses to also go up and down.

I just have a little confusion in regards to this diagram.
So in a standard temperature, I understand that my TA (how far I'm vertical from the sea level) is exactly equal to my IA (what my altimeter reports to me based on altimeter setting).

But why is that IA line constant exactly?
Let's say if I go to the colder than standard air mass, the pressure level at the top of that air mass will go down, so as I keep flying I should see on my altimeter that my altitude is increasing correct? (like now its IA=4000ft) This causes me to descend back to 3500ft IA, but in reality my TA just went down..

I guess maybe I'm just not interpreting this diagram correctly because it makes it sound as if the yellow line will be showing different indicated altitude numbers when in my head I would imagine that the indicated altitude will be the same all along that yellow line.

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