wiedehopf
15th Mar 2019, 22:15
I didn't want to continue this in the ET302 thread since it's a bit offtopic, i'll just ask the original question that emerged in the ET302 thread here as well:
Do you need to take field elevation into account when calculating MSL altitudes from pressure altitude?
(I don't think so, otherwise altimeters wouldn't work with just one input)
I found this pdf helpful and while it makes sense i can't be sure it's correct.
https://www.win.tue.nl/~jldejong/gliding/FLS-Disk/Reading Flight Levels from QNH-altimeters-booklet.pdf (https://www.win.tue.nl/~jldejong/gliding/FLS-Disk/Reading%20Flight%20Levels%20from%20QNH-altimeters-booklet.pdf)
Also i'm curious if the QNH is corrected for temperature difference from ISA.
Let's say you are on the ground in Denver with an ISA deviation of 10C and your altimeter set to local QNH, would your displayed altitude deviate from the field elevation?
I mean how is the QNH calculated? Couldn't i imagine it as just tuning a bog standard airplane altimeter until the field elevation is displayed and reading the correction value?
If that is the case then there must be a temperature correction embedded in the QNH, must it not?
That correction would only be valid for the field elevation though.
For altitudes above that, pilots would still need to apply the correction because the altimeter only has the correct offset so it works at field elevation. It does not "know" about the increased lapse rate with colder temperatures.
Now i'm not really sure about all of that, so please enlighten me :)
Do you need to take field elevation into account when calculating MSL altitudes from pressure altitude?
(I don't think so, otherwise altimeters wouldn't work with just one input)
I found this pdf helpful and while it makes sense i can't be sure it's correct.
https://www.win.tue.nl/~jldejong/gliding/FLS-Disk/Reading Flight Levels from QNH-altimeters-booklet.pdf (https://www.win.tue.nl/~jldejong/gliding/FLS-Disk/Reading%20Flight%20Levels%20from%20QNH-altimeters-booklet.pdf)
Also i'm curious if the QNH is corrected for temperature difference from ISA.
Let's say you are on the ground in Denver with an ISA deviation of 10C and your altimeter set to local QNH, would your displayed altitude deviate from the field elevation?
I mean how is the QNH calculated? Couldn't i imagine it as just tuning a bog standard airplane altimeter until the field elevation is displayed and reading the correction value?
If that is the case then there must be a temperature correction embedded in the QNH, must it not?
That correction would only be valid for the field elevation though.
For altitudes above that, pilots would still need to apply the correction because the altimeter only has the correct offset so it works at field elevation. It does not "know" about the increased lapse rate with colder temperatures.
Now i'm not really sure about all of that, so please enlighten me :)