MSL altitude calculation based on pressure altitude and QNH

[wiedehopf]

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 :)

[deltafox44]

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?
That's just as simple as that, forget any other math :ugh:

It can be the aircraft altimeter or any meteo station pressure sensor. No correction for temperature, since ISA is built-in in altimeters.

By construction, the altitude you read on a QNH-set altimeter is exact on the airport ground, but it may be different from the actual MSL altitude at any altitude above or below the airport, if temperature differs from ISA. Therefore it cannot be used to indicate height above terrain in mountain.

QNH alt will be higher than MSL alt if temperature is warmer than ISA, lower if colder. The actual height above the airport will be the QFE height
* actual temperature / ISA temperature (at least if this ratio is constant between the airport and your altitude)

[wiedehopf]

It can be the aircraft altimeter or any meteo station pressure sensor. No correction for temperature, since ISA is built-in in altimeters.

By construction, the altitude you read on a QNH-set altimeter is exact on the airport ground, but it may be different from the actual MSL altitude at any altitude above or below the airport, if temperature differs from ISA.

Ok that was probably a lot of bad wording on my part.

To get QNH you measure the absolute local pressure and calculate pressure at 0 ft MSL using ISA.
(If your measuring device already has an altitude input it will do the ISA based calculation for you)

And to get actual sea level pressure you would need to use ISA+X for the calculation.

(So you could say QNH is corrected for temperature by not applying the ISA temperature offset when calculating it, but let's not make it unnecessary complicated and just say there is no temperature correction applied deriving the QNH from local pressure)

[wiedehopf]

Posted in the ET302 thread:

Let's try a thought experiment: sitting on the runway at Bole in a helicopter on the day in question, if you set the QNH (1029) your altimeter should read the field elevation (7,625'). Now adjust the subscale to 1013.2 - how much lower will the altimeter read, and why ? How much must you climb in order that the altimeter once more reads 7,625' ?


Either you use one of the approximations, for example in the pdf i linked at the start of this thread, or you use the barometric pressure formula using values for the 0 to 11km part of the atmosphere (https://en.wikipedia.org/wiki/Barometric_formula)

First you need to calculate the local pressure from the field elevation and QNH:
p = 102 900 Pa * (288.15 K / (288.15 K - 0.0019812 K/ft * 7625 ft))**(exponent)
with
exponent = (9.80665 m/s2 * 0.0289644 kg/mol) / ( 8.3144598 J/mol/K * -0.0065 K/m) = -5.255787

p = 102 900 Pa * 0.753499183275923 = 77535 Pa

Then you do the whole process backwards with 1013.25 mbar instead of 1029 mbar, calculating height from a pressure ratio.

77535 Pa / 101325 Pa = 0.7652116058138907
0.7652**(1/-5.255787) = 1.0522343388858144
(288.15K/1.0522)-288.15K = -14.304 K
-14.304 K / -0.0019812 ft/K = 7220 ft

So the altimeter will read 405 ft lower.
The approximation would suggest 430 ft, which isn't too far off.
(Also not sure how precisely the ISA model is built into altimeters)

Maybe i made some error in the calculation but i don't think so.
Anyway i'm gonna stick to the approximation now i think :)

Someone in the other thread suggested a difference of 525 ft, which i think is wrong.
(Still in the right ballpark though)

[eckhard]

The QNH for high elevation airports will differ from the barometric pressure shown on meteo charts. The isobars on the charts are actually QFF values, which are surface pressure corrected down to sea-level using actual temperature conditions.

As mentioned, airfield QNH is the surface pressure corrected down to sea-level using ISA temperatures, as that’s what’s built in to the altimeter mechanism.