I thank you for the link, but it's of no help at all.
I have read ICAO doc 7488, and I think I now know enough about ICAO atmosphere.
Obviously my English is not good enough and my question was not understood.
My questions refers to aviation altimeters.
When set to 1013.25 setting, a perfect aviation altimeter should read if the static pressure is P<264.36hPa
Altitude(in feet)=(1-(P/1013,25)^(1/5.2561))*283.16/.0065/.3048
I call below that function Altitude=ICAO(P)
That's the ICAO definition of ICAO atmosphere for altitude <11km, ie pressure<264.36hPa (see Doc 7488)
The derivative of that function when altitude=0 is 27.31ft/hPa
My question is
What a perfect aviation altimeter should read if the altimeter setting is s and the static pressure P?
My guess is it should read
Altitude(in feet)=ICAO(P)+27.31*(s-1031.25)
Am I right, and where is it officially documented?
My other question is: what is the accuracy required to legally fly IFR? And where is it documented.
May last question is: What is the QNH?
My guess is QNH=round down ((Altitude of the station - ICAO(P))/27.31+1031.25)
Am I right? And where is it officially documented?