Originally Posted by
DaveReidUK
Thanks for that.
Can you clarify - are you using the value for hPa/ft at SL, or the value at 8000' ?
Hi Dave. It depends.
My answer to your first question was:
Originally Posted by
Luc Lion
Originally Posted by
DaveReidUK
Let's put it another way - what pressure difference at Bole's elevation (i.e. QFE) would you expect to result from a difference at SL between 1029 mb and ISA?
∆p = (1029 – 1013.25) hPa * (1 - 0.0019812(°K/ft) * 7657 ft/288.15 °K)^5.2561
∆p = 11.85 hPa
So, physically, your question is equivalent to "If a translation is applied to the ISA pressure model so that 0 ft is moved from 1013.25 hPa to 1029 hPa, by how much pressure is translated the point at altitude 7657 ft ? As the whole model is translated everywhere by the amount of feet, 420 ft, it means applying the pressure lapse rate that exists at 7657 ft to the height difference that matches 1029 hPa - 1013.25 hPa = 15.75 hPa at sea level pressure lapse rate.
As the pressure lapse rate at 7657 ft is about 75% of the lapse rate at sea level, I found 15.75 hPa * 0.75 = 11.85 hPa.
So it is pressure lapse rate at 7657 ft for your first question.
Your second question was more straight forward:
Originally Posted by
Luc Lion
Originally Posted by
DaveReidUK
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' ?
∆h = 1/(0.0375 hPa/ft) * (1029 – 1013.25) hPa
∆h = 420 ft
Again, we know that the whole ISA model is translated everywhere by the amount of feet when the reference isobaric surface is moved.
You move the 0 ft surface from 1013.15 to 1029, so basically these are atmospheric layers very close to the 1013.25 reference surface where the pressure lapse rate is 0.0375 hPa per feet.
And 0.0375 hPa/ft * 15.75 hPa = 420 ft
So it is pressure lapse rate at seal level for your second question.
There is a common misconception, even in the pilot community, about the altitude difference versus pressure difference (aka pressure lapse rate) when adjusting the value in the Kollsman window.
It is well known that pressure lapse rate of the
real atmosphere is decreasing, but it is less known that the altitude/pressure ratio for each hPa tuned in the Kollsman window also decrease with decreasing pressure.
It follows an exponential curve with (almost) the same coefficient as the pressure/altitude relation of the ISA model.
When one changes the altimeter setting with a value very close to 1013.25 hPa, each 3.75 hPa moves the altitude by 100 ft.
When one changes the altimeter setting with a value very close to 800 hPa (setting only possible with SAE AS8002A altimeters) only 3.03 hPa is enough to move the altitude by 100 ft.