Can someone provide me with an idiots guide to calculating the ‘Airport’ Pressure Altitude when determining aircraft performance figures for landing and take off calcs. I think I’m right in saying that the PA is the airfield altitude displayed on the ground when 1013 is set on the altimeter.
For example, if the airfield elevation is 100 foot amsl, what would be the PA when the QNH is say 1005 or say 1030? I’m embarrassed to say that I keep getting confused whether to add or subtract the difference (mb from 1013 × 27).
Perhaps this should be under the student section even though I’ve got over 1000hrs!
Not 100% sure I understand your Q, but if you set the known airport elevation in the altimeter, that gives you the current pressure at that location.
Yes, I was not very clear – the Q was how to calculate the PA prior to arriving at the airfield, the link from GRIFF is perfect – many thanks!
Rule of thumb:
airport elevation + (1013.25 – QNH) × 27 ft
for mental math you can round to (is close enough)
airport elevation + (1013 – QNH) × 30 ft
However, pay attention that your aircraft performance depends on the density altitude and not only pressure altitude. If you start from pressure altitude and have a temperature correction, then fine, you are using density altitude. Else, you have to apply density altitude correction yourself by adding about 225 ft per °C above ISA (according to AOPA). Take into account the ISA temperature for the actual pressure altitude, that is 15° C – 2° C/1000 ft.
Thanks Lionel, that simple algebraic formula determines whether it’s a positive or negative adjustment to field elevation. The performance tables for my spam can convert PA to DA via temperature, I have an IR renewal Friday (expired a few years ago) and am attempting to appear like a well prepared knowledgable student on the day, haha!
PeterD wrote:
I think I’m right in saying that the PA is the airfield altitude displayed on the ground when 1013 is set on the altimeter.
Correct.
PeterD wrote:
if the airfield elevation is 100 foot amsl, what would be the PA when the QNH is say 1005 or say 1030?
Think of the pressure figure as a line or area of equal pressure.
Then remember that pressure decreases when height increases.
So where would the line be if the pressure is 1005? Above or below standard as a height? – > Above the field elevation.
And where would the line of pressure be if it’s above standard such as 2030? → below the field elevation.
Therefore you need to ADD the correction factor if QNH < Standard.
And you need to deduct the correction factor if QNH is > Standard.
For your examples:
ELEV 100 ft, QNH 1005 means PA would be at 316 ft. (Elevation + (1013-QNH) x 27ft)
ELEV 100 ft, QNH 1030 means PA would be at at -359 ft. (Elevation – (1013 – QNH) x 27ft)
The important figure however is hardly ever pressure altitude but Density Altitude.
And here, the “easy” formula depends on temperature and the factor is 120 ft per degree Celsius above or below ISA.
So first you need to find out the standard ISA temperature at your elevation, which is calculated from MSL = 15°C minus 2°C per 1000 ft elevation.
So an airport of 1000 ft elevation has an ISA Standard temp of 13°, one of 2000 ft of 11 degrees and so on.
In your case, it’s close enough not to matter at 100 ft elevation.
Now you take the temperature of the day and see if it’s higher or lower than standard. The easy way is, if it’s lower, forget it and use the ISA Tables. If it’s higher, take the difference and multiply with 120 ft and add that to the pressure altitude.
So let’s say in your example with 1005 hpa: Elevation 100 ft, standard temp 15°C, PA 316 ft. OAT 25°C.
So the ISA Difference is 10°C times 120 ft is 1200 ft. Therefore DA that day would be 316 + 1200 ft = 1516 ft. For OAT 35°C you’d be at 2716 ft.
Personally I do use an app to calculate it out, but if I need a quick idea:
I turn the altimeter to 1013 and read off PA.
I check the standard Temp by calculating 15° – (2 x (Elevation/100)) (or 1° per 500 ft whatever is easier)
If Temp is lower than standard, forget it. If temp is higher than standard, DA=PA + (OAT-ISA temp) x 120ft.
(btw I used x instead of * in the formulas because otherwise I get bold letters…)
Wow, thanks for the detailed explanation! Hopefully this will be useful info for others as well 😊.
As a lowlander I’ve not needed to use it for a couple of years, but there’s a Telegram bot that does density altitude for you: https://t.me/AviatorsBot
Pressure altitude is that standard barometric sensors (e.g. the one in your transponder, or feeding your transponder) return.
It has to be that way because these sensors have no way of knowing the local QNH.
It then has to be adjusted with the local QNH to get altitude AMSL for flying.
