PPRuNe Forums calculating pressure altitude

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 25th Oct 2002, 15:17 #1 (permalink) Join Date: Nov 2000 Location: St Albans, herts, UK Posts: 120 calculating pressure altitude I'm doing a flight planning speadsheet, more of an exercise in learning about flight planning and spreadsheets than anything else. Anyway, can anyone tell me if either or both of the following equations are valid for calculating pressure altitude? 1) Pressure altitude = ((1-(aerodrome QNH/1013.25)^0.190284) x 145366.45) + aerodrome elevation 2) Pressure altitude =(1-(aerodrome QFE/1013.25)^0.190284) x 145366.45 Cheers
 25th Oct 2002, 15:35 #2 (permalink) Join Date: Jan 2001 Location: EU Age: 35 Posts: 59 hmm... maybe I didn't quite get the point, I learnt to calculate PA by multiplying the difference between QNH and 1013 by 30, then adding or subtracting that from aerodrome elev. PA= elev. - (QNH-1013)*30
 25th Oct 2002, 15:49 #3 (permalink) Join Date: Nov 2000 Location: St Albans, herts, UK Posts: 120 I don't think you missed the point it's just that air pressure doesn't decrease by exactly 1mB per 30ft. It's a good approximation though, perhaps I'm being to picky....
 25th Oct 2002, 22:04 #4 (permalink) Join Date: Jan 2001 Location: EU Age: 35 Posts: 59 true... ...the altitude difference per hPa increases with height ( approx. 50ft at FL 180 according to textbook...) but for the calculation of PA only the height adjustment of the 1013,25 level is calculated, this being more or less in sea level the temperature variation and logarthmic change of pressure shouldn't make that much a difference... for temperature, it was 4% per 10K change. just a thought...
 26th Oct 2002, 10:10 #5 (permalink) Join Date: Aug 2000 Location: UK Posts: 3,495
 26th Oct 2002, 14:55 #6 (permalink) Join Date: Aug 2000 Location: UK Posts: 3,495 Using the relationship there, I think you're on the right track. I make it: Pressure_altitude = a*(1-(QNH/P0)^b) + Indicated_altitude*(QNH/P0)^b where a = 145000 ft, b = 0.190 and P0 is 1013.25 hPa. The correction factor to the second term makes little difference. So for, say, a QNH of 1000 hPa at an airport at 400 ft, the first term is 355 ft, the second 399 ft, for an overall pressure_altitude of 754 ft. For small QNH - P0, Pressure_altitude = Indicated_altitude + a*b*(QNH - P0)/P0 which is an additive correction of 27.2 ft/hPa. Last edited by bookworm; 26th Oct 2002 at 15:01.
 29th Oct 2002, 06:56 #7 (permalink) Join Date: Aug 2002 Location: East Anglia Posts: 183 To the moderators I posted an innocuous looking question in reply to this thread on 28/10/02 re Air Pressure/Altimeters the post has seemingly disappeared, anyone any reasons for doing so? May I respectfully try again? Situation: Year 1960, 4 Engine Piston job, about to land, distance from runway 1.5 miles, height on finals 420', cloud base 450', fierce tropical storm blowing, aircraft LANDS in sea, could rapidly falling barometric pressure cause altimeters to give an incorrect height reading? Last edited by John (Gary) Cooper; 30th Oct 2002 at 06:39.

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