I know this can be done by setting altimeter to 29.92 but this willgive you various readouts depending on what altitude you input 29.92 at--so how do you find a "pressure altitude" for specific operations that need to be calculated by performance charts such as takeoff, ground roll distance, etc...?
Take your current altimeter setting and find the difference between it and 29.92
So since our pressure lapses at one inch of mercury per thousand we need to take that .22 and multiply it by 1000 to come up with 220ft.
Now since our pressure is higher than standard, it means our pressure altitude is actually lower. So if our field elevation was 700ft, simply subtract that 220 to find that your pressure altitude is 480.
I'm having the same problem understanding the question as CJE. You're sitting on the ground and you put 29.92 in the Kollsman window. That =is= the pressure altitude you want to use for all the calculations you mention.
Originally posted by Ironeagle feedboy what happens if you are at a low elevation and come up with a negative pressure altitude?
Use sea level figures. If you want to extrapolate you can, but unless the aircraft is brand new with no bugs on the wings, humidity is low, etc., you want to have some cushion in your figures. Never take those numbers as fact. They were created by using a brand new aircraft with a test pilot at the controls. They are suppose to simulate an average pilots response, but what the heck is an average pilot?
Originally posted by midlifeflyer I'm having the same problem understanding the question as CJE. You're sitting on the ground and you put 29.92 in the Kollsman window. That =is= the pressure altitude you want to use for all the calculations you mention.
Perhaps he is trying to calculate pressure altitude without having to sit in the plane (i.e. for use in flight planning). If that is the case, then Feedboy's estimate is useful. This calculator might also be useful:
thanks jeff thats exactly what im talking about---i had to calculate press alt yesterday for a X-C country flight in order to use with the performance charts for takeoff distance and ground roll, and was at a total loss how to figure it out without sitting in the airplane
To recover, I geeked out and plotted the information in that website in Excel to determine a formula. The equation:
Pressure = 32.678e^(-.00005 x Altitude)
Fits with an R-squared value of .999, which is a very close fit.
So this equation is easy to carry around in your flight bag and use whenever you need it!
(This coming from the guy who, in IR training, divided 100 by 3 (for a timed turn) and got 20. Never mind how. Yes, my instructor noticed but didn't say anything.)
Also looking at the web site, it appears that the 1"/1000' rule starts breaking down pretty quickly. If you consider 10% difference to be "good enough", then it works until only about 6500 feet. 10,000 ft is about 20% different. I would use it up to about 5000 feet, myself, now that I've seen the data.