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 atso 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...?
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How to find Pressure altitude
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Take your current altimeter setting and find the difference between it and 29.92
example 30.1429.92=.22
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.

Guest
I am not sure what you are looking for, but if you put 29.92 in your altimeter, thats the pressure altitude for that day and elevation and that is what you want to use for performance calculations.
If you want to know what the actual pressure is at any pressure altitude, you can use a chart such as:
http://www.usatoday.com/weather/wstdatmo.htm
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feedboy what happens if you are at a low elevation and come up with a negative pressure altitude?
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Guest
Originally posted by Ironeagle
feedboy what happens if you are at a low elevation and come up with a negative pressure altitude?
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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.
http://www.7atc.army.mil/weather/stnpresspa.html
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Guest
The 1 inch per thousand feet rule of thumb only work to about 10,000 feet. If it worked all the way up, we would be in space at 30,000 feet.
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Guest
thanks jeff thats exactly what im talking abouti had to calculate press alt yesterday for a XC 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
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Guest
Originally posted by C.J. Eliassen
The 1 inch per thousand feet rule of thumb only work to about 10,000 feet. If it worked all the way up, we would be in space at 30,000 feet.
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Guest
Okay, duh, I should have looked at CJE's link first.
For anyone who's interested, it's right on his link. Scroll down to see the "English" units.
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Guest
Originally posted by C.J. Eliassen
Ski, I use a chart similar to this:
http://www.usatoday.com/weather/wstdatmo.htm
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 Rsquared 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.
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