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How to find Pressure altitude

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  • How to find Pressure altitude

    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...?

  • #2
    Take your current altimeter setting and find the difference between it and 29.92
    example 30.14-29.92=.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.

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    • #3
      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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      • #4
        feedboy what happens if you are at a low elevation and come up with a negative pressure altitude?

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        • #5
          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.

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          • #6
            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?

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            • #7
              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:

              http://www.7atc.army.mil/weather/stnpresspa.html

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              • #8
                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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                • #9
                  If its negative...I would use 0.

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                  • #10
                    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

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                    • #11
                      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.
                      I was going to mention this myself. Does anyone know what the actual formula for pressure lapse rate on a standard day is? 1" = 1000' is just a close rule of thumb.

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                      • #12
                        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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                        • #13
                          Ski, I use a chart similar to this:

                          http://www.usatoday.com/weather/wstdatmo.htm

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                          • #14
                            FWIW, the calculator above takes the lapse rate into consideration.

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                            • #15
                              Originally posted by C.J. Eliassen
                              Ski, I use a chart similar to this:

                              http://www.usatoday.com/weather/wstdatmo.htm
                              Teach me to read before writing!

                              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.

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