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Density altitude

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General Aviation

Hello. 
The question says determine pressure altitude with an indicated altitude of 1380 ft msl with an altimeter setting of 28.22 at standard temperature.

a.3010 ft msl

b.2991 ft msl

c.2913 ft msl.

The answer is B

but how did they get to that answer. The answer break down says for 28.3 you must add  1533ft. For an altimeter setting of 28.2 add 1630. Using interpolation, you must subtract 20%….. I’m trying to figure out where did that’s 20% came from? You’re help will be appreciated!

 

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1 Answers



  1. Bryan on Jul 09, 2022

    Look at the PHAK, page 11-3, figure 11-3. For the question you have, you can call 1,380 your field elevation and then just use the chart.

    If the altimeter setting were 28.20, you would add 1,630 feet to 1,380 and get 3,010. If you had an altimeter setting of 28.30, you would add 1,533 feet to 1,380 and get 2,913. Those two answers are there–but neither is correct.

    The reference to interpolation is there because you have an altimeter setting (28.22) that doesn’t actually appear on the chart. But, 28.22 is 20% of the way from 28.20 to 28.30 because it is two-tenths, right? So you take the two correction numbers and subtract them (1,630 – 1,533 = 97) for a difference of 97 feet. You need 20% of that less than the 1,630 figure to account for 28.22 instead of 28.20. 97 * .2 = 19.4. The corrected altitude for 28.20 was 3,010 so subract 19.4 and you get 2,990.6 which rounds nicely to 2,991.

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