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I am currently going through PPL and sitting for the meteorological paper and I see this question in the mock exam.

What is the approximate International Standard Atmosphere (ISA) standard pressure at FL 180?

The answer has it as 500 mba but I can't seem to get the answer anywhere. What's the best formula I should use in this sort of questions?

DeltaLima
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acvon
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  • I remember studying from an EASA book that used the rule of thumb where 1 mb=30’ change. That way, FL180 would have 413 mb. If the answer is 500mb, they took 1mb=35’. I used the an iPhone app based CX3, and it seems to be take 30’ as well. Strange! – PapaMike99 Nov 10 '22 at 07:15
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    @PapaMike99 the pressure gradient changes over altitude. At sea level, the gradient is approximatly 27 ft / mb, at FL180 it is approximately 48 ft / mb. – DeltaLima Nov 10 '22 at 08:07
  • @DeltaLima I had totally forgotten about that! Thanks! – PapaMike99 Nov 20 '22 at 08:12

1 Answers1

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For altitudes in the troposphere, use this, which I give here in Excel format:

= 1013.25 * ( 1 - ( 0.0000068755856 * 18000 ) ) ^5.2558797

This yields a result of 505.99 mB at 18,000 feet.

The generalized form of this is:

= isoSLpressure * ( 1 - ( 0.0000068755856 * altFt ) ) ^5.2558797

Where:

  • isoSLpressure is your choice of any ISO sea level pressure (29.92", 1013.25mB, 760.0mm, or 14.696psi, they all work the same.)
  • altFt is altitude MSL in feet.

This formula basically calculates the pressure RATIO of the atmosphere at any given altitude, which is a different way of applying the pressure lapse rate.

All credit goes to Ed Williams.

DeltaLima
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Max R
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  • I had a typo in my answer, was missing the second closing parenthesis in the formulas. Fixed. – Max R Nov 10 '22 at 16:07
  • Hi Max, which Ed Williams are you referring to? Wikipedia gives me an actor and several football players. I also found a pilot boat but that is as close to aviation as I got. – DeltaLima Nov 10 '22 at 17:26
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    @DeltaLima Ed has been the long time de facto keeper of the Aviation Formulary, which has been a goldmine for many years. https://edwilliams.org/avform147.htm Not unlike other references (like the CRC math tables) the formulary is not a tutorial and is of no use in learning the concepts. But if you already understand the concepts and just need to find a formula or constant now and then, it's a treasure chest that you'll keep a link to the rest of your life. – Max R Nov 11 '22 at 05:41