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Another density altitude question

Posted on 2013-06-29
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Last Modified: 2013-06-29
I asked a similar question to this on experts exchange. I am trying out a few more just to make sure I am doing the calculation correctly. I am close to the answer but not getting the exact answer:

"You are planning your departure from your regular aerodrome at 1032ft elevation. The temperature is quite high at 30 degrees Celsius. The QNH is 1014mb. What is the approximate density altitude"


1. I need to calculate the pressure altitude.
PA= airfield elevation +(30x(1013-1014))
PA=1032 + (30x-1)
=>1002

2. Calculation of density altitude
DA=PA + (120xISA deviation)

When calculating ISA deviation, should I use the calculated pressure altitude or the aerodrome altitude(I think it should be the pressure altitude, just wanted to check).

15 degrees at sea level and -2 for every 1000 feet so isa temperature should be 13 degrees but in fact it's 30 degrees giving a difference of 17 degrees.

DA = 1002 + (120x17)
1002 + 2040
=>3042

I wanted to check that my calculation is correct as the book is 3000 feet giving a difference of 42 feet?

Cheers,

Ross
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Question by:ross13
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Expert Comment

by:aadih
ID: 39286764
Use the formula:

DA=1.24 PA + 118.8 OAT - 1782

where DA=density altitude and PA=pressure altitude where PA=Hgt+30(1013-QNH)

Reference:

< http://en.wikipedia.org/wiki/Density_altitude >

You do the calculation. ;-)
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Accepted Solution

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aadih earned 500 total points
ID: 39286818
Got it yet?

The differences in answers are because of approximations (used in the formulas).

Your answer (3042), my answer (3024), and the book answer (3000) are all correct. :-)

The ~1.5% difference is within the acceptable margin for approximation errors.
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Author Closing Comment

by:ross13
ID: 39286840
I downloaded an app onto my ipad and got a slightly different answer as well:-)

Thanks for the help.

Cheers
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Expert Comment

by:aadih
ID: 39286845
Great.  A good (re)learning experience for me also.  So the pleasure is all mine, i.e., ours.  :-)
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