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# Density altitude/pressure altitude help

Posted on 2013-01-06
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Last Modified: 2013-01-14
Hi,

I am currently doing some revision on aircraft performance and want to confirm my understanding on calculating density altitude.

You first need to work out the pressure altitude. So for example the airfield is 399 feet amsl.

PA =airfield elevation +(30x(1013-ambient pressure). I was wanting to clarify is ambient pressure the QNH pressure setting.

PA = 399 +(30x(1013-1012))
=>399+30
PA=429 feet

So now I can work out the density altitude.

DA=PA + (120 x isa dev)

Is the term isa deviation the difference in the actual temperature against the ISA standard lapse rate of 2 degrees C?

Is the lapse rate calculated from the QNH height or the known elevation. So the value of 399 feet amsl would be used. So if for 1000 feet its 2 degrees for 499/500 1 degree drop from standard 15 degrees C could be used.

So would the isa deviation be the 14 degrees - the actual temperature / forecasted temperature. So say the outside air temperature at the airfield is 10. The difference would be 4. The DA would then be:

DA= 429 +(120x4)
DA=429 +(480)
DA = 909 feet.

Thanks for your help. I am just really wanting to check my understanding of the above.
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Question by:ross13
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2 Comments

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Accepted Solution

-Mystique- earned 250 total points
ID: 38750047
Do you know there's an android app available for calculating density altitude?

https://play.google.com/store/apps/details?id=com.bagelboysoftware.bbdenaltfree&hl=en
Offers 2 methods to calculate density altitude using Imperial or Metric units
Density altitude can be dangerously high, even at sea level. Just look at the promotional video to understand why calculating density altitude is so important, and how this app can save your life!

Both easy to use and highly accurate, Bagelboy Software's density altitude calculator offers 2 methods of determining density altitude using any combination of English and Metric units.
Method 1: Air Temperature, Dew Point, Altimiter Setting (Barometric Pressure), and Airfield Elevation
Method 2: Pressure altitude, Air Temperature, and Airfield Elevation

This app is useful for aircraft performance calculations, engine tuning, and motor racing.
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Assisted Solution

Flyster earned 250 total points
ID: 38752873
Is the term isa deviation the difference in the actual temperature against the ISA standard lapse rate of 2 degrees C?
No. ISA Deviation is what the standard temperature would be at any given altitude. On a "Standard Day" at sea level, the pressure would be 29.92 inHg (1013.25 millibars) and a temperature of 59°F (15° C). With a standard lapse rate of 2° per 1000 feet, you can use this formula to find out ISA Deviation:

15-(Altitude/1000)*2

Flyster
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