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manual calc of density altitude vs e6b, density altitude chart

Asked by: 1708 views ,
Aerodynamics, Weather

can anyone explain why this chart from the test supplement (pg 42) for density altitude has values which don't match the results of the DA calculation formula? is the formula correct? am I doing the math right?

ex: 3000' @ 50° F (10° C)
  1. e6b (old school) is just above 3000' but not quite 3500, let's say ~3200'
  2. electronic e6b = 3112'
  3. DA chart in the test supplement (see screenshot) says 2850'
DA calc:
3.5° F per thousand 
3000 = 3 * 3.5 = 10.5
subtract 10.5 from ISA (59° F @ MSL)
59 - 10.5 = 48.5° F (ISA @ 3000')
OAT = 50
50-48.5 = 1.5
1.5 * 120 = 180
3000 + 180 = 3180
 
ex: 30° F (-1.1° C)
  1. e6b: just under 2000'
  2. elec e6b: 1767'
  3. DA chart: ~1550', almost 1600'
DA calc:
59 - 10.5 = 48.5° F (ISA @ 3000')
OAT = 30
30-48.5 = -18.5
-18.5 * 120 = -2220
3000 -2220 = 780

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1 Answers



  1. Mark Kolber on Apr 14, 2022

    The real calculation of density altitude includes relative humidity. In can make a difference of a few hundred feet but the calculations by pilots discount that as inconsequential. So we are really talking about differences in rounding functions. So I’m not surprised to see discrepancies.

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