ISA deviation and density altitude help!

[dumbatplquestions]

Can anyone tell what is the easiest way to calculate ISA deviation and density altitude

Question:

If the pressure altitude at an airfield is 5500ft and the prevailing temperature is +30 calculate the approximate density altitude at the airfield?

[OhNoCB]

120' per 1 degree C different from ISA tends to work well.

In your example, 30C at 5500ft is ISA + 26

so 5500 + (26 * 120) = 5500 + 3120

DA = 8620'

Just noticed you asked for a quick way to calculate ISA deviation too. I am not sure if there is a 'quick way' but temperature should reduce by 2C per 1000' above sea level. so take your altitude and multiply the thousands by 2 and take the difference from 15 and that's the ISA temp. then you can just work out the difference.

Again using your example, 5500ft with 30C

5.5 * 2 = 11.
15 - 11 = 4 so ISA would be 4C
Actual temp is 30 C which is 26 more than 4, hence ISA + 26

[gpiper]

Nicely done OhNoCB, good job!

dumbatplquestions this is the easiest way to do it(OhNoCB's way).

Basically you first need to calculate the ISA temp at the specified altitude which is 5500 ft. In order to calculate that, you always need to remember two things, the temp drop of 2 degree C every 1000 ft gained, and 1mb drop every 30 ft gained for pressure(although you do not require this info for this kind of prob).

These two point are essential and you have to remember them. The other thing you need to remember for the density altitude problems is that for every degree C deviation ABOVE ISA temp, you gain 120 ft.

Once you have the ISA temp for 5500 ft, (4 degree C) you find the ISA deviation by doing 30-4 = 26 therefore ISA + 26.

Then all is left is to add the pressure altitude you are given with the ISA deviation to get the density altitude.

5500 + (26 X 120) = 8620 ft.

[B737900er]

gpiper - If you edit the part " any deviation from ISA you gain 120ft…"to any deviation ABOVE ISA gains 120ft you would have a perfect answer.