Like the above posts point out it is not something that you really will need to know all that often. But if you're curious anyway I'll show you the method I got from Trevor Thom's book 3. It's not that easy at first but with practice becomes second nature .
I did actually use it once on a really hot day at Netherthorpe with its short grass runways but I could have like those posting above just looked at the graph for the plane I would be flying in. However, my instructor believes that knowing about something is one thing, but understanding what you know is another so he made me learn it.
Perhaps you already know this method but just in case you don't....
For example, Airfield elevation 1000ft, QNH is 980. Temp 8C.
Firstly you need to work out your pressure altitude.
1000 + [(1013-980) x 30] (30 being no. of feet per millibar)
1000 + [33 x 30]
1000 + 990 = 1990 ft Pressure Altitude.
Now work out ISA Temperature at this Pressure Altitude.
ISA Temp@ MSL - (Temp change per 1000 ft x Pres. Alt in1000's ft.
15 - (2 x 2) I have rounded 1990 ft to 2000ft for simplicity
15 - 4
Which makes ISA at this Pressure Altitude 11 degrees Celsius.
Now we take the actual Temperature away from ISA for the calculated Pressure Altitude;
11 - 8 = 3 or ISA +3.
Now we multiply the deviation by 120,
120 x 3 = 360.
Next, add this value to the pressure altitude;
1990 + 360 = 2350
So now we have a Density Altitude of 2350 ft.
To sum up this method;
Density Altitude = Pressure Altitude + (ISA deviation x120)
If you have already tried this method I apologise. But it is the one that after practicing time and time again that I can do in my head. Its in Book 3 of the Trevor Thom manuals on page 129 although I have changed the example slightly in case of copyright issues.
Again, this method is harder that it looks but with practice becomes a rule of thumb. Hope this helps.
