I had the privilege of attending a lecture last year by Barry Schiff (of the Proficient Pilot column in AOPA Pilot magazine) on Mountain Flying at Camarillo airport. I truly enjoyed the manner in which this massively experienced pilot conveyed his information to such a wide range of pilots and pilot wanna-bees; experience-wise.

Reading his column on Density-altitude discussions in this month's magazine, I realized that perhaps since our group last year didn't screw up the calculations thrown at them like this year apparently, I missed out on some good stuff. In this article, he details a GREAT way of quickly determining density altitude by knowing airport elevation and temperature and even an easy way to refine for non-standard pressure, all without using an E6b or electronic calculator.

Since we know that standard day is 15°C and for every 1,000 feet of elevation you lose 2°C in temperature you can take the airport elevation times 2 and subtracting that from 15°C should give you the standard temperature at that altitude. The difference between that and the actual temperature times 100 added to the original elevation gives you the new altitude corrected for temperature (density altitude).

So, for Big Bear airport with an elevation of almost 7,000 feet, the standard temperature would be 7x2°=14° subtracted from sea level standard of 15° or 1°C.  If you have a 15°C day at Big Bear it is not a "standard" day but really is 14°C warmer than standard. So 7,000 + (1400) = 8,400 feet DA which is close enough to the actual computed of 8,600 feet. Obviously that 200 foot difference is less important in density calculations than altitude calculations since you don't use this value for obstacle clearance but for performance calculations. It is not uncommon to see 32°C during the summer at big bear which puts it at over 10,000 DA (32-1=31*100=3,100+7,000=10,100 feet). That is higher than some planes can handle, especially with full fuel and passengers.

So just remember, figure out the standard day for the airport elevation (15°C - (elevation * 2) and subtract that from actual temperature in °C and multiply the result times 100 to get the density increase you would add back into the airport elevation to get density altitude. It is actually easier to do in practice than to describe. I thought perhaps writing this down would help firm it up in my head.