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DENSITY ALTITUDE: Airplanes perform different at different altitudes. At higher altitudes the air is less dense and that makes the engine, wings and prop to all be less efficient than they are at lower (more dense) altitudes. An increase in temperature makes the air less dense too, just like an increase in altitude makes air less dense. So an increase in temperature makes the airplane less efficient too. To make it easier to understand (and compute) the effects of temperature, we have a thing called density altitude that converts the temperature's effect on the airplane's efficiency to an equivalent altitude. The POH charts demonstrate this. The distances increases as the temperature increases (which is the same as the density altitude increasing).

The altimeter setting also effects density altitude. The lower the altimeter setting the higher the density altitude. Also the higher the dew point, the higher the density altitude. You can see the effect of these variables at: http://wahiduddin.net/calc/calc_da.htm.  It's interesting to note that as the altitude increases, the density altitude increases faster - when everything else stays the same. i.e. you might show a density altitude of 219 feet at sea level, but just by increasing your field elevation to 6000', does not give you a 6219' density altitude.

Density altitude is pressure altitude corrected for variations from standard temperature. When conditions are standard, pressure altitude and density altitude are the same. If the temperature is above standard, the density altitude is higher than pressure altitude. If the temperature is below standard, the density altitude is lower than pressure altitude. This is an important altitude because it is directly related to the airplane's performance. 

Density Altitude Matrix

Density Altitude Chart

This article is interesting, but long and technical

Also see the PowerPoint on cold air and altimeter errors

 
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 Dick Fechter
(507) 775-7686
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139 13th St NE
Byron, MN 5592
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