| 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.
This article
is interesting, but long and technical
Also see the PowerPoint
on cold air and altimeter errors |
