![]() ![]() In this configuration, the density altitude could possibly be, for example 5000 feet, while the airport field elevation is actually at sea level. If the barometric pressure decreases, and the temperature and dew point increase, so does the density altitude. To take this one step further, density altitude can occur at any location regardless of the altitude, time, or weather conditions. When one or more of these variables varies, the density altitude varies. These variables are directly affected by the weather. The chief components, in the order that they affect density altitude, are pressure, temperature, and dew point. Each increment on the barometric scale represents ten feet.Īlong with the different types of densities come different types of altitudes and temperatures. As a refresher, barometric scale on the altimeter is calibrated from 28.00″ to 31.00″ and is read in inches Hg. Add this to the field elevation or altitude at which you are flying, and this will result in pressure altitude. Take the difference and, referring to Table 1 below, convert altitude to feet. Pressure altitude can also be seen as the height the aircraft would be if the actual atmospheric pressure at sea level were 29.92 in Hg.Īnother way of determining pressure altitude is to subtract the altimeter setting from standard barometric pressure, 29.92 in Hg. It would be the height above the imaginary plane, which would be displayed as either above, at, or below mean sea level. This, however, would not necessarily be the distance above MSL. Then the altimeter will indicate altitude in feet above the standard datum plane or the height the aircraft is above the actual pressure of 29.92 in Hg. Pressure altitude is obtained by setting the altimeter to 29.92 in Hg. As atmospheric changes occur, the datum plane changes to either above or below sea level. The standard datum plane, which by international agreement is considered to be representative of the atmosphere for pressure altimeter calibrations and other purposes, is a level where the atmospheric weight is 29.92 in Hg on the barometer. Your “book numbers” are going to be wrong when it’s this hot. The important part about pressure altitude is that the aircraft performance data under non‑standard conditions is obtained using pressure altitude.ĭensity altitude is how the airplane thinks it is performing and is not meaningful when considering height above any given point. As the temperature rises above standard and altitude increases, the density of the air decreases, resulting in an increase in density altitude. Density altitude is pressure altitude corrected for non-standard temperature, so pressure and density altitude are the same in standard day conditions. A standard day is barometric pressure of 29.92 inches of mercury (in Hg) and sea level air temperature of 15☌ (59☏). There was an incident in which four people died because they failed to account for density altitude: three Marine Corps helicopter pilots went up to a high altitude airport to pick up a passenger with their baggage, and, on a hot day, took off and tragically never got out of ground effect.ĭensity altitude is the measure of air density relative to a standard day. However, it is something that must not be ignored. ![]()
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