![]() ![]() 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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