Density Altitude and Pressure Altitude
Why a 5,000 ft strip flies like 8,600 ft on a hot day.
Example
You enter
- Field / station elevation (ft) 5000
- Altimeter setting (in Hg) 29.92
- Outside air temperature (°F) 95
You get
- Pressure altitude 5,000 ft
- Isa c 5
- Da (ft) 8600
Checked against published worked examples from: FAA.
Details, formula, and sources
ScopeThe FAA density-altitude method turns field elevation, altimeter setting, and temperature into the performance altitude a chart is entered with. PA = elevation + 145,366 x (1 - (altimeter/29.92)^0.190284), the standard-atmosphere conversion the FAA tabulates; ISA = 15 - 2 x (PA/1000) C; DA = PA + 120 x (OAT - ISA). A 5,000 ft field at 29.92 and 95 F is 30 C warmer than standard -> DA 8,600 ft, so lift, engine power, and prop thrust all fall as if 3,600 ft higher; a cold -5 F day drops DA to about 1,930 ft, the winter bonus. Humidity is included when entered (it lowers density and raises DA); the aircraft flight manual and the pilot in command govern.
Formula or tableoat_c = (oat_f - 32) x 5/9; PA = elevation + 145366.45 x (1 - (altimeter/29.92)^0.190284); ISA = 15 - 2 x (PA/1000); DA = PA + 120 x (T - ISA), with T the OAT, or with relative humidity entered the virtual temperature Tv = (T + 273.15)/(1 - 0.378 e/p) - 273.15, e = RH x 6.112 exp(17.67 T/(T + 243.5)) hPa and p the station pressure = altimeter x (1 - 6.8756e-6 x elevation)^5.2559.
Source and editionFAA density-altitude method (ISA lapse correction), per FAA AC 00-6 / the ICAO Standard Atmosphere, by name; the aircraft flight manual performance charts and the pilot in command govern.
Free accessThe density-altitude relation is a published FAA/ICAO standard-atmosphere approximation; the inputs come from the field elevation, the altimeter setting, and a thermometer.
What governsEstimate. AHJ and licensed professional govern.
API field namesfield_elevation_ft, altimeter_in_hg, oat_f, pa_ft, isa_c, da_ft
- Pressure altitude PA = elevation + 145,366 x (1 - (altimeter/29.92)^0.190284); equals the field elevation at a standard 29.92 settingFAA-H-8083-25, altimeter-setting conversion table
- ISA lapse standard temperature falls 2 degrees C per 1000 ft from 15 C at sea levelICAO Standard Atmosphere
- Humidity included through the vapor pressure when a relative humidity is entered; it lowers density and raises DAscope of this tile