Density Altitude and Pressure Altitude
Why a 5,000 ft strip flies like 8,500 ft on a hot day: the FAA density-altitude method turns field elevation, altimeter setting, and temperature into the performance altitude a chart is entered with. PA = elevation + (29.92 - altimeter) x 1000; 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. Dry-air model (humidity lowers density further); the aircraft flight manual and the pilot in command govern.
Formula and source
oat_c = (oat_f - 32) x 5/9; PA = elevation + (29.92 - altimeter) x 1000; ISA = 15 - 2 x (PA/1000); DA = PA + 120 x (oat_c - ISA).
FAA 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.
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