Density Altitude

Density altitude: DA = PA + 120·(OAT − ISA)

Density altitude (DA) takes the geographic altitude and adjusts it for temperature, plus a bit for humidity, to express how dense the air actually feels to the aircraft. A handy approximation is DA = PA + 120·(OAT − ISA_temp), where PA is pressure altitude in feet, OAT is outside air temperature in °C and ISA_temp = 15 − 1.98·PA/1000. As DA climbs, the air thins out and everything suffers at once: engines make less thrust, propellers and wings bite less air, the take-off roll stretches and the climb rate drops. Take El Alto airport in La Paz, sitting at 4060 m (13,300 ft geographic). On a 25 °C day its DA lands near 18,000 ft, which already rules out a fair number of aircraft. Brazilian regulation uses the ISA standard, and in the sertão a hot summer afternoon can push DA well past the actual field elevation. Worked example: PA = 1000 ft, OAT = 25 °C, ISA = 13 °C → DA = 1000 + 120·12 = 2440 ft.

Applications

Flight planning at high-elevation strips, sertão operations on hot summer days, general-aviation weight-and-balance, helicopter hover ceiling checks, and take-off and landing performance tables. ASTAR and Garmin G1000 flight computers work out DA on their own.

FAQ

Why does heat hurt aircraft performance? Warm air is less dense. At the same true airspeed the engines ingest less oxygen and the wings make less lift, so you end up needing longer runs and shallower climb gradients.

Does humidity matter? Yes. Water vapour weighs less than dry air, so high humidity drops the density a little more. It's a smaller effect than temperature, but it still counts when your performance margins are tight.

How is pressure altitude obtained? Set the altimeter to 29.92 inHg (1013.25 hPa) and read off the indication. That reading is the altitude the aircraft would sit at in the ISA standard atmosphere.