Density altitude
• The efficiency of the propeller or rotor — which for a propeller (effectively an airfoil) behaves similarly to lift on wings.
• The power output of the engine — power output depends on oxygen intake, so the engine output is reduced as the equivalent “dry air” density decreases and produces even less power as moisture displaces oxygen in more humid conditions.
Aircraft taking off from a "hot and high" airport such as the Quito Airport or Mexico City are at a significant aerodynamic disadvantage. The following effects result from a density altitude which is higher than the actual physical altitude:[2]
• The aircraft will accelerate slower on takeoff as a result of reduced power production.
• The aircraft will need to achieve a higher true airspeed to attain the same lift - this implies both a longer takeoff roll and a higher true airspeed which must be maintained when airborne to avoid stalling.

Density Altitude Computation Chart[1]

Density altitude is the altitude relative to the standard atmosphere conditions (ISA) at which the air density would be equal to the indicated air density at the place of obser• The aircraft will climb slower as the result of revation. In other words, density altitude is air density given duced power production and lift. as a height above mean sea level. “Density altitude” can also be considered to be the pressure altitude adjusted for
Due to these performance issues, a plane’s takeoff weight non-standard temperature. may need to be lowered or takeoffs may need to be schedBoth an increase in temperature, decrease in atmospheric uled for cooler times of the day. Wind direction and pressure, and, to a much lesser degree, increase in runway slope may need to be taken into account. humidity will cause an increase in density altitude. In hot and humid conditions, the density altitude at a particular location may be significantly higher than the true altitude.

2 Calculation

In aviation the density altitude is used to assess the aircraft’s aerodynamic performance under certain weather conditions. The lift generated by the aircraft’s airfoils and Density altitude can be calculated from atmospheric presthe relation between indicated and true airspeed are also sure and temperature (assuming dry air). subject to air density changes. Furthermore, the power delivered by the aircraft’s engine is affected by the air
[
(
) ΓR ] density and air composition.
P /PSL gM −ΓR
TSL
1−
DA = γ T/TSL

1

Aircraft safety

where

Air density is perhaps the single most important factor affecting aircraft performance. It has a direct bearing on:[2]

DA = density altitude in feet
P = atmospheric (static) pressure

• The lift generated by the wings — reduction in air density reduces the wing’s lift.

PSL = standard sea level atmospheric pressure
(1013.25 hPa ISA or 29.92126 inHg US))
1

2

6
T = true (static) air temperature in kelvins (K)
[add 273.15 to the Celsius (°C)] figure

EXTERNAL LINKS

3 Notes

TSL = ISA standard sea level air temperature in kelvins (K) (288.15 K)

[1] http://www.faa.gov/documentLibrary/media/Advisory_
Circular/AC-0045G_chg1_fullDocument.pdf page 13-1

γ = lapse rate (0.0019812 K/ft)

[2] AOPA Flight Training, Volume 19, Number 4; April
2007; Aircraft Owners and Pilots Association; ISSN
1047-6415

Γ = lapse rate (0.0065 K/m)
R = gas constant (8.31432 J/mol·K) g = gravity (9.80665 m/s²)
M = molar mass of dry air (0.0289644 kg/mol) National Weather Service Equation
The National Weather Service uses the following dry-air approximation of the above equation in their standards.
[
DA = 145442.16 1 −

(

17.326P
459.67 + T

)0.235 ]

where
DA = density altitude in feet
P = Is the station pressure (atmospheric static pressure) in inches of mercury (inHg)
T = T is the station temperature (atmospheric temperature) in Fahrenheit (F)
Note that the NWS standard specifies that the density altitude should be rounded to the nearest 100 feet.
Easy formula to calculate density altitude from pressure altitude
This is an easier formula to calculate (with great approximation) density altitude from pressure altitude ..and International Standard Atmosphere temperature deviation
Density altitude in feet = pressure altitude in feet + 118.8 x (OAT - ISA_temperature)

4 References
• Air Navigation. Departments of the Air Force and
Navy. 1 December 1989. AFM 51-40 / NAVAIR
00-80V-49.
• “Air Density and Density Altitude”. Retrieved 9
January 2006.
• Advisory Circular AC 61-23C, Pilot’s Handbook of
Aeronautical Knowledge, U.S. Federal Aviation Administration, Revised 1997
• http://www.tpub.com/content/aerographer/14269/ css/14269_74.htm This article incorporates public domain material from the
United States Government document "Pilot’s Handbook of Aeronautical Knowledge".

5 See also
• Outside air temperature
• Barometric formula
• Density of air
• Hot and high
• List of longest runways

Where:
OAT = Outside air temperature in °C
ISA_temperature = 15 °C - 1.98ºC / 1000ft x
PA
considering that temperature drops at the rate of 1.98 °C each 1000 ft of altitude until the Tropopause (36000ft), usually rounded to 2ºC
Or simply:
DA=PA+118.8([PA/500]+OAT-15)
Or even simpler
DA=1.24 PA + 118.8 OAT - 1782 where DA=density altitude and PA=pressure altitude where PA=Hgt+30(1013-QNH) and QNH = QNauticalHeight = Height above sea level

6 External links
• Density Altitude Calculator
• Density Altitude influence on aircraft performance
• Newbyte Atmospheric Calculator, Android Version

3

7

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