Density altitude

Density Altitude Computation Chart.

Density altitude is the altitude in the International Standard Atmosphere at which the air density would be equal to the actual air density at the place of observation, or, in other words, the height when measured in terms of the density of the air rather than the distance from the ground. "Density Altitude" is the pressure altitude adjusted for non-standard temperature.

Both an increase in temperature and humidity will cause a reduction in air density. Thus, in hot and humid conditions, the density altitude at a particular location may be significantly higher than the true altitude.

Aircraft safety

Air density is perhaps the single most important factor affecting airplane performance. It has a direct bearing on:^[1]

• The lift generated by the wings — reduction in air density reduces the wing's lift.
• The efficiency of the propeller — 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.

As a result of a density altitude that is higher than the actual physical altitude, the following effects are observed:^[1]

• The aircraft will accelerate more slowly down the runway 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 roll down the runway before liftoff and a higher true airspeed which must be maintained when airborne to avoid stalling.
• The aircraft will climb more slowly as the result of reduced power production and lift.
• The aircraft service ceiling (maximum altitude which can be attained) will be lower due to both lower power output and reduced lift, reducing the ability to fly above obstacles such as mountains

Calculation

Density altitude can be calculated from atmospheric pressure and temperature (assuming dry air).

where

DA = density altitude in feet

P₀ = atmospheric (static) pressure

P_SL = standard sea level atmospheric pressure (101.325 kPa)

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

T_SL = standard sea level air temperature (288.15 K)

b = 0.235

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 + (120 x (OAT - ISA_temperature))

Where:

OAT = Outside air temperature in °C

ISA_temperature = 15 °C - (1.98 °C/1000 ft × pressure altitude in feet)

References and notes

• Air Navigation. Departments of the Air Force and Navy. 1 December 1989. AFM 51-40 / NAVAIR 00-80V-49. 
• "Air Density and Density Altitude". http://wahiduddin.net/calc/density_altitude.htm. 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".

See also

• Outside air temperature
• Barometric formula
• List of longest runways

External links

• Density Altitude Calculator
• Density Altitude Calculator
• Density Altitude influence on aircraft performance