Density altitude

From Wikipedia, the free encyclopedia

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. "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.

1 Aircraft safety
2 Calculation
3 References and notes
4 See also

[edit] 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 wings lift, while the effect of humidity on lift is negligible.
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.
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

[edit] Calculation

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

$\mathrm{DA} = 145426 \left[1-\left(\frac{P_0/P_{SL}}{\mathrm{T}/T_{SL}}\right)^b\right]$

where

DA =

density altitude in feet

P 0 =

atmospheric (static) pressure

P S L =

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 S L =

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 (OTA - ISA_temperature))

Where:

OTA = Outside Air Temperature in °C

ISA_temperature = 15°C - (1.98°C/1000ft x Pressure Altitude in feet)

[edit] References and notes

^ ^a ^b AOPA Flight Training, Volume 19, Number 4; April 2007; Aircraft Owners and Pilots Association; ISSN 1047-6415

(1 December 1989) Air Navigation. Departments of the Air Force and Navy. AFM 51-40 / NAVAIR 00-80V-49.
Air Density and Density Altitude. Retrieved on 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 text from Pilot's Handbook of Aeronautical Knowledge, a public domain work of the United States Government.