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# Subsonic

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Submitted By aerosibin
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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

3 Notes

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

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
United States Government document "Pilot’s Handbook of Aeronautical Knowledge".

• 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

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

3

7

Text and image sources, contributors, and licenses

7.1

Text

• Density altitude Source: http://en.wikipedia.org/wiki/Density%20altitude?oldid=628296667 Contributors: Hike395, Psb777, Bobblewik,
Mormegil, R.123, AJP, Gene Nygaard, Dennis Bratland, Lifung, GregorB, Rjwilmsi, Bruce1ee, Vegaswikian, Williamborg, RexNL, Gaius
Cornelius, Bovineone, Ospalh, Aarky, SmackBot, Ephraim33, Htra0497, TheGerm, Ligulembot, Basawala, AN2597, Uruiamme, Dhaluza,
Uhai, PaplooTheLearned, Fliteshare, Flyinalex, TJRC, TonyTiger22, Prof. ATP, Pschluter, Someone the Person, Ctlanni, MelonBot,
Addbot, XerozoreX, Lightbot, Luckas-bot, 2D, Materialscientist, Abuk SABUK, FrescoBot, Arlen22, DrilBot, Pinethicket, Nolween,
TheAustinMan, Full-date unlinking bot, DexDor, Smashingly, ZéroBot, M. David. Watkins, DescryVA, JustBerry, RookTorre, Meliscar and Anonymous: 58

7.2

Images

Contributors: Transfered from en.wikipedia Original artist: Original uploader was Dhaluza at en.wikipedia
• File:Text_document_with_red_question_mark.svg Source: http://upload.wikimedia.org/wikipedia/commons/a/a4/Text_document_ with_red_question_mark.svg License: Public domain Contributors: Created by bdesham with Inkscape; based upon Text-x-generic.svg from the Tango project. Original artist: Benjamin D. Esham (bdesham)

7.3

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