Density Altitude
03-14-2007, 07:26 PM
#2
Gets Weekends Off
Joined APC: Jan 2007
Position: FO
Posts: 117
From http://tpub.com/content/aerographer/...s/14269_74.htm
DA = PA + (120 Vt),
DA = density altitude
PA = pressure altitude at the level you desire density altitude
120 = a temperature constant (120 feet per 1°C)
Vt = actual temperature minus standard temperature at the level of the pressure altitude.
For example, let's say the surface temperature is 30°C and your pressure altitude is 2,010 feet. Look at table 1-6 and find the standard temperature corresponding to 2,000 feet. You should find 11°C. Plug these values into the formula to find the following: DA = PA + (120 Vt) DA = 2,010 feet + [120(30°C - 11°C)] DA = 2,010 + 120(19) DA = 2,010 + 2,280 DA = 4,290 feet
DA = density altitude
PA = pressure altitude at the level you desire density altitude
120 = a temperature constant (120 feet per 1°C)
Vt = actual temperature minus standard temperature at the level of the pressure altitude.
For example, let's say the surface temperature is 30°C and your pressure altitude is 2,010 feet. Look at table 1-6 and find the standard temperature corresponding to 2,000 feet. You should find 11°C. Plug these values into the formula to find the following: DA = PA + (120 Vt) DA = 2,010 feet + [120(30°C - 11°C)] DA = 2,010 + 120(19) DA = 2,010 + 2,280 DA = 4,290 feet
03-14-2007, 08:53 PM
#4
Gets Weekends Off
Joined APC: Dec 2005
Position: FO, looking left
Posts: 1,060
It really depends on how accurate you want to be. The formula above is a big APPROXIMATION, and it only works decent at lower altitudes.
I use the Ideal Gas law
Pressure = density * R * Temperature
where R is the gas constant.
Solve for density.
Look at my standard atmosphere table and find what altitude that density occurs at for STP.
This will give you an exact answer. Any temperature. Any pressure. Any altitude. Any time. The other equation is simple, but it is approximating an exponential function (density) with a linear slope. And that is Bass Ackwards!
I use the Ideal Gas law
Pressure = density * R * Temperature
where R is the gas constant.
Solve for density.
Look at my standard atmosphere table and find what altitude that density occurs at for STP.
This will give you an exact answer. Any temperature. Any pressure. Any altitude. Any time. The other equation is simple, but it is approximating an exponential function (density) with a linear slope. And that is Bass Ackwards!
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