Density Altitude

I would like to share a little rule of thumb to determine Density Altitude.

I am pretty sure most of you fellow pilots know this one, but it never hurts to bring up something that could

be easily overlooked. Especially so many years after graduating flightschool.

Density Altitude adversely affects the performance of our aircraft even at low elevation fields, making our engines and airframe behave as if they were performing at a higher altitude airport. 

To determine Density Altitude, we first have to figure out Pressure Altitude.

To do that, we use the following formula:

In this example, we will be using numbers we commonly see at high elevation airports.

FIELD ELEVATION:4,600'

ALTIMETER SETTING: 30.25

TEMPERATURE: 35C

PA = FE + (29.92 - ALT) X 1,000'

Where PA= Pressure Altitude.

      FE= Field Elevation.

     ALT= Altimeter Setting. 

In this case:

Field elevation is= 4,600'

Altimeter setting= 30.25

PA= 4,600 +( (29.92-30.25) X 1,000)

PA= 4,600 +(-0.33 X 1,000')

PA= 4,600 - 330

PA= 4,270

*Remember, on a standard day pressure loses 1" of mercury 

for every thousand feet above sea level

E.G. Sea Level= 29.92

     1,000'MSL= 28.92

     2,000'MSL= 27.92

*Tip: If you are sitting in a cockpit and don't have time to put your math

      Skills to test, set your altimeter to 29.92, and the indicated altitude

      Will be the same as your pressure altitude..

*Tip#2: Aircraft performance charts use Pressure Altitude , not actual field elevation. 

Now that we have our Pressure Altitude figured out, let's determine our

Density Altitude. 

The formula to determine Density Altitude reads as follows:

DA = PA + 120(OAT-ISA)

DA = Density Altitude.

PA = Pressure Altitude.

OAT = Outside Air Temperature.

ISA = International Standard Atmosphere.(In this case for temperature)

*ISA Decreases at a rate of 2 degrees per 1,000'increase in altitude.

 Sea Level= 15 degrees.

 1,000'MSL= 13 degrees.

 Formula for ISA= 15-(2(altitude/1'000))

             ISA= 15-(2(4,600/1'000))

             ISA= 15-(2(4.6))

             ISA= 15-9.2

             ISA= 5.8

Back to our density altitude formula:

DA= 4,270 + 120(35-5.8)

*Notice the difference between the standard temperature for the given altitude 

 And the actual temperature at the station. Its huge. And not uncommon. 

DA= 4,270 +120(29.2)

DA= 4,270 + 3,504

DA= 7,774'

That's right, our aircraft will perform as if it were at a 7,774'elevation field.

Something to really consider before loading fuel and passengers or cargo, 

Since single engine performance will be dramatically affected.

Thanks for your attention.

I hope you find this post helpful.

Please feel free to leave a comment.