Ever wonder why our airplane performs better some days more than others? It all comes down to density altitude. Density Altitude is best defined as: Pressure altitude corrected for non-standard temperature variations. Air density is the single most important factor affecting aircraft performance.
So what does this mean exactly? It’s how high the aircraft feels like it is. We can both agree that my little Cessna 150 will perform better at sea level than it will at 10,000 feet! (If you can even get it up there) This is what density altitude is. Your airplane sitting on the ramp will “feel” like or perform like it’s at a certain altitude depending on density altitude.
Example
Typical density altitude in the Summer here in Florida is 2,000-3,000 feet. So my airplane rolling down the runway will perform like it’s at 3,000 feet because of the air density.
What Factors Effect Air Density?
Humidity
Humidity increases density altitude. Why? Because it decreases the density of the air! Confusing huh?
Think of it like this. If there is a high humidity that water in the air has to take the place of something. So the water vapor takes the place of the air molecules thus making the air “less dense” So if there is less air molecules for our propeller to bite and to flow over our wing we’ll have a decrease in performance.
Temperature
Remember the warmer the air gets the less dense it is. This is why our airplane always performs better in the winter months. So warm air also decreases performance because it increases density altitude.
Altitude
This one should be a given. We know that at a higher altitude the air is less dense. Which means a decrease in performance.
Calculating Density Altitude
As long as you know the pressure altitude. This can be done on your e6b. However I’ve always used this formula.
Density Altitude = (OAT – Standard Temp) x 120 + Pressure Altitude
Note: OAT = Outside Air Temperature
For example:
PA = 600
OAT = 19
Standard Temp = 15
(19-15) x 120 + 600 = 1,080ft
So your airplane will feel like it’s at 1,080 feet just sitting on the ground.
