What Is Density Altitude?

Density altitude is formally defined as the “pressure altitude corrected for nonstandard temperature variations.” That’s just a fancy way of saying density altitude is where the airplane “feels” like it is. For example, if the density altitude is 5,000 feet at sea level, your airplane is going to take off like it’s already flying at 5,000 feet. This is due to the fact that air density decreases at higher altitudes. 

Pilots need to know the density altitude to operate aircraft safely during flight. It directly impacts an airplane’s ability to fly effectively. 

High air density altitude has a negative impact on aerodynamic performance. It decreases the horsepower output of an aircraft’s engine. High air density altitude requires airplanes to increase their takeoff distance. It reduces an airplane’s rate of climb. High air density altitude also requires increased distance for a landing roll. 

The most dangerous combination of density altitude conditions include a heavy load, high temperature, high airport elevation, and high humidity. High, hot, humid, and heavy  is a very bad combination, but an aircraft needs only be experiencing one to have its performance decreased by density altitude.

When density altitude increases, it decreases the amount of available power that can be produced by an airplane’s engine. It also decreases the effectiveness of airplane propellers. For a typical non-turbocharged, light, single-engine airplane, this can result in a takeoff roll that’s 25 percent longer for every 1,000 feet of elevation above sea level. 

Pilots are taught to determine their takeoff roll using performance charts in the Pilots Operating Handbook—sort of the owner’s manual for the airplane that indicates what performance they can expect given selected power settings and the atmospheric conditions.

Here’s the difference between density altitude and pressure altitude: Pressure altitude is the altitude read off the face of the altimeter when the 29.92 in Hg is set in the Kollsman window of the instrument (or 1013 hPa if you are going metric.) Density altitude is pressure altitude corrected for nonstandard temperature variations. 

The standard temperature is 15 degrees Celsius at sea level or 59 degrees Fahrenheit.

An E6B flight computer (or E6B app) can be used as a density altitude calculator as well as a way to quickly access key data for flight planning and navigation By entering specific information for outside air temperature (OAT) and pressure altitude, the E6B flight computer will help you determine the current density altitude.

To determine the pressure altitude, set the window in the altimeter to 29.92. Whatever value it reads is the pressure altitude. 

If you don’t have access to an altimeter, you can determine pressure altitude by using this formula: (standard pressure minus your current pressure setting) times 1,000 plus field elevation equals pressure altitude.

For example: Let’s say the current altimeter setting is 29.45 and the field elevation is 5,000 feet. Plugging these numbers into the pressure altitude formula, you get: (29.92 – 29.45) x 1,000 + 5,000 = 5,470 feet. 

According to the formula, your pressure altitude is 5,470 feet. 

To determine the outside air temperature (OAT) in degrees Celsius, simply check the outside air temperature gauge or obtain the information by the airport terminal information system or aviation weather briefing.

If you don’t have an E6B flight computer, it’s easy to determine the density altitude by using a density altitude chart. Here’s how: 

Now that you know the important details surrounding density altitude, why it’s important, and how to calculate the density altitude, here’s a well-known and reliable resource for all your pilot interests and questions: FLYING Magazine. Click here to start your subscription.