Those of you who are new to aviation might have heard the term density altitude thrown around by your flight instructor. While I have every confidence in your understanding of altitude, density altitude deserves a bit more attention for us to accurately plan for our airplanes’ performance capabilities. In this post, we’ll spend a little time defining density altitude, examining the factors that affect it, and discussing ways in which you can determine the density altitude of any plane you fly under any given set of circumstances.
Standardization in Aviation
As aviation is a global activity, many aspects of its operations have been standardized for simplicity. Weather is one such standardized aspect, from which we get the term international standard atmosphere (ISA). For pilots, ISA is the sea level temperature of 15°C (59°F) and the sea level atmospheric pressure of 29.92 inches of mercury (in Hg, equivalent to 1013.2 millibars [mb]). It is from these standards that we calculate all performance parameters, which include adjusting for non-standard atmospheric conditions. As a general rule, pilots use a lapse rate of 2°C decrease in temperature for every 1,000-ft increase in altitude. For atmospheric pressure, a 1” decrease for every 1,000-ft gain in altitude is the standard.
Standards for Non-Standard Conditions
As you might imagine, ISA conditions rarely occur in the real world. In most cases, pilots need to adjust for the differences from standard in order to calculate performance expectations. The first stop here is determining pressure altitude, which we’ll subsequently use in calculating density altitude for your conditions.
Quite simply, pressure altitude (PA) is our aircraft’s difference from the ISA pressure standard. Determining our PA is a simple matter of setting our altimeter to 29.92 in Hg. With this setting, pressure altitude is the height indicated by the altimeter’s dials. You can also calculate your PA by subtracting 29.92 from your airport’s reported pressure (or vice versa) and adding/subtracting the difference to/from the airport elevation (ask your instructor to show you).
With pressure altitude in hand, we now need to adjust for non-standard temperature (if applicable). We can determine the standard temperature for our altitude by applying the 2°C/1,000’ lapse rate to our airport’s elevation above sea level. Then, we can compare this standard to the ambient temperature at the airport to determine the number of degrees above or below standard temperature.
Calculating Density Altitude
When we know how far above or below standard temperature we are, we can use that information to compute our density altitude (DA). Density altitude is simply pressure altitude corrected for non-standard temperature. Under standard temperature conditions (infrequent), pressure altitude and density altitude are identical. During non-standard temps, a bit more math is necessary.
Determining our DA is slightly more involved than calculating PA. Thankfully, the process has been simplified by aircraft manufacturers and pilot supply companies. All airplane Pilot’s Operating Handbooks (POHs) contain a density altitude chart (often several) for the various phases of flight. These charts can be found in Section 5, the performance chapter (POH formats are also standardized), and allow us to determine density altitude by inputting pressure altitude and temperature. Depending on the aircraft manufacturer, these charts can be in graph or table format. Either way, it’s a relatively simple matter of following the instructions to determine your DA.
Besides using POH charts, pilots can also compute DA using an aviation flight computer (sometimes referred to as an E6B or whizwheel). In this case, simply line up the ambient temperature with the pressure altitude and note where the “Density Altitude” needle is pointing (ask your CFI for a demonstration). To get 21st Century, plug the necessary info into an online density altitude calculator. For the Smartphone savants, I’m sure there’s an app for that.
What is Density Altitude Used For?
At this point, you might be wondering just what practical applications DA serves. This is an extremely important question; as density altitude is the measure used to compute your airplane’s performance parameters. Everything from takeoff & landing distances, climb rates, airspeeds, fuel burn, aircraft range, and endurance is predicated on the aircraft’s density altitude. Without getting too detailed, the basic result is that a high DA decreases airplane performance while a lower DA improves performance characteristics.
A Final Caveat
Now that you understand the basics of density altitude and how it’s determined, be careful not to view the POH numbers as gospel. This data was obtained by highly skilled test pilots flying pristine planes, so you’re very unlikely to be able to match their numbers. Instead, add a bit of a safety margin to your calculations (50% is a common buffer for takeoff & landing distances). Doing so will give you a more realistic idea of what to expect from your aircraft.