Don’t be “Dense”

The summer months have taken hold on the Central Coast, bringing with them their trademark heat and other interesting weather.

Though us Slocal pilots don’t deal with many summer storms, we do have to contend with high temperatures and density altitude.

Density altitude is pressure altitude corrected for non-standard temperatures (“non-standard” being anything above or below 15°C at sea level.) There are several ways to calculate it, but at AKS we like using the equation:

Density Altitude = Pressure Altitude + [120 x (OAT – ISA)]

Note: OAT represents your outside air temperature, and ISA represents the standard temperature, 15°C, corrected for pressure altitude. Remember that standard temperature is reduced by about 2°C per 1,000 feet of altitude gained above sea level.

Why is density altitude important? Well, as varying factors cause density altitude to read higher and higher, the air surrounding you and your airplane begins to lack molecules. Less-dense air flows over the plane’s control surfaces differently than air packed with molecules, making it harder to generate lift, and ultimately causing the plane to feel heavier when climbing through the air. Because of this, you can expect to experience increased takeoff distances and slower rates of climb in elevated density altitude situations.

Additionally, “loosely-packed” air can decrease an engine’s performance. If there are fewer air molecules than normal in an engine’s cylinders, then there’s less material there to mix with fuel during combustion, resulting in an overall lack of power. Propeller performance can also dwindle in direct relation to increased density altitude; the less dense the air is, the less molecules the propeller has to grab at as it swings through the air.

Consider this example situation in which acknowledging and calculating density altitude is crucial for safety:

A pilot flies a C172 to an airport at 5,500’ with a 6,500’ runway in the middle of California in July. The pilot arrives in the early morning, stops for breakfast, and picks up two friends to go for a fun flight with her around noon.

When the pilot arrived at this airport in the early morning, the air temperature was 20°C, and there was a low-pressure system (29.86) sitting over the airport with a predicted drop in pressure as the day continued.

Now, at noon, with two passengers in the plane, the pilot notices that the temperature has risen to 33°C. The plane will be departing loaded to max ramp weight. The winds are variable, and the airport is located in a valley whose peaks sit at 7,500’ and must be cleared by 1,000’ per legal ground clearance limitations.

Would you take off if you were this pilot? What resources would you use to check your potential performance given these variables?

As we continue to enjoy the summer season in the Golden State, just remember that the warmer the air is, the less dense it is. So check your density altitude!