Density Altitude on Aircraft Performance

Density altitude (DA) is pressure altitude corrected for non-standard atmospheric conditions (Federal Aviation Administration, 2016). Standard atmospheric conditions at sea level are when the pressure is 29.92" Hg, and the temperature is 15 degrees Celsius. When the actual conditions outside differ from standard, DA changes and thus affects performance. One way DA is explained is to think of it as the altitude the plane actually feels and performs like it's at.

When you have a higher than standard pressure outside, air molecules are more compact for any given volume of air measured. With that being said, there are more air molecules and force attacking the surfaces of the aircraft which helps increase performance. The increase in pressure increases the amount of lift and thrust that is produced. The exact opposite occurs with lower pressure.

With lower temperatures, air is denser. Since it is denser, again, there's more air molecules acting upon the surfaces of the aircraft to cause a reaction producing more trust and lift. The exact opposite happens with higher, less dense temperatures. 

Density altitude can be calculated using the formulas provided in the picture below.

When a high DA is calculated, the aircraft will perform like it is at a higher altitude. At higher altitudes, the air is less dense and thus, performance is decreased. For pilots, this is an important factor to consider because high DA's might be outside of the limitations of the aircraft and if you fall outside the limitations, hazards are likely.

The best way to mitigate hazards due to high DA is to calculate your performance data which is directly affected by density altitude. This data includes takeoff and landing distances, engine performance, true airspeed, climb rate, and much more. With and increase in DA, there is a 3% reduction in power and a 15% increase in takeoff distance per 1000ft (Goodman & Small Griswold, 2018). The picture below is a performance chart example I use for the aircraft that I use -the Piper Cherokee. 

Federal Aviation Administration (FAA). (2016). Pilot’s Handbook of Aeronautical Knowledge (PHAK). Retrieved https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/

Goodman, C. J., Small Griswold, J. D. (2018, March 1). Climate impacts on density altitude and aviation operations. Journal of Applied Meterology and Climatology. https://doi.org/10.1175/JAMC-D-17-0126.1