Environmental Effects on Aircraft Performance
An aircraft's performance relies upon a wide array of factors including the wing design, the power plant, the weight of the airplane and the drag that is created, among other things. No matter the aircraft design, however, there is one thing that every aircraft's performance relies upon, and that is density altitude. Density altitude can be broken down to a simple and somewhat meaningless definition, yet still has a meaningful and practical application of the idea in relation to aircraft performance. Simply put, density altitude is pressure altitude corrected for nonstandard temperature variations. This doesn't mean much unless we understand pressure altitude as well as nonstandard temperature and what that does to the air we choose to fly in.
Altitude and Temperature
Altitude, as in the height above some standard plane, and temperature, as in the measurement of the heat energy that an element possesses, both have something in common that seriously effects the performance capabilities of an aircraft - pressure. Air pressure will tend to decrease as the altitude and temperatures increase. Therefore, as the pressure altitude increases or the temperatures increase, so will density altitude.
Pressure altitude can be thought of as the height above sea level when standard pressure exists: 29.92 inches of mercury (or 1013.25 millibars for those of us not in the US). These numbers are based on the International Standard Atmosphere, or ISA. "ISA is a hypothetical model – representative of an ideal atmosphere based on the thermodynamic equation, as defined by the International Civil Aviation Organization, devoid of water vapor, wind, and turbulence" (Arbogast, 2018). This model of the atmosphere also contains a standard temperature of 59 degrees Fahrenheit or 15 degrees Celsius. This also comes along with standard decreases in temperature and pressure as you increase in altitude. Those would be a decrease in pressure by 1 inch of mercury for every 1,000 feet of altitude above sea level and a decrease in temperature by 2 degrees Celsius for every 1,000 feet of altitude above sea level.
Effects on Density Altitude
In order to calculate density altitude then, you would need to know a few things. The pressure altitude, the standard temperature at that altitude, and the actual reported temperature. When the temperature increases, it will increase the density altitude for any given pressure altitude because it will correspond to a decrease in pressure. If my pressure altitude is 3,000 feet, yet my temperatures are higher than standard, my airplane will "feel" like it is operating at relatively higher altitude, even though I am not any higher above sea level. The decrease in air density will have a negative effect on performance, that is, an increase in takeoff and landing distance and a decrease in climb performance.
The reason for this relies on the way that the engines produce thrust and the way that the wings produce lift. Relatively lower pressure renders a wing less efficient at producing lift because of the smaller number of air molecules that are traveling around the wing. The density of the fluid that an airfoil is surrounded by is a key factor in the equation for lift. The same will be said for the production of thrust on a reciprocating engine with a propeller. The propeller is, in effect, a spinning airfoil and produces thrust by creating lift in the horizontal plane rather than the vertical plane. With less dense air flowing around the propeller, there will be less thrust produced just as there will be less lift produced by the wings. The engine power is also effected, as there is less air available to mix with fuel for combustion inside of the cylinders and the overall power produced is decreased.
What can be done?
Density altitude and its effects on aircraft performance are inescapable and as such should not be taken lightly. Taking off on short runways with obstacles in high density altitude conditions can be frightening in the most favorable of outcomes or deadly in the least favorable. As pilots, we must understand what we can change and how we can maintain safety in challenging scenarios. We can't change the altitude of the airport we are operating at but we can change the time of day or the time of year that we go into said airport. Taking off in the early morning when temperatures are cooler is a great way to mitigate the risks of operating at high density altitude. Additionally, avoiding high altitude airports in the heat of the summer may also be a good idea. Moreover, we can alter the performance characteristics of our airplane by choosing how much weight to carry. Fuel, baggage, and passengers are are not always easy things to limit, but it may just save your life one day.
The best way to mitigate risk when operating out of high density altitude airports is to build the habit of running performance numbers out of the airplanes approved Airplane Flight Manual or Pilots Operating Handbook before every flight as well as becoming familiar with a specific airplane's characteristics. I have found in my experience that not all airplanes, especially older ones, do exactly what the numbers say it should. We must remember that those numbers were derived from a brand new airplane, a new propeller, a competent test pilot, and great conditions. By being diligent with weight and balance numbers, takeoff and landing data, and becoming familiar with an airplane and your own piloting capabilities, we can mitigate the risks of hot temperatures, high altitudes, and short runways with obstacles.
Thanks for reading and may your views from the cockpit always be as beautiful as this one!
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