Ever wonder how air density affects airplanes? Airplanes generate lift primarily by deflecting air downward. Their wings are designed to push air downward, resulting in an equal and opposite reaction that pushes or “lifts” the airplane up. The density of the surrounding air, however, can affect an airplane’s performance in several ways.
An Introduction to Air Density
Air density is a measurement of how dense or compacted the air molecules are in a given space. It represents the mass of air per unit volume in the space. When air molecules are packed tightly together, the density is high. When air molecules are spread out, resulting in less mass, the density is low.
Density Altitude Explained
In aviation, density altitude is typically used to measure air density. It represents the altitude at which the air currently feels like. More specifically, density altitude is the pressure altitude corrected for nonstandard temperature. It takes into the atmospheric pressure, temperature and humidity.
When the atmospheric pressure increases, so does the air density. When the air temperature or humidity increases, on the other hand, the air density decreases. Density altitude is calculated using these three factors to provide a more complete look at the density of air molecules.
Lift
Air density will affect an airplane’s ability to generate lift. As previously mentioned, airplanes generate lift primarily by deflecting or pushing air downward. The more air molecules they push downward, the greater the lift. Thin air means there are fewer air molecules, so the wings must move faster to generate lift. This is why most commercial airliners fly no higher than 42,000 feet — where there is still a sufficient amount of air molecules to produce lift.
Engine Performance
The performance of an airplane’s engines is influenced by air density. Nearly all types of aircraft engines use oxygen. Piston and turbine engines, for instance, mix fuel and oxygen together in a combustion chamber. When ignited, the fuel-oxygen mixture powers the engines and propels the airplane forward. Low air density means there’s less oxygen to feed the engines, resulting in poorer performance.
Landing Distance
Another way in which air density affects airplanes is by changing the required landing distance. Airplanes landing at high-elevation airports where the air is particularly thin may require up to 30% more runway length when compared to airports at sea level. This is because thin air increases true airspeed (TAS) and ground speed but not indicated airspeed (IAS).
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