Aircraft Performance: Air Density

Takeoff and landing are perhaps two of the most labor intensive tasks involved in piloting an airplane, and they start long before the wheels leave the ground.

Takeoffs involve much more than smooth piloting skills; they involve careful planning and preparation. A very smooth takeoff is of little value if the airplane, once airborne, is faced with obstacles impossible to avoid. The takeoff performance of the airplane needs to be matched to the runway and the surrounding obstacles prior to actually taking off. Today, we’ll take a look at one of the factors affecting takeoff performance: air density. This post is excerpted from the new fourth edition of The Pilot’s Manual: Ground School (PM-2C).

One cause of an increase in density altitude is a decrease in air density. This results in a longer ground run and takeoff distance to clear a 50-foot obstacle. A decrease in air density can be caused by a number of factors.

A lower air pressure will decrease the density and this can occur as a result of a different ground-level ambient pressure or as a result of a higher airport elevation. This effect is covered by pressure altitude, which relates the actual pressure experienced by the airplane to a level in the standard atmosphere that has an identical pressure. High elevation airports lead to longer takeoff distances.

A higher air temperature will also decrease the air density, reducing airplane and engine performance.


Hot, high and humid means decreased performance. (Click to view full-size!)

If the air density decreases, the engine–propeller combination will not produce as much power and so the takeoff distance will increase. In addition to the power-producing performance of the engine–propeller decreasing, the aerodynamic performance of the airplane will also decrease as air density becomes less.

To produce the required lift force (L = Lifting ability ½ρV2 × S), a decrease in air density (ρ) means that for the same required indicated airspeed, an increase in the velocity (true airspeed, V) is required and a longer takeoff distance will result. Not only does a lower air density affect the aerodynamic performance of the airframe (controlled by ½ρV2), it also decreases the weight of the fuel/air mixture in the engine cylinders, causing a decrease in engine power.

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