July 2009Departments

The Weather Never Sleeps:

Summer's challenge

Dealing with density altitude

The heat that accompanies summer's arrival in most of the country results in thinner air, effectively raising the pressure altitude higher than the actual altitude--sometimes much higher. This higher density altitude decreases lift and robs aircraft engines of power.

The somewhat abstract concept of density altitude represents a real danger to aircraft. To fight it, sometimes a pilot should lean the engine's fuel/air mixture for takeoff--an idea that might seem strange to pilots who learn to fly from airports at relatively low elevations.

Understanding density altitude begins with the fact that the lift a wing creates depends on the wing's size and shape, and the mass of the molecules of nitrogen, oxygen, and other gases in the air flowing around the wing. This, in turn, depends on the airspeed and on the air's density. Density refers to the mass of a substance per a unit of volume, such as the number of kilograms in each cubic meter of a solid. In American units, density is often described in pounds per cubic foot, which works well enough for ordinary uses. But pounds are a measure of force, not mass. Scientists and engineers working in American units use the slug for mass. Near the Earth's surface, a slug is about 32.2 pounds.

Since lift depends on the air's density as well as airspeed, anything that decreases the air's density decreases lift. Decreased lift is only the beginning. An airplane's engine produces power by burning the correct mixture of fuel and oxygen from the air. As air density decreases, each cubic foot of air the engine sucks in has fewer molecules, including oxygen. Thus, power decreases as air density decreases.

Thrust depends on the mass of air that an engine can push toward the aircraft's rear; decreased air density reduces thrust. Drag also decreases as air density decreases, but not enough to offset the decreases in power, thrust, and lift.

The density of any gas, including air, depends on its temperature, pressure, and mass of the gases in the air. Since nitrogen and oxygen account for approximately 99 percent of the molecules in the air, and the mass of these molecules doesn't change, air density depends mostly on pressure and temperature. (The amount of water vapor in the air also changes the air's density, with humid air being less dense than dry air, but this change is minor compared to the effects of temperature and pressure.)

Atmospheric pressure is caused by the weight of the air above any particular point pushing down on the air below it. This is why air pressure decreases with altitude. Air density also decreases as the air becomes warmer and increases as it grows colder.

Since the air aloft is usually cooler than at the surface, you might wonder why the air aloft isn't denser than the warmer air below. Lower atmospheric pressure and the resulting lower density aloft more than make up for the colder air. Air pressure also changes with the weather. Storms, in fact, are areas of relatively low atmospheric pressure at all levels of the atmosphere.

As you can imagine, ever-changing air density complicates life for someone trying to design an aircraft. For example, how will a particular combination of air pressure, altitude, and temperature affect an airplane's rate of climb? But pilots need to be able to calculate performance, such as the runway length needed for a takeoff, using the temperature and pressure measurements available to them. (No direct measurements of density are available; it has to be calculated.)

Early in the twentieth century, aeronautical engineers and meteorologists developed the standard atmosphere as a way to characterize aircraft performance in the atmosphere's ever-changing density. You can think of the standard atmosphere as a global average atmosphere, with values of air pressure, temperature, and density for each altitude. Figure 1 is an excerpt from a standard atmosphere table that shows altitude in feet, air pressure in inches of mercury, temperature in degrees Fahrenheit, and density in slugs per cubic foot.

Altitude (in feet) Pressure (in. Hg) Temperature (F) Density (slugs per cubic foot)
0 29.92 59.0 0.002378
1,000 28.86 55.4 0.002309
2,000 27.82 51.9 0.002242
3,000 26.82 48.3 0.002176
4,000 25.84 44.7 0.002112
5,000 24.89 41.2 0.002049
Standard temperature and pressure decrease with an increase in altitude.

The chart shows that a 1,000-foot-elevation airport on a "standard" day would have an air temperature of 55.4 degrees F; the air pressure would be 28.86 inches of mercury, and the air density 0.002309 slugs per cubic foot. But on a hot day with a temperature of 100 F, and the pressure of 28.86 inches of mercury, the air's density would be very close to 0.002112. When we look at the chart, we see that this density is the same as the density at 4,000 feet in the standard atmosphere. We say that the density altitude in this case is 4,000 feet.

Dealing with density altitude is all about understanding its effects and altering the airplane's configuration based on it. A piston engine runs well only when an exact amount of gasoline is mixed with the air flowing into engine's carburetor, or the amount of fuel injected directly into the cylinders. As the airplane climbs into the less dense air aloft, too much gasoline is present for the thinner air, making the engine run rich and reducing its power. The pilot uses the mixture control to "lean" the mixture; that is, reduce the amount of gasoline combining with the air going into the engine.

A pilot attempting to take off on a hot summer afternoon will not succeed if the air's density is too low to produce the needed power, thrust, and lift. Summer is a good time for students to learn about performance calculations and for experienced pilots to become reacquainted with them.

Jack Williams, a freelance science writer specializing in weather and climate, is an instrument-rated private pilot. The latest of his six books is The AMS Weather Book: The Ultimate Guide to America's Weather. Visit his Web site.