The Weather Never Sleeps

All this hazard needs is a warm day

Most of the time in most of the United States, summer is a nice time to fly. The cold temperatures, snow, and ice of winter are forgotten, and spring's sometimes extremely violent thunderstorms are fading from memory. But one of summer's main dangers is so subtle it usually catches pilots by surprise. High density altitude doesn't announce itself with dark clouds, gusty wind, or thunder. And it's by no means limited to high-elevation airports.

To understand density altitude, all you need to know is that lift depends on the mass of the molecules of nitrogen, oxygen, and other gases in the air that flow over and under the wings each second. This, in turn, depends on the mass of the molecules of the various gases in each cubic foot of air (known as the air's density) and how fast the air is flowing over the wing.

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 that the engine sucks in has less oxygen; thus power decreases as air density decreases. The amount of thrust depends on the mass of air that a propeller or jet engine can push toward the aircraft's rear. Decreasing air density reduces thrust. Drag also decreases as air density decreases, which helps a little, but the decrease in power, thrust, and lift more than offset the drag reduction as air density decreases.

Air, like anything else, expands as it heats up, which means that a cubic foot of hot air has fewer molecules--that is, it's less dense--than a cubic foot of cooler air. As we go higher in the atmosphere, the air becomes less dense because there is less air above us squeezing down and compressing the air.

Generally, as you climb, air temperature decreases. If the colder air aloft were denser than the warm air below, it would descend. While this happens in thunderstorm downdrafts, most of the time the air's decreasing pressure lowers its density more than enough to make up for the density increase as the air cools.

Obviously, the amount of power, thrust, drag, and lift your airplane will produce changes as the air's density changes, no matter whether increasing altitude or increasing temperature is causing the density reduction.

Now, we're ready to see what the term high density altitude means. When aviators talk about high density altitude, they are describing the atmosphere at a particular time and place as having a high-altitude density. In other words, you could be preparing to take off from an airport that's only 2,000 feet above sea level, but the air's density--and thus your airplane's performance--could be like that normally found at 4,000 feet or higher.

The concept of density altitude is based on a set of figures known as the standard atmosphere, which you can think of as a global average atmosphere, with values of air pressure, temperature, and density for each altitude. The table at left shows standard atmosphere figures for sea level up to 4,000 feet above sea level using the units commonly used in the United States, including density in slugs per cubic foot.

The table shows us that on a "normal" day, the air pressure at 1,000 feet is 28.86 inches of mercury, the temperature 55.4 degrees F, and the density is 0.002309 slugs per cubic foot. But, if on a particular day a higher temperature at an airport with an elevation of 1,000 feet has made the density only 0.002112 slugs per cubic foot, we would say that the density altitude is 4,000 feet, no matter what the atmospheric pressure is at the time.

Pilots planning a takeoff use the different kinds of performance charts in their airplane's pilots operating handbook to calculate takeoff distance and other performance figures for a particular airplane.

The handbooks for different airplanes use a variety of charts to aid performance calculations. Often these charts don't mention density altitude. For example the takeoff distance chart in the Cessna 172M pilot's operating handbook uses the airplane's takeoff weight, the temperature, and the "pressure altitude" to calculate takeoff distance. The pressure altitude is the figure your altimeter reads when it's set at 29.92 inches of mercury. With this setting, it would read zero altitude at a sea level airport in the standard atmosphere. This chart shows that if you were at sea level with the standard atmosphere pressure in a Cessna 172M weighing 2,300 pounds, you would need 775 feet of runway to lift off if the temperature were 32 degrees. If the temperature were 104 degrees at the same air pressure, you'd need 1,030 feet of runway before the wheels leave the pavement.

Some airplane POH charts use air pressure and temperature to give density altitude. A pilot then uses the density altitude for performance calculations. In such cases, you can use an electronic or E6B computer to easily find the density altitude, or use the weather calculator on the El Paso National Weather Service office Web site.

Using this calculator with the figures we obtained from the Cessna 172 chart shows why the takeoff distance is so much greater at 104 degrees. This temperature and a pressure of 29.92 inches of mercury give a density altitude of 2,904 feet above sea level. If the temperature drops to 32 degrees and the pressure remains at 29.92 inches of mercury, the density altitude falls to 1,745 feet below sea level. Just as high temperatures harm performance, low temperatures improve it.

Reading National Transportation Safety Board reports of accidents involving high density altitude shows that it's usually listed as a contributing cause, not the major cause of accidents. One NTSB report of an instructional flight said that the instructor reported the airplane had stopped climbing and "the ridge was getting closer" when the pilot "attempted a shallow bank at a direction 90 degrees from their original course." At this time "the left wing dipped and we lost control." The NTSB determined that the accident's probable cause was: "the flight instructor's failure to maintain airspeed while maneuvering resulting in an inadvertent stall and subsequent impact with terrain. Contributing factors were the high density altitude, rising terrain, and improper in-flight planning."

Another NTSB report tells of a private pilot who lost control after takeoff and flew into the tops of several trees before landing in water 300 feet beyond the end of the runway. The report cites the "failure to properly lean the mixture, which resulted in a power deficiency, a degraded climb capability, and the inability to attain/maintain an adequate airspeed that led to a stall/mush condition...Also causal was the pilot in command's inaccurate preflight performance and weight and balance calculations." This accident illustrates an important point about high density altitude flying: When the air's density is low you need to lean the airplane's air/fuel mixture.

When planning a flight you should remember that your airplane's performance charts are based on figures obtained by test pilots flying new airplanes. It's a good idea to allow for the fact that you aren't a test pilot and your airplane, even if very well maintained, has lost a little pep with age.

Since potentially dangerous air density isn't going to announce itself the way a thunderstorm would, you need to obtain the latest weather data and use it with your airplane's performance charts before going flying on a nice, warm day.

Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.