While airplanes don?t suffer from heat exhaustion the way that people do, they are affected by the heat and other conditions that lower the air?s density.
A National Transportation Safety Board report on a Cessna 182 accident at Willcox, Arizona, is typical of many in the NTSB files: ?The pilot reported that the takeoff ground roll was unusually long. After liftoff the aircraft stayed in ground effect until just before the end of the runway, then climbed to approximately 100 feet. The pilot landed in a soft field. During the landing roll the aircraft nosed over. The density altitude was 5,800 feet. There were no mechanical failures or malfunctions reported.? The report identified the probable causes of the accident as poor preflight planning and preparation, and a late decision to abort the takeoff. The report also cited high density altitude as a contributing factor.
Fortunately, in this particular accident the pilot and both passengers suffered only minor injuries.
Density altitude describes the effects of air pressure, temperature, and humidity on aircraft performance. The way we describe the potentially dangerous condition, high density altitude, can get in the way of understanding the concept because the term violates the normal placement of modifiers in the English language. The word high describes the altitude. A native English speaker who?s not familiar with aviation speak would assume that high refers to density. The condition that leads to NTSB accident reports is really high-altitude air density.
The American Meteorological Society?s Glossary of Weather and Climate defines density altitude as: ?The altitude in the standard atmosphere at which the air has the same density as the air at the point in question.? The definition goes on to say that density altitude is the same as pressure altitude corrected for non-standard temperature.
You can think of the standard atmosphere as a list of the average temperatures, air pressures, and air densities at various altitudes from the Earth?s surface up to outer space. Figure 1 is a simplified depiction of the lower part of the atmosphere where most general aviation airplanes fly.
The first aeronautical engineers defined the standard atmosphere early this century to give them an agreed-upon set of figures for the atmospheric factors that determine aircraft performance. These figures are used to calculate a wing?s lift, an engine?s power, a propeller?s or jet?s thrust, and an aircraft?s drag at various altitudes.
Pilots need to adjust these theoretical values to take into account differences between the standard atmosphere and the real atmosphere at a particular time and place. Generally, you will use tables in the pilot?s operating handbook (POH) for your aircraft to make the adjustments. With most of these charts, you don?t calculate the density altitude, but go directly from pressure altitude and temperature to a performance factor such as takeoff distance.
To see what the charts are really doing, look at Figure 1 and imagine that you have some kind of device that directly measures the air?s density. Imagine that your device tells you that the air?s density is 0.001812 slugs per cubic foot. (A slug is a unit of mass approximating 32.2 pounds or 15 kilograms.) You?d find that figure on the chart and then see that it?s the same as the air?s density at 9,000 feet in the standard atmosphere.
This means the aircraft will perform as though it were at 9,000 feet regardless of the true altitude. For example, if you were planning to take off from Colorado Springs, Colorado, where the airport elevation is 6,090 feet above sea level, on a day when the temperature was 86 degrees Fahrenheit, you?d be at a density altitude of approximately 9,000 feet. The performance chart for a Cessna 172M (Figure 2) shows that at this density altitude you?d need a ground run of 1,700 feet for takeoff if the airplane was loaded to its maximum weight of 2,300 pounds. At a higher ambient temperature, you?d be off the takeoff performance chart and should not attempt a takeoff. In contrast, if you were at sea level and the temperature was 32 degrees Fahrenheit, the same Cessna 172M would need a ground roll of only 775 feet for takeoff. By the way, under these conditions the density altitude would be around minus 1,700 feet, that is, 1,700 feet below sea level.
This explains why small training airplanes, such as Cessna 152s, perform so much better in cold weather than during the summer. The cold, dense air increases lift, thrust, and power. In a very basic way, you can think of lift depending on how many molecules of air hit the wing each second, how many molecules the propeller hurls toward the rear, and how many molecules the engine pulls in to combine with gasoline. The denser the air, the more molecules in each cubic foot and the more molecules that hit the wings. Denser air, of course, increases drag. But the extra lift, power, and thrust it provides more than make up for this penalty. Conversely, true airspeed increases with altitude?to a point?because the air is less dense and creates less drag at higher altitudes.
Using most POH charts requires finding the pressure altitude. If you?re in an airplane, you can do this simply by setting the altimeter at 29.92 and reading the altitude the instrument shows.
If you are not in an airplane, there are three ways to find pressure altitude. One is to find the station pressure, that is, the figure read directly from a barometer at your location. You could then use a standard atmosphere table to get the pressure altitude. For example, if the station pressure is 26.82, Figure 1 shows that you are at a pressure altitude of 3,000 feet, even though your true altitude could be higher or lower.
The second method is to use a ?density altitude chart? like the one shown in Figure 3, which comes from the FAA?s recreational and private pilot test booklet. For instance, if the altimeter setting is 30.10 and you are at an airport with an elevation of 1,000 feet, take the pressure altitude conversion factor for 30.10, which is minus 165 and add it to the field elevation to obtain a pressure altitude of 835 feet.
For altimeter settings below 29.92 the conversion factors are positive. Again, you add them to the field elevation to obtain the pressure altitude. This makes sense because lower air pressure reduces the air?s density, giving you a higher-altitude density.
The third way to find pressure altitude requires access to the Internet. Go to the Weather Calculator on the El Paso, Texas, National Weather Service office Web site (http://nwselp.epcc.edu/elp/wxcalc.html) and have it do the calculation. Finding pressure altitude requires that you first convert the 30.10 altimeter setting to a station pressure, which turns out to be 29.04 for a 1,000-foot elevation. You use this to find the pressure altitude of 824.6 feet. The chart in Figure 3, like all such charts, gives you an estimate. Such estimates are good enough for performance planning because none of the figures involved is exact.
Let?s use the chart in Figure 3 to find the density altitude when the temperature is 90 degrees and the altimeter setting is 30.10 at a 1,000-foot elevation airport. The red lines on Figure 3 show how it?s done. You come up from 90 degrees to a slanting line parallel to the lines for sea level and 1,000 feet, about eight-tenths of the way between them for the 835-foot pressure altitude. You then follow the horizontal line to the left to read a density altitude of about 3,100 feet. The El Paso NWS Weather Calculator gives a density altitude of 3,148.58 feet for these numbers.
If you go to the Weather Calculator you will see that you also have to supply a dew point as well as the temperature and station pressure to obtain density altitude. Even though most density altitude charts and calculators do not take humidity into account, it affects density altitude because humid air is less dense than dry air. Aircraft performance will suffer on a humid day, but the effect of humidity isn?t as great as the effects of temperature and air pressure.
You can use the Weather Calculator to see what difference humidity makes. To find the 3,148-foot density altitude for a 90-degree temperature and 30.10 altimeter setting, a dew point of 50 degrees was used. This would be a fairly comfortable 90-degree day. If you make the dew point 75 degrees, which is about as humid as it gets even along the Gulf of Mexico, and keep everything else the same, the density altitude becomes 3,373 feet. Apparent temperature is a measure of the danger of hot-weather exercise because high humidity slows the evaporation of sweat from the body. Evaporation cools the body. If sweat doesn?t evaporate not only do you begin feeling sticky, your body?s thermostat can be thrown out of whack. This can lead to problems ranging from annoying heat cramps to life-threatening heat exhaustion.
Fortunately, high apparent temperatures make us uncomfortable. If you listen to your body, drink plenty of water, slow down, and get into a cool place, you?ll be OK. Apparent temperature can also work the other way. When the temperature is 101 degrees at Colorado Springs, the density altitude is 10,049 feet. Yet, in the dry air of Colorado, the 101-degree temperature would have felt like 97 degrees because sweat evaporates quickly into dry air. While 97 degrees is still warm, it wouldn?t be as uncomfortable as a slightly cooler day in a humid place such as Mobile, Alabama.
Pilots can?t count on feeling uncomfortable to warn of potential performance losses from a high-altitude air density. Their best bet is to calculate the density altitude themselves or, better yet, to ask during a weather briefing. It?s always a good idea to use performance charts to calculate takeoff distances and climb rates before taxiing onto the runway.
Once you?ve made your calculations, throw in a generous fudge factor to allow for the fact that the charts are based on figures obtained by a test pilot flying a new airplane. Some flight instructors recommend doubling takeoff distances obtained from performance charts. Even if the humidity is low and the temperature is comfortable, flying an airplane that doesn?t want to leave the runway and then won?t climb is a good way to produce a lot of pilot sweat.