DENSITY ALTITUDE. Density altitude is defined as the altitude at which a given density is found in the standard atmosphere. If, for example, the pressure at Cheyenne, Wyoming, (elevation 6,140 feet) is equal to the standard atmosphere pressure for 6,140 feet, but the temperature at that station is 101F, the density there is the same as that found at 10,000 feet in the standard atmosphere. The air is less dense than normal; therefore, an aircraft on takeoff (at approximately constant weight and power setting) will take longer to get airborne. Air density also effects airspeed. True airspeed and indicated air-speed are equal only when density altitude is zero. True airspeed exceeds indicated air-speed when density altitude increases.

No instrument is available to measure density altitude directly. It must be computed from the pressure (for takeoff, station pressure) and the virtual temperature at the particular altitude under consideration. This may be accomplished by using the Density Altitude Computer (CP-718/UM) or from Table 69, Density Altitude Diagram, of Smithsonian density altitude.

DENSITY ALTITUDE CALCULA-TIONS. The quickest method of calculat-ing density altitude is to use the Density Altitude Computer (CP-718/UM). Specific instructions are printed on the device. Density altitude results from the computer may be estimated to the nearest 10 feet between the marked increments of 100 feet. If you are in a situation where you do not have a density altitude computer or the Smithsonian

  For example, lets say the surface temperature is 30C and your pressure altitude is 2,010 feet.

Look at table 6-1-6 and find the standard temperature corresponding to 2,000 feet. You should find 11C, Plug these values into the formula to find the following:

For an acceptable result with slightly less precision, you may use the density altitude diagram (fig. 6-1-9) to obtain density altitude to the nearest 200 feet. Enter the bottom of the diagram with your air temperature and proceed vertically to the intersection of the pressure altitude line, then horizontally to the left side of the diagram to find the density altitude. The light dashed line shows an example using 22C and a pressure altitude of 10 feet, resulting in a density altitude of about 1,000 feet.

You may interpolate for more precise values, but this precision isnt often necessary for most density altitude calculations. (A quick method of determining standard temperatures in degrees Celsius for all levels up to 35,000 feet is to double the altitude in thousands of feet, subtract 15, and change the sign.)

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