For example, using the formula for the same casewe just calculated with the table, we find the following:PA= H_{A} + PAVPA = 1,500 + 1,000(29.92 - 29.41)PA = 1,500 + 510PA = 2,010 feetBy comparison, you can see that this value is 34 feethigher than we found by using the table, but it is a closeenough approximation when nothing else is available.And, it may be done quickly in your head. With thepressure reduction computer, the same case yields apressure altitude of 1,979 feet.Pilots of aircraft, especially rotary wing aircraft,frequently ask for maximum pressure altitude fortakeoff and for all destinations. This is calculated usingthe lowest expected altimeter setting (QNH) for thedestination. The forecaster may have to interpret theother station’s forecast to determine if the forecast QNHwill be valid during the time the aircraft will be in thevicinity. Many rotary wing aircraft have a table in theiraircraft technical data that is entered using maximumpressure altitude and maximum temperature to find themaximum permissible load that can be carried.Maximum pressure altitude may be used by the pilot inlieu of density altitude.DENSITY ALTITUDEDensity altitude is defined as the altitude at which agiven air density is found in the standard atmosphere.For a given altitude, density altitude changes withchanges in pressure, air temperature, and humidity. Anincrease in pressure increases air density, so itdecreasesdensity altitude. Anincreaseintemperaturedecreasesair density, so it increases density altitude. An increasein humidity decreases air density, so itincreasesdensityaltitude. Changes in pressure and temperature have thegreatest effect on density altitude, and changes inhumidity have the least effect.If, for example, the pressure at Cheyenne,Wyoming, (elevation 6,140 feet) is equal to the pressureof the standard atmosphere at that elevation, and thetemperature is 101°F, the density would be the same asthat found at 10,000 feet. Therefore, the air is less densethan normal, and an aircraft on takeoff (atapproximately constant weight and power setting) willtake longer to get airborne. Air density also affectsairspeed. True airspeed and indicated airspeed are equalonly when density altitude is zero. True airspeedexceeds indicated airspeed when density altitudeincreases.No instrument is available to measure densityaltitude directly. It must be computed from the pressure(for takeoff, station pressure) and the virtualtemperature at the particular altitude underconsideration. This may be accomplished by using theDensity Altitude Computer (CP-718/UM) or fromTable 69, Density Altitude Diagram, of SmithsonianMeteorological Tables, NA-50-lB-521. Remember,virtual temperature is used in the computation ofdensity altitude.The quickest method of calculating density altitudeis to use the Density Altitude Computer (CP-718/UM),discussed in chapter 2.Density altitude must becomputed from the pressure (for takeoff, stationpressure) and the virtual temperature at the particularaltitude under consideration. Specific instructions areprinted on the device. Density altitude results from thecomputer may be estimated to the nearest 10 feetbetween the marked increments of 100 feet. If you arein a situation where you do not have a density altitudecomputer or the Smithsonian Meteorological Tablesavailable, you may ignore the humidity value andcalculate density altitude by the formulaDA = PA + (120 V_{t}),whereDA = density altitude,PA = pressure altitude at the level you desiredensity altitude,120 = a temperature constant (120 feet per 1°C),andV_{t} = actual temperature minus standardtemperature at the level of the pressurealtitude.For example, let's say the surface temperature is30°C and your pressure altitude is 2,010 feet. Look attable 1-6 and find the standard temperaturecorresponding to 2,000 feet. You should find 11°C.Plug these values into the formula to find the following:DA = PA + (120 V_{t})DA = 2,010 feet + [120(3°C - 11°C)]DA = 2,010 + 120(19)DA = 2,010 + 2,280DA = 4,290 feet1-62

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