For example, using the formula for the same case we just calculated with the table, we find the following: PA=  HA  +  PAV PA = 1,500 + 1,000(29.92 - 29.41) PA = 1,500 + 510 PA = 2,010 feet By comparison, you can see that this value is 34 feet higher than we found by using the table, but it is a close enough approximation when nothing else is available. And, it may be done quickly in your head. With the pressure reduction computer, the same case yields a pressure altitude of 1,979 feet. Pilots of aircraft, especially rotary wing aircraft, frequently ask for maximum pressure altitude for takeoff and for all destinations. This is calculated using the  lowest  expected  altimeter  setting  (QNH)  for  the destination. The forecaster may have to interpret the other station’s forecast to determine if the forecast QNH will be valid during the time the aircraft will be in the vicinity. Many rotary wing aircraft have a table in their aircraft technical data that is entered using maximum pressure altitude and maximum temperature to find the maximum  permissible  load  that  can  be  carried. Maximum pressure altitude may be used by the pilot in lieu of density altitude. DENSITY  ALTITUDE Density altitude is defined as the altitude at which a given air density is found in the standard atmosphere. For a given altitude, density altitude changes with changes in pressure, air temperature, and humidity. An increase in pressure increases air density, so it decreases density altitude. An increase in temperature decreases air density, so it increases density altitude. An increase in  humidity  decreases  air  density,  so  itincreases  density altitude. Changes in pressure and temperature have the greatest effect on density altitude, and changes in humidity have the least effect. If,   for   example,   the   pressure   at   Cheyenne, Wyoming, (elevation 6,140 feet) is equal to the pressure of the standard atmosphere at that elevation, and the temperature is 101°F, the density would be the same as that found at 10,000 feet. Therefore, the air is less dense than  normal,  and  an  aircraft  on  takeoff  (at approximately constant weight and power setting) will take longer to get airborne. Air density also affects airspeed. True airspeed and indicated airspeed are equal only when density altitude is zero. True airspeed exceeds  indicated  airspeed  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 Meteorological  Tables,  NA-50-lB-521.  Remember, virtual  temperature  is  used  in  the  computation  of density  altitude. The  quickest  method  of  calculating  density  altitude is to use the Density Altitude Computer (CP-718/UM), discussed in chapter 2. Density altitude must be computed  from  the  pressure  (for  takeoff,  station pressure) and the virtual temperature at the particular altitude  under  consideration.  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 Meteorological Tables available, you may ignore the humidity value and calculate  density  altitude  by  the  formula DA = PA + (120 Vt), where DA =  density  altitude, PA  = pressure altitude at the level you desire density  altitude, 120 = a temperature constant (120 feet per 1°C), and Vt  =  actual  temperature  minus  standard temperature at the level of the pressure altitude. For example, let's say the surface temperature is 30°C and your pressure altitude is 2,010 feet. Look at table  1-6  and  find  the  standard  temperature corresponding  to  2,000  feet.  You  should  find  11°C. Plug these values into the formula to find the following: DA = PA + (120 Vt) DA = 2,010 feet + [120(3°C - 11°C)] DA = 2,010 + 120(19) DA = 2,010 + 2,280 DA = 4,290 feet 1-62


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