COMPUTATION OF AIRCRAFT PERFORMANCE INDICATORS FROM OBSERVED DATA LEARNING  OBJECTIVES:  Identify  three aircraft performance indicators computed from observed  data.  Define  the  terms  pressure altitude,   density   altitude,   and   specific humidity.  Describe  the  procedure  used  to compute pressure altitude and density altitude. Identify the procedure used to find specific humidity. Air density and water vapor content of the air have an important effect upon aircraft engine performance and takeoff characteristics. In this section, we describe some of these effects and how they are computed. The three most common elements an Aerographer’s Mate must furnish information on are pressure altitude, density altitude, and specific humidity. All ofthesemay be determined by using a Density Altitude Computer, discussed in chapter 2, while pressure altitude and density  altitude  can  be  easily  obtained  from  ASOS. Pressure altitude and density altitude are given in feet; while specific humidity is provided in grams per gram or in pounds per pound.    Now let’s look at pressure altitude. PRESSURE  ALTITUDE l-60 Pressure altitude is defined as the altitude of a given  atmospheric  pressure  in  the  standard  atmosphere. The pressure altitude of a given pressure is usually a fictitious altitude, since it is rarely equal to true altitude. Pressure  altitude  is  equal  to  true  altitude  only  when pressure at sea level (or the flight-level pressure) corresponds  to  the  pressure  of  the  U.S.  Standard Atmosphere. Pressure altitude higher than the actual altitude indicates the air is less dense than normal, and the aircraft may not be able to carry a full (standard) cargo load. Pressure altitude lower than the actual altitude means the air is more dense than normal, and the aircraft may be able to takeoff successfully with a larger cargo load. Aircraft altimeters are constructed for the pressure- height  relationship  that  exists  in  the  standard atmosphere. Therefore, when the altimeter is set to standard sea-level pressure (29.92 inches of mercury), it indicates pressure altitude and not true altitude. Flight levels-an  indicated  altitude  based  on  an  altimeter setting of 29.92 inches-rather than true altitudes, are flown above 18,000 feet in the United States, and on over-water flights more than 100 miles offshore. The quickest  method  for  approximating  the  pressure  altitude is by using the Pressure Reduction Computer (CP- 402/UM), covered in chapter 2. Detailed instructions are listed on the computer. For your own station, you simply dial in the current station pressure and read the pressure altitude on the scale. The solution is more complex when converting forecast altimeter settings to pressure altitude, but the pressure reduction computer may still be used. On occasion, you may find yourself in a situation where this device is not available. Two alternate  methods  follow  that  will  enable  you  to calculate  approximations  of  the  pressure  altitude. Pressure altitude varies directly with the change in pressure  multiplied  by  a  complex  variable.  The  variable amount takes into account temperature and station elevation. Both methods simplify the equation but still give fairly close pressure altitude approximations. The first method uses a set of precalculated pressure altitudes based on pressure differences from standard pressure. These are listed in table 1-5. Using the table, you may find the pressure altitude value  corresponding  to  your  current  or  forecast altimeter setting or the current or forecast altimeter setting for any other station. This value must be added to your station elevation or the other station’s elevation to find the pressure altitude. For example, if your altimeter setting  is  29.41  inches  and  your  station  elevation  is 1,500 feet, you would enter the left side of the table with "29.4" and find the intersection of the column under "0.01" to find 476 feet. Add 476 feet to your station elevation, 1,500 feet, to find the pressure altitude 1,976 feet. You may also use the table to find pressure altitude by using station pressure. Station elevation should NOT be added to the value when using station pressure. The second method is useful when you do not have ready  access  to  the  table.  To  calculate  pressure  altitude, use the formula where PA = H_A+PAV, PA = pressure altitude, H_A  =  station  elevation,  and PA V = pressure altitude variation approximation (or 29.92 minus the current altimeter setting times  1,000).