COMPUTATION OF AIRCRAFTPERFORMANCE INDICATORS FROMOBSERVED DATALEARNING OBJECTIVES: Identify threeaircraft performance indicators computed fromobserved data. Define the terms pressurealtitude, density altitude, and specifichumidity. Describe the procedure used tocompute pressure altitude and density altitude.Identify the procedure used to find specifichumidity.Air density and water vapor content of the air havean important effect upon aircraft engine performanceand takeoff characteristics. In this section, we describesome of these effects and how they are computed. Thethree most common elements an Aerographer’s Matemust furnish information on are pressure altitude,density altitude, and specific humidity. All ofthesemaybe determined by using a Density Altitude Computer,discussed in chapter 2, while pressure altitude anddensity altitude can be easily obtained from ASOS.Pressure altitude and density altitude are given in feet;while specific humidity is provided in grams per gramor in pounds per pound. Now let’s look at pressurealtitude.PRESSURE ALTITUDEl-60Pressure altitude is defined as the altitude of agiven atmospheric pressure in the standard atmosphere.The pressure altitude of a given pressure is usually afictitious altitude, since it is rarely equal to true altitude.Pressure altitude is equal to true altitude only whenpressure at sea level (or the flight-level pressure)corresponds to the pressure of the U.S. StandardAtmosphere. Pressure altitude higher than the actualaltitude indicates the air is less dense than normal, andthe aircraft may not be able to carry a full (standard)cargo load. Pressure altitude lower than the actualaltitude means the air is more dense than normal, andthe aircraft may be able to takeoff successfully with alarger cargo load.Aircraft altimeters are constructed for the pressure-height relationship that exists in the standardatmosphere. Therefore, when the altimeter is set tostandard sea-level pressure (29.92 inches of mercury), itindicates pressure altitude and not true altitude. Flightlevels-an indicated altitude based on an altimetersetting of 29.92 inches-rather than true altitudes, areflown above 18,000 feet in the United States, and onover-water flights more than 100 miles offshore. Thequickest method for approximating the pressure altitudeis by using the Pressure Reduction Computer (CP-402/UM), covered in chapter 2. Detailed instructionsare listed on the computer. For your own station, yousimply dial in the current station pressure and read thepressure altitude on the scale. The solution is morecomplex when converting forecast altimeter settings topressure altitude, but the pressure reduction computermay still be used. On occasion, you may find yourself ina situation where this device is not available. Twoalternate methods follow that will enable you tocalculate approximations of the pressure altitude.Pressure altitude varies directly with the change inpressure multiplied by a complex variable. The variableamount takes into account temperature and stationelevation. Both methods simplify the equation but stillgive fairly close pressure altitude approximations.The first method uses a set of precalculated pressurealtitudes based on pressure differences from standardpressure. These are listed in table 1-5.Using the table, you may find the pressure altitudevalue corresponding to your current or forecastaltimeter setting or the current or forecast altimetersetting for any other station. This value must be added toyour station elevation or the other station’s elevation tofind the pressure altitude. For example, if your altimetersetting is 29.41 inches and your station elevation is1,500 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 stationelevation, 1,500 feet, to find the pressure altitude 1,976feet.You may also use the table to find pressure altitudeby using station pressure. Station elevation shouldNOT be added to the value when using station pressure.The second method is useful when you do not haveready access to the table. To calculate pressure altitude,use the formulawherePA = H_{A}+PAV,PA = pressure altitude,H_{A}= station elevation, andPA V = pressure altitude variation approximation(or 29.92 minus the current altimeter settingtimes 1,000).