It must be emphasized that the manufacturers’ information  and data furnished in the AFM/POH is not standardized.  Some provide the data in tabular form, while others use  graphs. In addition, the performance data may be presented  on the basis of standard atmospheric conditions, pressure  altitude, or density altitude. The performance information in  the AFM/POH has little or no value unless the user recognizes  those variations and makes the necessary adjustments.  To be able to make practical use of the aircraft’s capabilities  and limitations, it is essential to understand the significance  of the operational data. The pilot must be cognizant of the  basis for the performance data, as well as the meanings of  the various terms used in expressing performance capabilities  and limitations.  Since the characteristics of the atmosphere have a major  effect on performance, it is necessary to review two dominant  factors—pressure and temperature.  The atmosphere is an envelope of air that surrounds the  Earth and rests upon its surface. It is as much a part of the  Earth as is land and water. However, air differs from land  and water in that it is a mixture of gases. It has mass, weight,  and indefinite shape.  Air, like any other fluid, is able to flow and change its shape  when subjected to even minute pressures because of the  lack of strong molecular cohesion. For example, gas will  completely fill any container into which it is placed, expanding  or contracting to adjust its shape to the limits of the container.  The atmosphere is composed of 78 percent nitrogen, 21  percent oxygen, and 1 percent other gases, such as argon  or helium. Most of the oxygen is contained below 35,000  feet altitude.  Though there are various kinds of pressure, pilots are mainly  concerned with atmospheric pressure. It is one of the basic  factors in weather changes, helps to lift the aircraft, and  actuates some of the most important flight instruments in the  aircraft. These instruments often include the altimeter, the  airspeed indicator (ASI), the vertical speed indicator (VSI),  and the manifold pressure gauge.  Though air is very light, it has mass and is affected by the  attraction of gravity. Therefore, like any other substance, it  has weight; because it has weight, it has force. Since it is a  fluid substance, this force is exerted equally in all directions,  and its effect on bodies within the air is called pressure.  Under standard conditions at sea level, the average pressure  The pressure of the atmosphere may vary with time but more  importantly, it varies with altitude and temperature. Due to  the changing atmospheric pressure, a standard reference  was developed. The standard atmosphere at sea level has  a surface temperature of 59 degrees Fahrenheit (°F) or 15  degrees Celsius (°C) and a surface pressure of 29.92 inches  of mercury ("Hg) or 1013.2 millibars (mb).  [Figure 11-1]  exerted by the weight of the atmosphere is approximately 14.7  pounds per square inch (psi). The density of air has significant  effects on the aircraft’s performance. As air becomes less  dense, it reduces:  A standard temperature lapse rate is one in which the  temperature decreases at the rate of approximately 3.5 °F or  2 °C per thousand feet up to 36,000 feet. Above this point,  the temperature is considered constant up to 80,000 feet. A  standard pressure lapse rate is one in which pressure decreases  at a rate of approximately 1 "Hg per 1,000 feet of altitude gain  to 10,000 feet.  [Figure 11-2]  The International Civil Aviation  Organization (ICAO) has established this as a worldwide  standard, and it is often referred to as International Standard  Atmosphere (ISA) or ICAO Standard Atmosphere. Any  temperature or pressure that differs from the standard lapse  rates is considered nonstandard temperature and pressure.  Adjustments for nonstandard temperatures and pressures are  provided on the manufacturer’s performance charts.  •  Power, because the engine takes in less air  •  Thrust, because the propeller is less efficient in thin air  •  Lift, because the thin air exerts less force on the airfoils  Structure of the Atmosphere  Figure 11-1.  Standard sea level pressure.  Atmospheric Pressure  11-2  30  25  20  15  10  5  0  Inches of  Mercury  Millibars  1016  847  677  508  339  170  0  29.92"  Standard  Sea Level  Pressure  Hg  1013  Standard  Sea Level  Pressure  mb  Atmospheric Pressure