Aircraft Performance

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