Chapter 11: Aircraft Performance

Chapter 11 Aircraft Performance Introduction This chapter discusses the factors that affect aircraft performance, which include the aircraft weight, atmospheric conditions, runway environment, and the fundamental physical laws governing the forces acting on an aircraft. Importance of Performance Data The performance or operational information section of the Aircraft Flight Manual/Pilot’s Operating Handbook (AFM/ POH) contains the operating data for the aircraft; that is, the data pertaining to takeoff, climb, range, endurance, descent, and landing. The use of this data in flying operations is mandatory for safe and efficient operation. Considerable knowledge and familiarity of the aircraft can be gained by studying this material. 11-1 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 the AFM/POH has little or no value unless the user recognizes • Thrust, because the propeller is less efficient in thin air 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 Millibars percent oxygen, and 1 percent other gases, such as argon Standard Mercury Standard or helium. Most of the oxygen is contained below 35,000 Pressure 30 1016 Pressure feet altitude. 29.92Hg 25 847 1013 mb Atmospheric Pressure 20 677 Though there are various kinds of pressure, pilots are mainly 15 508 concerned with atmospheric pressure. It is one of the basic 10 339 factors in weather changes, helps to lift the aircraft, and 5 170 Atmospheric Pressure actuates some of the most important flight instruments in the 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 2. By applying a correction factor to the indicated Pressure Temperature altitude according to the reported “altimeter setting,” ("Hg) (°C) (°F) [Figure 11-3] 0 2992 150 590 1000 2886 130 554 3. By using a flight computer 2000 2782 110 519 3000 2682 91 483 Density Altitude 4000 2584 71 447 5000 2489 51 412 The more appropriate term for correlating aerodynamic 6000 2398 31 376 performance in the nonstandard atmosphere is density 7000 2309 11 340 altitude—the altitude in the standard atmosphere 8000 2222 −0.9 305 corresponding to a particular value of air density. 9000 2138 −2.8 269 10000 2057 −4.8 233 Density altitude is pressure altitude corrected for nonstandard 11000 1979 −6.8 198 temperature. As the density of the air increases (lower 12000 1902 −8.8 162 13000 1829 −10.8 126 density altitude), aircraft performance increases. Conversely, 14000 1757 −12.7 91 15000 1688 −14.7 55 16000 1621 −16.7 19 Altimeter Altitude 17000 1556 −18.7 −1.6 setting correction 18000 1494 −20.7 −5.2 280 1824 19000 1433 −22.6 −8.8 281 1727 20000 1374 −24.6 −12.3 282 1630 283 1533 Figure 11-2. Properties of standard atmosphere. 284 1436 285 1340 Since all aircraft performance is compared and evaluated 286 1244 using the standard atmosphere, all aircraft instruments 287 1148 are calibrated for the standard atmosphere. Thus, certain 288 1053 289 957 corrections must apply to the instrumentation, as well as the 290 863 aircraft performance, if the actual operating conditions do Add elevation 291 768 not fit the standard atmosphere. In order to account properly 292 673 for the nonstandard atmosphere, certain related terms must 293 579 be defined. 294 485 295 392 Pressure Altitude 296 298 297 205 Pressure altitude is the height above the standard datum 298 112 plane (SDP). The aircraft altimeter is essentially a sensitive 299 20 barometer calibrated to indicate altitude in the standard 2992 0 atmosphere. If the altimeter is set for 29.92 "Hg SDP, the 300 −73 altitude indicated is the pressure altitude—the altitude in the 301 −165 standard atmosphere corresponding to the sensed pressure. 302 −257 303 −348 304 −440 The SDP is a theoretical level at which the pressure of the 305 −531 elevation atmosphere is 29.92 "Hg and the weight of air is 14.7 psi. As 306 −622 Subtract atmospheric pressure changes, the SDP may be below, at, or 307 −712 above sea level. Pressure altitude is important as a basis for 308 −803 determining aircraft performance, as well as for assigning 309 −893 flight levels to aircraft operating at above 18,000 feet. 310 −983 Field elevation versus pressure. The aircraft is located The pressure altitude can be determined by any of the three Figure 11-3. on a field that happens to be at sea level. Set the altimeter to the following methods: current altimeter setting (29.7). The difference of 205 feet is added 1. By setting the barometric scale of the altimeter to to the elevation or a PA of 205 feet. 29.92 "Hg and reading the indicated altitude, 11-3 as air density decreases (higher density altitude), aircraft flight level. Density altitude can also be determined by referring performance decreases. A decrease in air density means a to the table and chart in Figures 11-3 and 11-4 respectively. high density altitude; an increase in air density means a lower density altitude. Density altitude is used in calculating aircraft Effects of Pressure on Density performance. Under standard atmospheric condition, air at Since air is a gas, it can be compressed or expanded. When each level in the atmosphere has a specific density; under air is compressed, a greater amount of air can occupy a standard conditions, pressure altitude and density altitude given volume. Conversely, when pressure on a given volume identify the same level. Density altitude, then, is the vertical of air is decreased, the air expands and occupies a greater distance above sea level in the standard atmosphere at which space. That is, the original column of air at a lower pressure a given density is to be found. Density altitude is computed using pressure altitude and 15000 temperature. Since aircraft performance data at any level is 14,000 based upon air density under standard day conditions, such 14000 performance data apply to air density levels that may not be identical to altimeter indications. Under conditions higher 13,000 or lower than standard, these levels cannot be determined 13000 directly from the altimeter. 12,000 12000 Density altitude is determined by first finding pressure 11,000 altitude and then correcting this altitude for nonstandard 11000 temperature variations.

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