Aircraft Performance

Pressure Temperature Altitude (ft) ("Hg) (°C) (°F) 0 29.92 15.0 59.0 1,000 28.86 13.0 55.4 2,000 27.82 11.0 51.9 3,000 26.82 9.1 48.3 4,000 25.84 7.1 44.7 5,000 24.89 5.1 41.2 6,000 23.98 3.1 37.6 7,000 23.09 1.1 34.0 8,000 22.22 −0.9 30.5 9,000 21.38 −2.8 26.9 10,000 20.57 −4.8 23.3 11,000 19.79 −6.8 19.8 12,000 19.02 −8.8 16.2 13,000 18.29 −10.8 12.6 14,000 17.57 −12.7 9.1 15,000 16.88 −14.7 5.5 16,000 16.21 −16.7 1.9 Altimeter Altitude 17,000 15.56 −18.7 −1.6 setting correction 18,000 14.94 −20.7 −5.2 28.0 1,824 19,000 14.33 −22.6 −8.8 28.1 1,727 20,000 13.74 −24.6 −12.3 28.2 1,630 28.3 1,533 Figure 11-2. Properties of standard atmosphere. 28.4 1,436 28.5 1,340 28.6 1,244 28.7 1,148 28.8 1,053 28.9 957 29.0 863 29.1 768 29.2 673 29.3 579 29.4 485 29.5 392 29.6 298 29.7 205 29.8 112 29.9 20 29.92 0 30.0 −73 30.1 −165 30.2 −257 30.3 −348 30.4 −440 30.5 −531 30.6 −622 30.7 −712 30.8 −803 30.9 −893 31.0 −983 11-3 ddA tcartbuS Method for Determining Pressure Altitude Alternate Method for Determining Pressure Altitude To field elevation To get pressure altitude From field elevation level aes si noitavele dleiF 2. By applying a correction factor to the indicated altitude according to the reported “altimeter setting,” [Figure 11-3] 3. By using a flight computer Density Altitude The more appropriate term for correlating aerodynamic performance in the nonstandard atmosphere is density altitude—the altitude in the standard atmosphere corresponding to a particular value of air density. Density altitude is pressure altitude corrected for nonstandard temperature. As the density of the air increases (lower density altitude), aircraft performance increases. Conversely, Since all aircraft performance is compared and evaluated using the standard atmosphere, all aircraft instruments are calibrated for the standard atmosphere. Thus, certain corrections must apply to the instrumentation, as well as the aircraft performance, if the actual operating conditions do not fit the standard atmosphere. In order to account properly for the nonstandard atmosphere, certain related terms must be defined. Pressure Altitude Pressure altitude is the height above the standard datum plane (SDP). The aircraft altimeter is essentially a sensitive barometer calibrated to indicate altitude in the standard atmosphere. If the altimeter is set for 29.92 "Hg SDP, the altitude indicated is the pressure altitude—the altitude in the standard atmosphere corresponding to the sensed pressure. The SDP is a theoretical level at which the pressure of the atmosphere is 29.92 "Hg and the weight of air is 14.7 psi. As atmospheric pressure changes, the SDP may be below, at, or above sea level. Pressure altitude is important as a basis for determining aircraft performance, as well as for assigning flight levels to aircraft operating at above 18,000 feet. Figure 11-3. Field elevation versus pressure. The aircraft is located The pressure altitude can be determined by any of the three 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,