Principles of Flight

Since aircraft performance is compared and evaluated with The computation of density altitude involves consideration respect to the standard atmosphere, all aircraft instruments are of pressure (pressure altitude) and temperature. Since aircraft calibrated for the standard atmosphere. In order to properly performance data at any level is based upon air density under account for the nonstandard atmosphere, certain related terms standard day conditions, such performance data apply to must be defined. air density levels that may not be identical with altimeter indications. Under conditions higher or lower than standard, Pressure Altitude these levels cannot be determined directly from the altimeter. Pressure altitude is the height above a standard datum plane (SDP), which is a theoretical level where the weight of the Density altitude is determined by first finding pressure atmosphere is 29.92 "Hg (1,013.2 mb) as measured by a altitude, and then correcting this altitude for nonstandard barometer. An altimeter is essentially a sensitive barometer temperature variations. Since density varies directly with calibrated to indicate altitude in the standard atmosphere. If pressure and inversely with temperature, a given pressure the altimeter is set for 29.92 "Hg SDP, the altitude indicated altitude may exist for a wide range of temperatures by is the pressure altitude. As atmospheric pressure changes, the allowing the density to vary. However, a known density SDP may be below, at, or above sea level. Pressure altitude occurs for any one temperature and pressure altitude. The is important as a basis for determining airplane performance, density of the air has a pronounced effect on aircraft and as well as for assigning flight levels to airplanes operating at engine performance. Regardless of the actual altitude of the or above 18,000 feet. aircraft, it will perform as though it were operating at an altitude equal to the existing density altitude. The pressure altitude can be determined by one of the following methods: Air density is affected by changes in altitude, temperature, and humidity. High density altitude refers to thin air, while 1. Setting the barometric scale of the altimeter to 29.92 low density altitude refers to dense air. The conditions that and reading the indicated altitude result in a high density altitude are high elevations, low 2. Applying a correction factor to the indicated altitude atmospheric pressures, high temperatures, high humidity, or according to the reported altimeter setting some combination of these factors. Lower elevations, high atmospheric pressure, low temperatures, and low humidity Density Altitude are more indicative of low density altitude. SDP is a theoretical pressure altitude, but aircraft operate in a nonstandard atmosphere and the term density altitude is used Effect of Pressure on Density for correlating aerodynamic performance in the nonstandard Since air is a gas, it can be compressed or expanded. When atmosphere. Density altitude is the vertical distance above sea air is compressed, a greater amount of air can occupy a given level in the standard atmosphere at which a given density is volume. Conversely, when pressure on a given volume of air to be found. The density of air has significant effects on the is decreased, the air expands and occupies a greater space. aircraft’s performance because as air becomes less dense, At a lower pressure, the original column of air contains a it reduces: smaller mass of air. The density is decreased because density • Power because the engine takes in less air is directly proportional to pressure. If the pressure is doubled, the density is doubled; if the pressure is lowered, the density is • Thrust because a propeller is less efficient in thin air lowered. This statement is true only at a constant temperature. • Lift because the thin air exerts less force on the airfoils Effect of Temperature on Density Density altitude is pressure altitude corrected for nonstandard Increasing the temperature of a substance decreases its temperature. As the density of the air increases (lower density. Conversely, decreasing the temperature increases density altitude), aircraft performance increases; conversely the density. Thus, the density of air varies inversely with as air density decreases (higher density altitude), aircraft temperature. This statement is true only at a constant pressure. performance decreases. A decrease in air density means a high density altitude; an increase in air density means a In the atmosphere, both temperature and pressure decrease lower density altitude. Density altitude is used in calculating with altitude and have conflicting effects upon density. aircraft performance because under standard atmospheric However, a fairly rapid drop in pressure as altitude increases conditions, air at each level in the atmosphere not only has usually has a dominating effect. Hence, pilots can expect the a specific density, its pressure altitude and density altitude density to decrease with altitude. identify the same level. 4-4