Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 59

Density altitude is the altitude used to calculate aircraft performance in most situations. Density altitude is found by correcting pressure altitude for nonstandard temperature conditions. Pressure altitude and density altitude are the same when conditions are standard. Once pressure altitude has been determined, the density altitude is calculated using outside air temperature. If the temperature is below standard, then the density altitude is lower than pressure altitude and aircraft performance is improved. If the outside air temperature is warmer than standard, the density altitude is higher than pressure altitude and aircraft performance is degraded.

Figure 2.4 is a commonly used chart for performance calculations to determine density altitude. As provided by the Pilot’s Handbook of Aeronautical Knowledge, using Figure 2.4, we will calculate the pressure altitude first, and then calculate the resulting density altitude based on temperature. With a given airport elevation of 5883 ft and an altimeter setting of 30.10″, as shown in Figure 2.3 the resulting pressure altitude is calculated to be 5718 ft. Since the density altitude is pressure altitude corrected for nonstandard temperature, we then use the provided 70 °C temperature and move from the X‐axis vertically until we reach the diagonal line (interpolated) for 5718 ft. Moving horizontally to the Y‐axis, the density altitude is determined to be 7700 ft, which makes sense as it is higher than the pressure altitude since the temperature is above standard for that altitude, resulting in lower air density (higher density altitude).

Figure 2.4 Pressure altitude conversion and density altitude chart.

Source: U.S. Department of Transportation Federal Aviation Administration (2016b).