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1.5  DENSITY ALTITUDE

Density altitude is the air density that exists at a given altitude for a given temperature. An airfoil's efficiency, either a wing or a rotor blade, is decreased at high altitudes by the lack of air density. All aircraft, regardless of design, have a ceiling (limit) where the air is too thin to sustain flight. The effect of air density on helicopter performance is vital because of the critical load limit and confined-area operation usually required of the helicopter.

Density altitude is pressure altitude corrected for temperature. Figure 1.3 shows the density altitude chart used in operating Army aircraft. An airfield may have a density altitude that varies several thousand feet from the mean sea level (MSL) elevation of the field. If the density altitude is higher than average for the field, this field has a high density altitude. An example of this is an airfield at 5,000 feet MSL with a density altitude of 10,000 feet. Aircraft operating from this field would be in air of the same density that would usually be found in the atmosphere at 10,000 feet. The efficiency of the aircraft can be seriously affected in high density altitudes, especially when the aircraft is critically loaded. If the density altitude is lower than average for a given altitude, the efficiency of the aircraft increased. A pilot operating from a field of 5,000 feet MSL with a density altitude of 1,000 feet is in the same air density field elevation that usually exists at 1,000 feet. Further, the density altitude on any airfield varies from hour to hour and day to day.

Figure 1.3 Density Altitude Chart.

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