5A - Performance and Limitations - Structure of the Atmosphere Flashcards
- DUPLICATE*
2. What is the standard atmosphere at sea level? (FAA-H-8083-25)
Standard atmosphere at sea level includes a surface temperature of 59°F or 15°C, and a surface pressure of 29.92 in. Hg or 1013.2 millibars.
- How is aircraft performance significantly affected as air becomes less dense?
(FAA-H-8083-25)
As air becomes less dense, it reduces
a. power because the engine takes in less air.
b. thrust because the propeller is less efficient in thin air.
c. lift because thin air exerts less force on airfoils.”
- What are standard atmosphere temperature and pressure lapse rates? (FAA-H-8083-25)
A temperature lapse rate is one in which the temperature decreases at the rate of approximately 3.5°F or 2°C per 1,000 feet up to 36,000 feet. Above this point, the temperature is considered constant up to 80,000 feet. A standard pressure lapse rate is one in which pressure decreases at a rate of approximately 1 in. Hg per 1,000 feet of altitude gain to 10,000 feet.
- DUPLICATE*
4. Define the term “pressure altitude.” (FAA-H-8083-25)
Pressure altitude is the height above a standard datum plane. An altimeter is a sensitive barometer calibrated to indicate altitude in the standard atmosphere. If the altimeter is set for 29.92 in. Hg Standard Datum Plane (SDP), the altitude indicated is the pressure altitude—the altitude in the standard atmosphere corresponding to the sensed pressure.
- DUPLICATE*
5. Why is pressure altitude important? (FAA-H-8083-25)
Pressure altitude is important as a basis for determining airplane performance as well as for assigning flight levels to airplanes operating above 18,000 feet.
- What are two methods of determining pressure altitude? (FAA-H-8083-25)
Pressure altitude can be determined by either of two methods:
a. By setting the barometric scale of the altimeter to 29.92 and reading the indicated altitude, or
b. By applying a correction factor to the indicated altitude according to the reported “altimeter setting.
- Define the term “density altitude.” (FAA-H-8083-25)
Density altitude is pressure altitude corrected for nonstandard temperature. It is the altitude in the standard atmosphere corresponding to a particular value of air density.
- How does air density affect aircraft performance? (FAA-H-8083-25)
As the density of the air increases (lower density altitude), airplane performance increases and conversely, as air density decreases (higher density altitude), airplane performance decreases. A decrease in air density means a high density altitude; an increase in air density means a lower density altitude.
- How is density altitude determined? (FAA-H-8083-25)
First find pressure altitude and then correct it for nonstandard temperature variations. Because density varies directly with pressure, and inversely with temperature, a given pressure altitude may exist for a wide range of temperatures. However, a known density occurs for any one temperature and pressure altitude. Regardless of the actual altitude at which the airplane is operating, it will perform as though it were operating at an altitude equal to the existing density altitude.
- What factors affect air density? (FAA-H-8083-25)
Air density is affected by changes in altitude, temperature, and humidity. High density altitude refers to thin air while low density altitude refers to dense air. The conditions that result in a high density altitude are high elevations, low atmospheric pressures, high temperatures, high humidity, or some combination of these factors. Lower elevations, high atmospheric pressure, low temperatures, and low humidity are more indicative of low density altitude.
- What effect does atmospheric pressure have on air density? (FAA-H-8083-25)
Air density is directly proportional to pressure. If the pressure is doubled, the density is doubled, and if the pressure is lowered, so is the density. This statement is true only at a constant temperature.
- What effect does temperature have on air density? (FAA-H-8083-25)
Increasing the temperature of a substance decreases its density. Conversely, decreasing the temperature increases the density. Thus, the density of air varies inversely with temperature. This statement is true only at a constant pressure.
- Since temperature and pressure decrease with altitude, how will air density be affected overall? (FAA-H-8083-25)
The decrease in temperature and pressure have conflicting effects on density as you go up in altitude, but the fairly rapid drop in pressure with increasing altitude is usually the dominating factor. Hence, the density is likely to decrease with altitude gain.
- What effect does humidity have on air density? (FAA-H-8083-25)
Water vapor is lighter than air, so moist air is lighter than dry air. As the water content of the air increases, the air becomes less dense, increasing density altitude and decreasing performance. It is lightest or least dense when it contains the maximum amount of water vapor. Humidity alone is usually not considered an important factor in calculating density altitude and airplane performance, but it does contribute.
- What is the definition of the term “relative humidity”? (FAA-H-8083-25)
Relative humidity refers to the amount of water vapor in the atmosphere, and is expressed as a percentage of the maximum amount of water vapor the air can hold. This amount varies with the temperature—warm air can hold more water vapor and colder air can hold less.
