THE AEROSPACE
ENVIRONMENT Atmospheric conditions greatly influence aviation operations. Influences like pressure, temperature, pre cipitation, and condensation, all affect the flight characteristics and safety of the aircraft, passenger comfort, and crewmember performance. As a professional crewmember, it is important to have an understanding of the weather phenomenon that can affect your flight and your performance.
THE ATMOSPHERE
The earth’s atmosphere is divided into upper and lower layers. The layer closest to the earth’s surface is called the “troposphere” and extends to about 10 miles above the Earth’s surface. Above the troposphere is the “stratosphere.” The majority civilian aircraft operate within the troposphere. (Some newer corporate aircraft are certified for flight into the fringes of the stratosphere!)
There are three important considerations to remember regarding the atmosphere and how it affects flight. They are pressure, temperature, and weather.
Atmospheric Pressure
Pressure is the gravitational force (or “push”) that keeps our atmosphere balanced for human survival. As we go up in altitude, pres sure decreases. At sea level, the air pressure applies around 14.7 pounds per square inch. At 18,000 feet, the pressure is about half that at sea level; and, at 35,000 feet it is less than one fourth. As altitude is in creased, pressure diminishes until it is no longer measurable. Therefore, aircraft flying at high altitudes (e.g. above 25,000 feet) are normally re quired to be “pressurized.” Pressurized aircraft are designed to keep a “cabin altitude” (an atmospheric pressure) of
35,000 30,000 25,000 20,000 15,000 10,000 5,000
approximately 6,000 to 8,000 feet.
Atmospheric Temperature Atmospheric temperature not only affects human comfort, but influences weather conditions and aircraft performance. For each thousand feet of altitude above sea level, the temperature drops approximately 3.5 degrees Fahrenheit. At about 36,000 feet, and into the stratosphere, temperature remains fairly constant at -69.7 degrees.
Temperature and pressure directly affect the performance of aircraft (especially high performance turbine powered corporate aircraft). The colder the temperature and the more pressure (more air), the better the air craft performs.
MEASURING ATMOSPHERIC PRESSURE
In the U.S. atmospheric pressure is measured in inches of mercury. The Standard Sea Level pressure is 29.92 inches of mercury. This means if we stood a tube containing mercury at sea level with “normal” pressure, the mercury would rise 29.92 inches. If the pressure is higher than normal, it is referred to as a “high” on weather charts. If the pressure is lower, it is referred to as a “low” and both cause the mercury levels to move from normal.
Aviation Basics 8
14.7lb
1
1 Sea lvel e
1Square Inh 1Square Inch
c
1Square Inh 1Square Inch
c
7.4 lb
18,000 feet
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