An Ocean Of Air
It can be a little hard to imagine that movement through air can produce these forces that act on an airplane. We are so accustomed to being surrounded by air, that it is easy to ignore its properties. We could be considered to constantly be immersed in an ocean of air. Like the ocean, air in our atmosphere has weight and flows as a fluid.
Air Is Compressible
One key characteristic of air is its compressibility. Unlike liquids, gases can be compressed. As a result, the air we fly through varies in density with altitude, temperature, atmospheric pressure, and humidity. These density variations affect the performance of the airplane significantly. The pilot uses an understanding of the effects of air density on airplane performance and performance charts issued by the airplane's manufacturer to determine how the airplane will perform during each flight. This knowledge allows the pilot to make safe decisions as to which airports and runways are acceptable for use, how much load can be carried, and how far the airplane can be flown.
Why Does Air Density Matter?
The airplane's wings generate lift as a result of movement through air. In thicker air, they will generate more lift at slower speeds. The propeller grabs the air to move the airplane forward. In thinner air, it has a harder time grabbing that air. As a result, it must spin faster and fails to do as good of a job. Additionally, the engine itself produces power by burning a mixture of fuel and air. If less air is available to mix with the fuel, the engine can't make as much power.
Air density affects the way almost everything performs, from the engine and propeller, to the wings and flight controls. Even people function less effectively in thinner air, since our brains use the oxygen contained in the air.
Atmospheric Pressure
Atmospheric pressure refers to the effects of weather systems on air density in a particular region. When flying through an area of low atmospheric pressure, the airplane is flying through an area of reduced air density. High atmospheric pressure results in higher air density. Atmospheric pressure is measured with a barometer, in which Mercury rises a certain distance through a tube as a result of the atmospheric pressure. These barometer measurements are given in inches of mercury and used by pilots in determining airplane performance before a flight.
The Standard Atmosphere
The International Standard Atmosphere (ISA) is a set of atmospheric conditions used for reference. Airplane performance charts are often based on the standard atmospheric conditions, and deviations from those standard conditions. The standard atmosphere has a temperature of 59 degrees Fahrenheit or 15 degrees Celsius, a barometric pressure of 29.92 inches of mercury, and no humidity.
The standard atmosphere also assumes specific changes in pressure and temperature with increasing altitude. These changes are called lapse rates. The standard temperature lapse rate is a two degrees Celsius drop in temperature with every thousand feet of altitude. The standard pressure lapse rate is a loss of one inch of pressure per thousand feet.
| Altitude | Standard Pressure | Standard Temperature |
| Sea Level | 29.92 inches | 15 degrees |
| 1,000 | 28.92 inches | 13 degrees |
| 2,000 | 27.92 inches | 11 degrees |
| 3,000 | 26.92 inches | 9 degrees |
| 4,000 | 25.92 inches | 7 degrees |
| 5,000 | 24.92 inches | 5 degrees |
| 6,000 | 23.92 inches | 3 degrees |
| 7,000 | 22.92 inches | 1 degree |
| 8,000 | 21.92 inches | -1 degree |
Every Airplane Has Its Limits
In order to be fly safely, the pilot must also understand the performance limitations of the airplane being flown and why these limitations must not be exceeded.