AIR-CONDITIONING AND PRESSURIZATION
by the atmosphere is much greater near the surface of
the Earth than it is at high altitudes. For example, the
air-conditioning of the cabin is the function of the
pressure of the atmosphere at sea level is 14.7 pounds
per square inch (psi), while the pressure at 40,000 feet
now in all naval aircraft. The inspection and
above sea level is 2.72 psi, and at 60,000 feet is 1 psi.
maintenance of this system is one of the important
duties of the AME.
As an aircraft ascends to higher altitude, the
resulting decrease in atmospheric pressure may affect
flight personnel in several ways. The most noticeable
effect is in breathing.
LEARNING OBJECTIVE: Recognize the
effect high-altitude flight could have on flight
Breathing is a mechanical process that depends
personnel because of decreased atmospheric
heavily on atmospheric pressure. When a person
inhales, they automatically raise their ribs and depress
their diaphragm so that the chest cavity is enlarged.
Transferring a human being from their natural
This reduces the air pressure within the cavity below
environment on the Earth's surface to the environment
that of the atmosphere outside. Air is thus pushed into
at 40,000 feet places them in surroundings in which
the lungs. When they exhale, they reduce the chest
they cannot survive without artificial aids. Even at half
cavity, increasing the pressure within it. This pushes the
that altitude, breathing becomes very rapid, and above
air out of the lungs.
25,000 feet, unconsciousness occurs, quickly followed
by death. A brief study of Earth's atmosphere tells us
When low atmospheric pressures are encountered,
why this condition exists.
the lungs are not filled so completely when inhaling.
With lower density, a person gets fewer molecules of air
The envelope of atmosphere surrounding the Earth
in each breath. If they get fewer molecules of air in each
is a gaseous mixture consisting chiefly of nitrogen and
breath, they also get fewer molecules of oxygen, and no
oxygen. There are traces of other gases, but they have
person can live unless they get a sufficient amount of
no significance as far as body functions are concerned.
nitrogen and oxygen are constant throughout the
This problem may be solved up to certain altitudes
thickness of the atmosphere, up through 200,000 feet or
by the proper use of oxygen equipment; however, at
extremely high altitudes (above 35,000 feet), the
atmospheric pressure is so low that the pressure of the
blood and other liquids in the body are no longer
balanced. The human body then tends to burst. In some
Although the chemical content of the atmosphere
cases, blood vessels near the surface may burst, causing
remains fairly constant, the density (mass per unit
hemorrhages in the ears, eyes, and breathing passages.
volume) of the atmosphere varies with altitude. At
The outside air temperature also changes with
18,000 feet, the density is about one-half of the density
altitude. For example, at approximately 18,000 feet, the
at sea level, and at 36,000 feet, it is only about
outside air temperature will be 4°F (20°C), and at
one-fourth of the density at sea level. The atmospheric
approximately 37,000 feet the outside air temperature
pressure also varies with altitude. The pressure exerted
will be 67°F (55°C). Above 37,000 feet, the air
by the atmosphere may be compared to the pressure of a
continues to thin, but the air temperature will remain
column of water. If holes are made in the container of
constant for several miles and then begin to rise slowly.
the column, the force with which the water spurts out of
Thus, the lowest outside air temperature to be
the upper holes will be considerably less than that at the
encountered by an aircraft could occur at a height of
bottom of the column. Similarly, the pressure exerted
about 7 miles.