Jetliner Cabin Air Systems
The cabin air system in
today's jetliners is designed to provide a safe, comfortable cabin environment
at cruising altitudes that can reach upwards of 40,000 feet.
At those altitudes, the cabin
must be pressurized to enable passengers and crew to breathe normally. By
government regulation, the cabin pressure cannot be less, at maximum cruise
altitude, than the equivalent of outside air pressure at 8,000 feet.
In addition to pressurization,
the cabin air system controls air flow, air filtration and temperature. Here's
how the system works:
Cabin Air System Operation
The cooled air then flows to a
chamber where it is mixed with an approximately equal amount of highly filtered
air from the passenger cabin. The combined outside and filtered air is ducted to
the cabin and distributed through overhead outlets.
Inside the cabin, the air
flows in a circular pattern and exits through floor grilles on either side of
the cabin or, on some airplanes, through overhead intakes. The exiting air goes
below the cabin floor into the lower lobe of the fuselage. The airflow is
continuous and quickly dilutes odors while also maintaining a comfortable cabin
About half of the air exiting
the cabin is immediately exhausted from the airplane through an outflow valve in
the lower lobe, which also controls the cabin pressure. The other half is drawn
by fans through special filters under the cabin floor, and then is mixed with
the outside air coming in from the engine compressors.
These high efficiency filters
are similar to those used to keep the air clean in hospitals. Such filters are
very effective at trapping microscopic particles as small as bacteria and
viruses. It is estimated that between 94 and 99.9 percent of the airborne
microbes reaching these filters are captured.
Characteristics and Overall Effectiveness
circulation is continuous. Air is always flowing into and out of the cabin.
cabin has a high air-change rate. All of the air in the cabin is replaced by the
incoming mixture of outside air and filtered air during intervals of only two to
three minutes, depending on airplane size. That's 20 to 30 air changes per hour.
mixing replenishes the cabin air constantly. The outside-air content keeps
carbon dioxide and other contaminants well within standard limits and replaces
oxygen far faster than the rate at which it is consumed. Replenishment also
assures that the recirculated portion of the air does not endlessly recirculate
but is rapidly diluted and replaced with outside air.
Compared to other modes of
transport, office buildings and other enclosed spaces occupied by large numbers
of people, today's jetliners provide an environment that is superior in many
respects. For example, in the heating and cooling seasons, most office buildings
provide a far lower percentage of outside air -- often as low as 20 percent or
even less. In addition, buildings typically have a much lower air-change rate,
and they're seldom equipped with high-efficiency filters like those found in
A further advantage for
airplane passengers: the outside air that enters jetliner cabins at cruising
altitudes is generally much cleaner than what is available for ventilating
buildings and surface modes of transportation.
Studies have confirmed the
overall safety and effectiveness of cabin air systems. One of the studies,
conducted for the U.S. Government, was the most comprehensive of all. It
involved an independent testing service taking air samples on 92 randomly
selected airline flights. The levels of pollutants such as fungi and bacteria
were found to be similar to or lower than those encountered in normal indoor
environments. Also, levels of carbon dioxide were found to average less than
one-third the limit recommended by the American Conference of Governmental
Industrial Hygienists. Studies conducted by Boeing and by airlines have shown
It is unlikely that cabin air
contains sufficient contaminants to cause such occasionally reported conditions
as fatigue headache, nausea or respiratory problems. It is more probable that
these conditions are caused by the complex interactions of such factors as the
individual's health, jet lag, medications, alcohol consumption and motion
sickness in combination with cabin altitude effects and low humidity. Boeing
supports industry efforts to develop a better understanding of how these factors
Between Older and Newer Cabin Air Systems
By contrast, most newer
jetliners are powered by high-bypass-ratio fan engines which are much quieter,
much cleaner burning, more powerful and much more efficient. At the front end of
this engine type is a large-diameter fan, which is powered by the core. The fan
moves a large volume of air past the core rather than through it, and actually
generates most of the thrust. Every unit of pressurized air extracted from the
engine core has the effect of reducing fan thrust by an even greater amount, and
that degrades fuel efficiency more severely on this type of engine than on the
older type. By providing the cabin with a mixture of about 50 percent outside
air taken from the compressor and 50 percent recirculated air, a balance has
been achieved that maintains a high level of cabin air quality, good fuel
efficiency and less impact to our environment.
However, that's only part of the rationale for the current design of cabin air systems. Cabin air is typically quite dry at cruise altitudes. With 50 percent recirculation, the cabin is provided with at least a modest level of humidity in newer jetliners compared to the very low levels in earlier models. In addition, recirculation of a portion of the cabin air reduces the ingestion of exhaust and other pollutants on taxiways and also reduces the level of ozone exposure at high altitudes.
Reference: "The Boeing Company"