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

Pressurized air for the cabin comes from the compressor stages in the aircraft’s jet engines. Moving through the compressor, the outside air gets very hot as it becomes pressurized. The portion drawn off for the passenger cabin is first cooled by heat exchangers in the engine struts and then, after flowing through ducting in the wing, is further cooled by the main air conditioning units under the floor of the cabin.

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 temperature.

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.

Key Characteristics and Overall Effectiveness

There are several characteristics of the cabin air system that deserve special emphasis:

  • Air circulation is continuous. Air is always flowing into and out of the cabin.
  • The 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.
  • Outside-air 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 Boeing airplanes.
  • 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 similar results.
  • 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 interact.

Differences Between Older and Newer Cabin Air Systems

Engines that produced all or most of their thrust directly from the engine core powered early-generation jetliners. Air extracted from the compressor in these older aircraft provided the cabin with 100 percent outside air with only a modest impact on fuel economy. But by today’s standards, the engines themselves were very noisy, emitted much higher levels of pollutants into the atmosphere and were much less fuel-efficient.

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