Cabin Air Systems

Frequently Asked Questions about Cabin Air Systems

Is the same supply air used over and over?

No. Approximately 50 percent of the supply air is outside air and 50 percent is filtered recirculation air

Back to Top

How frequently does air flow into the cabin?

Ventilation is continuous. Air is constantly flowing in and out of the cabin.

Back to Top

What happens when the air filters get dirty?

As filters get dirty, two things happen:

  1. The particulate capture efficiency increases because the trapped particles make it more difficult for other matter to pass through, and
  2. The filter resistance increases, which leads to a reduction in recirculation flow.

Boeing specifies a scheduled replacement interval for the High Efficiency Particulate Air (HEPA) filters to assure ventilation performance is maintained.

Back to Top

Aren't viruses too small to be captured by the high efficiency filters?

The HEPA filters are rated according to their ability to remove particulates measuring 0.3 microns, an industry standard.

Because of the way these filters are designed, their efficiency actually increases for particles both smaller and larger than the most penetrating particle size, which is about 0.1 to 0.2 microns.

The efficiency of HEPA filters to remove bacteria and viruses (.01 to .1 microns) is greater than 99 percent.

Back to Top

How long have recirculation systems been used on passenger airplanes?

Recirculation was in use before the jet age began.

For example, the Boeing Stratocruiser of the late 1940s was equipped with an air recirculation system but it did not include HEPA filters.

In jet airplanes, filtered or recirculated air combined with outside air came into use principally with the introduction of high-bypass-ratio fan engines.

At Boeing, this began with the 747 back in 1970. Keep in mind that air recirculation is common in building ventilation systems.

Back to Top

How does the air-flow rate on current jetliners compare to earlier models?

Each Boeing airplane model, from the earliest to the latest, have been designed to deliver approximately the same total ventilation rate per passenger.

The principal difference is that on newer versions, the cabin air is a mixture of about 50 percent outside air and 50 percent filtered/recirculated air.

Among the benefits of this design is a lower potential exposure to atmospheric ozone and reduced fuel burn and associated engine emissions.

Back to Top

Doesn't the recirculated air just keep recirculating?

No. Outside-air mixing replenishes the cabin air constantly.

Replenishment assures that the recirculated portion does not endlessly recirculate but is rapidly diluted and replaced with outside air.

During cruise or on the ground, the outside air is drawn in at the same rate that cabin air is exhausted out of the airplane.

Back to Top

Do pilots turn off air conditioning units to save fuel?

Pilots have the ability to turn off air conditioning units but this is intended only as a safety feature in the event of an equipment failure and is not intended as a means to save fuel.

If one air conditioning unit must be turned off during an equipment failure, the remaining unit or units on most jetliners will increase flow to partially recover the total air ventilation rate.

Older 747 model aircraft did have an economy feature for lightly loaded flights.

Back to Top

Does combustion air make it into the supply air?

In systems that use air from engines, the air is taken from the engine compressors that are well upstream of any combustion.

The air is simply compressed outside air.

Back to Top

If re-circulated air is filtered, why isn't bleed air off the engine filtered before it comes into the passenger cabin?

The ambient air outside the airplane at altitude cruise levels is very clean, cold (below -35 F/-37 C) and low in partial pressure of oxygen, too low to sustain life. Consequently, the air must be compressed to a density that is healthy for passengers and crew. Airplanes with a traditional bleed air system "bleed" or divert air from the airplanes' engine compressors to accomplish the task of warming and pressurizing the air. The air taken from the engine compressors is upstream of the combustion chamber where fuel is added. The bleed air is essentially dry, sterile and dust free. It is cooled in air conditioning packs and is then mixed with approximately 50 percent filtered recirculated air. The mixed air is then supplied to the airplane cabin at the proper temperature.

Back to Top

How dangerous are the fumes from jet fuel or oil that sometimes get into the passenger cabin?

On the very rare occasions where bleed air contaminants may enter the cabin, the contaminant levels are expected to be lower than occupational health thresholds established by toxicologists who have studied these contaminants extensively. We fully support the studies being conducted by the U.S. Federal Aviation Administration Center of Excellence for Airliner Cabin Environment Research (ACER) and by the U.K. Department of Transportation Aviation Health Working Group (AHWG).

Back to Top