Thin air outside – comfortable breathing inside
Modern jet aircraft cruise in altitudes of more than ten kilometers. Up there, the air is too thin for a human being to breathe. However, the pressurized cabin of today's airliners provides passengers with an environment in which they can respire normally. But where does the pressure come from that provides the passengers with breathable air? The primary air supply in aircraft cabins of nearly all modern commercial jet aircraft originates in their engines. Inside the engines, numerous stages compress the entering air for the final combustion process that powers the aircraft forward. Before that happens, a small part of the compressed air is rerouted from the engine's duct. This so called "bleed air– not only has a high pressure but also a high temperature, which makes it as well applicable for de-icing purposes of the wings and other aircraft parts.
To provide breathable air for passengers and crew, the hot "bleed air– is first pre-cooled by heat exchangers down to 200 degrees Celsius. Through valves, which can be controlled by the cockpit crew, the airflow is afterwards directed to the air condition, where it is cooled down even further to temperatures comfortable for the passengers. The cooled air is now fed into a mixing chamber. In here, the bleed air is mixed with previously used, recirculated air from the aircraft cabin. Finally, regulating valves feed the air mixture to the aircraft cabin, which is divided into several zones.
Climate zones onboard the aircraft
The separation of the aircraft cabin into zones enables an independent adjustment of the temperature in different areas or booking classes. The latter often see huge differences in seat pitch, for example between First Class and Economy Class, and hence a big variation in the heat which is emitted by the passengers. As a consequence, it might be necessary for the cabin crew to cool down one zone while another one has to be heated up a little bit in order to ensure the best passenger comfort. To heat up a cabin zone, the cooled air from the air condition is enriched by hot bleed air which is routed past the cooling process. In case a cabin zone needs to be cooled, the percentage of this hot air is simply decreased. The cabin temperature is pre-set from the flight deck but can be individually checked by the cabin crew through the ECAM system (Electronic Centralised Aircraft Monitor) and afterwards adjusted to the individual cabin zone by push-buttons. The number of different cabin zones is given by the aircraft type. For example, a short- and medium range aircraft like the Airbus A320 is divided into two different cabin zones; the larger long-range A330/A340 series have up to six, supplemented in every aircraft by a special zone for the cockpit. As well as the passengers, the flight crew also receives their breathing air from the mixing chamber, in which external and recirculated air is mixed in a pre-set ratio. The usage of recirculated air increases the humidity, thus leading to an improved comfort for the passengers.
Filtration systems eliminate viruses and bacteria in recirculated air
The constant admixing of recirculation air from the cabin further improves the purity of the breathing air. Within the recirculation process the air is passed through so called HEPA filters (High Efficiency Particulate Air). Filters certified to this standard are microbially tested to remove more than 99,999 percent of all known viruses and bacteria from the cabin air. However, such filtration can only be used with recirculated air because the HEPA filters are unable to withstand the hot bleed air. Additional activated carbon filters however are insensitive to these temperatures and further eliminate unpleasant smells. To further improve the efficiency and usability of air filters, research and development activities are carried out permanently.
In an Airbus A340-300, the ratio between bleed air and recirculated air stands at around 60 to 40 percent, but in every generation of newly developed aircraft the percentage of recirculated air is further increased. Despite its much bigger cabin the Airbus A380-800, for example, consumes around 60 percent less bleed air than the Boeing 747-400 it is going to replace in many airlines' fleets. However, the aviation authorities require a certain percentage of fresh air in the cabin to ensure an adequate supply of oxygen which, independent of the altitude, accounts to 21 percent of the outside air. In many aircraft types, the passengers are supplied with more than 80 times as much oxygen as required by the authorities, even in case of the admixing of 40 percent recirculated air. One example: In the given Airbus A340-300, the cabin air is completely replaced 30 times per hour, once every two minutes.
Permanently monitored cabin pressure
In upcoming aircraft generations like the Boeing 787, the usage of bleed air will be completely abandoned in order to further optimize the engines' fuel efficiency. The necessary supply of external air will hence be handled differently, for example by a ram air supply outside of the fuselage or by additional compressors.
To ensure that despite the permanent external air supply the cabin pressure is not getting higher and higher, a computer-controlled outflow valve permanently monitors the cabin pressure and keeps it at a comfortable level by gently and constantly releasing cabin air to the outside, taking into account both the flying altitude and the static pressure outside the aircraft. In this way, the uncomfortable feeling of pressure on the passengers` ears is minimized as far as possible. The technology of modern aircraft hence provides passengers with comfortable conditions at any time, even at altitudes in which the air is much too thin to breathe.