Environmental control system
The systems described below are specific to current production Boeing airliners, although the details are essentially identical for passenger jets from Airbus and other companies.
[1] On jetliners, air is supplied to the ECS by being bled from a compressor stage of each gas turbine engine, upstream of the combustor.
For this reason, air is commonly drawn from one of two (or in some cases such as the Boeing 777, three) bleed ports at different compressor stage locations.
This water removal process prevents ice from forming and clogging the system, and keeps the cockpit and cabin from fogging on ground operation and low altitudes.
Nearly all jetliners use a modulating door on the ram-air inlet to control the amount of cooling airflow through the primary and secondary ram air heat exchangers.
Modern jetliners use high-efficiency particulate arresting HEPA filters, which trap more than 99% of all bacteria and clustered viruses.
[a] A revolving control on the vent can be turned to adjust ventilation between no air output at all and a fairly substantial breeze.
Gaspers[a] usually receive their air from the AC PACKs aboard the aircraft, which in turn receive compressed, clean air from the compressor stages of the aircraft's jet engines or when on the ground from the auxiliary power unit (APU) or a ground source.
Airflow into the fuselage is approximately constant, and pressure is maintained by varying the opening of the out-flow valve (OFV).
Most modern jetliners have a single OFV located near the bottom aft end of the fuselage, although some larger aircraft like the Boeing 747 and 777 have two.
The new airliners such as the Airbus A350 and Boeing 787 will have lower maximum cabin altitudes which help in passenger fatigue reduction during flights.
Although low cabin humidity has health benefits of preventing the growth of fungus and bacteria, the low humidity causes drying of the skin, eyes and mucosal membranes and contributes to dehydration, leading to fatigue, discomfort and health issues.
[6] In a statement to US Congress in 2003 a member of the Committee on Air Quality in Passenger Cabins of Commercial Aircraft said "low relative humidity might cause some temporary discomfort (e.g., drying eyes, nasal passages, and skin), but other possible short- or long-term effects have not been established".
[8] Furthermore, the Boeing 787 and Airbus A350, by using more corrosion-resistant composites in their construction, can operate with a cabin relative humidity of 16% on long flights.
However, on occasions carbon seals can leak oil (containing potentially hazardous chemicals) into the bleed air, in what is known in the industry as a fume event.
Oil contamination from this and other sources within the engine bay has led to health concerns from some advocacy groups and has triggered research by several academic institutions and regulatory agencies.
