*Troposphere: The lowest region of the atmosphere between the earth's surface and the tropopause, characterised by decreasing temperature with increasing altitude.
Tropopause: The boundary layer between the troposphere and stratosphere. It is defined as the lowest level at which the temperature lapse rate decreases by 2°C km-1 or less, to reach an average of -56°C.
*Stratosphere: The region of the atmosphere above the troposphere and below the mesosphere.
Cond: Condenser; CMP: Compressor; FCV: Flow Control Valve; MHX: Main Heat Exchanger; PCKV: Pressure ChecK Valve; PHX: Primary Heat Exchanger; Reh: Reheater; TCV: Temperature Control Valve; TRB: Turbine; WE: Water Extractor.
A mixer unit, installed below the cabin floor in front of the centre wing box, mixes outside air with cabin air. The cabin air is taken from the under-floor area and drawn through recirculation filters by recirculation fans (figure 3). The quantity of recirculated cabin air mixed with the outside air varies from 40% to 60% and improves efficient removal of heat loads at a moderate temperature gradient and increases the humidity by making use of the cabin air. The latter contains humidity contributed by the passengers whilst outside air, being very cold, is almost completely dry. After leaving the mixer unit, the air is distributed to different cabin zones. For each cabin zone a different temperature can be selected. Trim air valves regulate the cabin outlet temperature by injecting small amounts of hot bleed air from the pre-cooler outlet (trim air).
Besides the air exchange with the outside air for efficient contaminant removal (most importantly CO2 and other bio-effluents), additional technologies are necessary to avoid exposure to ozone from the operational environment and to exclude distribution of biological pathogens in the cabin by recirculated air.
During cruise at high altitudes - in particular on polar routes - the outside air may contain significant concentrations of ozone. To ensure a sufficiently low ozone concentration in the cabin, long-haul service aircraft, and even most of the short-haul service aircraft, are equipped with catalytic ozone converters (figure 4). The ozone converters are installed in the bleed air, ducting upstream to the air conditioning pack to ensure sufficient temperature levels for the catalytic process. Modern ozone converters, as introduced by Airbus in 2004, combine the ozone conversion with the ability to mitigate odorous compounds from kerosene external vapours, which might occur during ground operations, to improve the olfactory* perception.
* Olfactory: Relating to, or contributing to the sense of smell.
* Olfactory perception: The sensation that results when olfactory receptors in the nose are stimulated by particular airborne chemicals.
Within the aircraft, air supply system areas with high temperatures are most critical with regard to air quality because high temperatures would lead to evaporation and possibly to the breakdown of organic substances, if they found their way into the high temperature areas. Consequently, special care needs to be taken to prevent contamination with materials susceptible to evaporation or deterioration at this area during design, manufacturing and operation of the aircraft.
A top priority is clean air supply starting with the sources of pressurized air for the aircraft ventilation, engines, APUs and air conditioning systems. Since the 1990’s, Airbus has defined additional requirements for bleed air cleanness on top of the requirements used for certification of engines and APUs by authorities. Airbus is supporting international research projects and working groups, to acquire independent external expertise and scientific knowledge by promoting best standards for an ideal cabin environment and its verification. Airbus is also involved during validation of the verification strategy with APU and engine manufacturers, while supporting with its own measurements.
These analytical capabilities can also be used to tackle problems that could occur during production flights, which are conducted for each aircraft prior to delivery by Airbus. Any contamination introduced during manufacturing will become apparent when the aircraft air supply system is operated as a whole, and some parts of the bleed air system are subjected to the highest operational temperatures for the first time.
Airbus developed a sophisticated measurement strategy consisting of combinations of online measurements and traditional air quality sampling according to international standards. Online mass spectrometry, as well as multifunctional sampling systems such as Fraunhofer IBP* sampling trolley, can be used to identify and track even small concentrations of pollutants in order to elucidate their origin.
*An automated and self-sustaining sampling system developed by Fraunhofer Institute for Building Physics (Fraunhofer IBP)
The online mass spectrometer used is a unique combination of a Proton Transfer Reaction (PTR) ionisation method with a Time Of Flight-Mass Spectrometer (TOF-MS).
With this analytical system a marker substance and finally the root cause for a specific contamination during first flights of a particular aircraft type could be identified unambiguously, based on the results of air quality measurements on the aircraft. Knowing the precise root cause and having clear evidence at hand, corrective actions could be implemented at the supplier’s, and be further followed up.
Airbus is working with industry partners and regulatory agencies to examine the end-to-end travel experience and ensure it continues to be healthy and safe.
Everyone - from airlines, pilots and crew, to members of the airport community, authorities, and travellers - needs to adopt health and safety routines when travelling. Whether it is on the way to or at the airport, behind the scenes at baggage, ground, and cargo handling or for cabin crew and pilots, everyone needs to play an active role to get back to the skies and maintain trust in air travel.
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