Presentation Open Access
Air conditioning in aviation means temperature control, pressure control and ventilation. The cabin is vented with a certain percentage (e.g. 50%) of fresh outside air. The remaining part of the air for cabin ventilation is provided as air from the cabin, filtered and recirculated back into the cabin. At cruise altitude, ambient pressure is below cabin pressure. Hence, the outside air needs to be compressed before it is delivered into the cabin. The air is compressed in the engine compressor and tapped off as "bleed air" at temperatures reaching 400 °C or more. Hence, bleed air cooling is necessary. The engine shaft is supported by lubricated bearings. They are sealed against the air in the compressor usually with labyrinth seals. It is explained why jet engine seals leak oil by design in small quantities. The amount of oil leakage can be estimated with a new equation. The estimate shows the same order of magnitude as measured in flight (Cranfield study, EASA study). The oil leaking into the compressor contains problematic additives which get pyrolized (burned) at the elevated temperatures in the compressor, leaving more than 100 substances behind, some of them hazardous and some known as Volatile Organic Compounds (VOC). An alternative source for the compressed air is the Auxiliary Power Unit (APU). Like the aircraft's jet engine, it is a gas turbine, built much in the same way when it comes to bearings and seals. For this reason, also compressed air from the APU is potentially contaminated. Engineering standards from SAE contain guidance about sound engineering design principles for air conditioning systems of airplanes. Also certification standards give some guidance, however, more general. In essence, bleed air systems as we see them on today's passenger jet aircraft should not be built the way they are. For immediate action, hints are given: In case of smoke in the cockpit pilots should read the carbon monoxide (CO) concentration from a personal CO detector as an objective indicator in addition to their human senses. The present CO concentration should be compared with values obtained under normal conditions. If pilots are alerted and it is suitable (fuel reserves, terrain clearance), pilots should consider to descend to 10000 ft, reduce speed and ventilate the aircraft by means of the ram air inlet. This is the only source of fresh air in flight, independent of engines or APU. If smoke is present, checklists tell pilots to put on their oxygen mask. Cabin crew should consider wearing a personal breathing mask in such cases. Technically the easiest way to install carbon filters to filter VOCs in existing aircraft is in the recirculation path, where HEPA filters are already in use. Unfortunately the physics are such that filters in the recirculation path cannot remove substances fully. It is only possible to reduce the concentration down to a value depending on filtration rate and recirculation rate. With typical values the incoming VOC concentration can be reduced to about 60%. In case of full filtration (including ducts from the bleed air sources) incoming VOC concentration can be reduced to about 18%. Aircraft from the beginning of the jet age (B707, DC-8) used turbocompressors keeping bleed air and outside air compressed for cabin ventilation separate. Based on past experience, turbocompressors cannot be considered a solution for future aircraft. A final solution to the problem of contaminated cabin air is seen in electric (bleed free) cabin air supply architectures. Here, outside air for cabin ventilation is compressed separately in dedicated clean compressors. Bleed free cabin air architectures have the additional advantage of much improved fuel economy. So far, the Boeing 787 is the only passenger aircraft in service with a bleed free cabin air architecture. Airbus could follow with related technology already available and checked in test flights.