Saturday, September 19, 2009

Cleaning aircraft-cabin air

ONE concern of travellers is that flying can make you ill. Despite soothing reassurances from airlines that the air inside an aircraft’s cabin is as clean as it can be, hundreds of people cooped up in a small space for a long time increases the risk of catching an infection. Viral diseases such as swine flu have spread quickly around the world by air. There are also pollutants to worry about: some airline staff claim to have been made seriously ill by engine fumes leaking into the cabin. A new development could help passengers and crew breathe more easily. It can be fitted during a routine overnight service and uses less power than a light bulb, but is capable of zapping just about all the bacteria, viruses and other biohazards in cabin air—as well as destroying chemical contaminants and pollutants. And it also removes nasty smells.
Staying alive
Air has to be pumped into the fuselage of an aircraft once it flies above 3,000 metres (9,800 feet) because from there the air outside starts to become thin with too little oxygen to keep people alive. In the early days of commercial flight, when piston engines turned propellers, electrical generators were used to provide the power to pressurise the cabin. But with the arrival of faster, higher-flying jets, aircraft-makers found it more efficient to pressurise the fuselage by “bleeding” some of the air entering the compressor stage of the jet turbine (before it is mixed with fuel and ignited). At first only fresh air was taken from the engines. But as jets have become more fuel-efficient, air bled from the engine has been mixed with recycled air from the cabin. This is because the big fans mounted on modern “high bypass” jet engines achieve thrust more efficiently by sending a larger volume of air around the turbine rather than through it. With less air available in the compressor, any that is bled off means the turbine has to work harder, which in turn increases fuel consumption. Typically an airline will strike a balance by using a 50:50 mixture of fresh and recirculated cabin air, although pilots can reduce the amount of fresh air to save fuel. Some are thought to cut it back to only 20%. The mixed air is maintained in the cabin at a pressure which is equivalent to an altitude of 2,500 metres and is passed through what are called high-efficiency particulate arrest (HEPA) filters. Provided they are well maintained, HEPA filters will trap most microscopic particles. But some things can get through, especially tiny viruses. The new AirManager system is said to kill 99.999% of pathogens in a single pass. Even on a short flight of about one hour, the cabin air will pass through the filters around 30 times. David Hallam, inventor of the technology and a director of Quest, says the device works by generating a non-thermal plasma (sometimes called a “cold plasma”) using a high voltage to strip electrons from some of the molecules in a gas. The plasma is confined using an electric field and the cabin air is passed through it. The free electrons disrupt the molecular bonds of any particles in the air, causing them to break up into electrically charged pieces. An electrically charged filter then traps the bits like fly paper. It has tested it with five European airlines that operate BAE’s small regional jet and recently obtained certification for an airline to fit the units to a Boeing 757. “There is a very tangible difference in the quality of air being breathed according to aircrew,” says Sean McGovern, who runs BAE’s regional-aircraft business. The companies will now offer the system to operators of other Boeing and Airbus aircraft. Each unit costs about $16,000. Two are needed on a BAE regional jet and five on an aircraft like the 757. By allowing air to be recycled more safely, the devices could pay for themselves within a year by reducing the amount of engine-bled air that is needed and hence fuel consumption, says Mr McGovern. Another advance that will make flying more comfortable in the coming years is the greater use of carbon fibre, especially by Boeing and Airbus, to make aircraft fuselages. Carbon fibre is stiffer than aluminium, which has long been used to make aircraft, and does not suffer from metal fatigue. One reason cabin pressures are kept low at the moment is to avoid stressing the fuselage too much. The use of carbon fibre will make it possible to pressurise cabins to a higher, more comfortable level. Nor does carbon fibre corrode, so the air inside the cabin will not have to be kept quite so dry—which often leaves passengers with a wrung-out feeling after a long flight. And with recent advances in power electronics, Boeing has decided that it is now more efficient to go back to using electrical generators to pressurise the air in the cabin. This change will start with its forthcoming (and much-delayed) 787, which is supposed to fly for the first time by the end of the year. That will put an end to the trade-off between air quality and fuel consumption. As all these developments start to reach the market, passengers may start checking not just flight schedules and fares before booking a journey but also their airline’s cabin-air quality.

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