Enlarge   A computer-generated image of the Boeing 787 Dreamliner, scheduled to begin service in 2008.    FUSELAGE MAKES 787 DIFFERENT The 787 fuselage will be made of man-made composite material that doesn't corrode or fatigue from repeated pressurizations. That's key to improving passenger comfort. Here's why: Today's planes are built mostly of aluminum, and the expansion and contraction of metal from repeated pressurizations weakens the plane's structure over time. Cabin humidity is kept low to reduce the threat of corrosion. Planes now are pressurized to what a person would experience at 8,000 feet above sea level. Because it can tolerate greater air pressure, the 787 will be pressurized to a more comfortable 6,000 feet above sea level. It's not feasible to maintain a cabin pressure equal to sea level. That would require a heavily reinforced cabin that would be too heavy to fly.

It's so dry it can make throats itch and eyes water. It's so low in oxygen that it can induce headaches or dizziness or it can make us tired.

Airplane maker Boeing (BA) heard all those gripes and is promising to do something about them. After much research, it set out to build a more comfortable cabin air system for its 787 Dreamliner, the new long-range, midsize jet scheduled to begin service in 2008.

Boeing says cabin air in the 787, which will seat about 300, will contain more oxygen, more humidity and fewer pollutants than on current jetliners. As a result, it should make passengers feel better during and after their flights.

Where air comes from now

On airplanes flying today, cabin air comes from a very unlikely place: the inside of the jet engine.

This so-called bleed air is diverted to the inside of the plane before it mixes with fuel and combusts. It is so hot and under so much pressure that it must be vented into the airplane through ducts made of titanium, one of the strongest metals.

Before the air enters the cabin, it is cooled and, in most cases, filtered through high-efficiency particulate air (HEPA) filters that remove most dust, pollen, bacteria and viruses. About 75% of the U.S. commercial fleet is equipped with HEPA filters for cabin air.

But about 25% of the fleet — the oldest planes — are not, and the Federal Aviation Administration has not required airlines to retrofit those planes.

The FAA says studies to date have found that airplane cabin air, even without HEPA filters, is at least as clean as the air in most homes and offices. Nonetheless, the FAA is continuing to fund research at universities on cabin air pollutants, filters and other subjects.

In existing planes, cabin air is compressed to the pressure that exists at 8,000 feet above sea level. Airplanes pressurize only to that high altitude because they're built from numerous aluminum pieces held together by rivets and other fasteners.

Higher air pressure pushing against the inside walls of the metal fuselage would increase wear and tear on the plane and hasten metal fatigue, which leads to costly repairs for airlines.

The high heat of engine air — up to 530 degrees Fahrenheit right after takeoff, when the engines are working hardest — makes it extremely dry. The only humidity in a plane comes from the breath of the occupants.

And because metal planes are susceptible to corrosion, which requires costly repairs, airlines keep cabin humidity very low — at about 4% — to keep costs down. Siphoning off engine-bleed air also reduces the available engine power, increasing fuel consumption.

How the 787 will be different

The 787's cabin air system will be fundamentally different.

Cabin air will be vented directly from the outside through dedicated inlets on each side of the plane's belly and won't pass through the engines, says Mike Sinnett, the 787 project director.

The 787 system will be driven by electricity generated by the engines. An electrical system makes it easier to humidify the cabin air because it's not starting with the hot, dry air from the jet engines, says physicist Hans Weber of consulting firm Tecop International in San Diego.

With the 787 fuselage made of composite material, it's possible to increase the humidity without corroding the airplane over time. On the 787, the manmade composite material is formed and baked into large barrels that are linked together to form the fuselage. The composite material doesn't corrode as aluminum does. As a result, the 787 cabin air system will allow 15% humidity, a more comfortable level than the current 4%.

Humidity's adjustable

The 787 crew will be able to program the cabin air system for optimal humidity based on the number of passengers aboard, Sinnett says.

The cabin air will be compressed to resemble an altitude of 6,000 feet above sea level, instead of 8,000 feet, increasing the air pressure and oxygen inside the cabin.

As a result of its lower simulated cabin altitude, the 787's air will have 8% more oxygen for absorption into the blood. In studies, only 15% of passengers breathing 787-type air reported throat irritation, vs. 30% of passengers breathing traditional cabin air.

Put simply, "The lower you go, the better people feel," Sinnett says.

"The impact is more pronounced the longer the flight."

Boeing-sponsored studies have found volatile organic gases build up inside the tight confines of a plane, making some passengers feel ill. Sources include hand wipes, cologne and vinyl luggage.

As a result, Boeing has designed the 787 with not only the HEPA filters now in widespread use, but also a second filtering system that removes as many of the bad gases as possible.

Airlines wild about it

The plane is wildly popular so far, with about 400 now on order. But most of the industry fixation with the 787 is related to economics.

The new jet will be lighter, faster and more fuel-efficient than its predecessors or competitors, which should save airlines money.

John Greenlee, flight planning director at Houston-based Continental Airlines, says the favorable economics of operating the plane were the top considerations in ordering 20 787s.

But the innovative ventilation system was also a factor, he says, because it will help differentiate Continental from competitors.

"On flights of eight, 10 or more hours, we think people will arrive feeling fresher," Greenlee says.

Aerospace consultant Richard Aboulafia of the Teal Group in Fairfax, Va., says other airlines are likely to view the improved ventilation as Continental does: a secondary but important consideration in buying the plane.

"If the economics are good, comfort can help with marketing and buzz," he says.

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