Boeing Dreamliner designed to help fliers breathe easy
By MARILYN ADAMS
By MARILYN ADAMS
Like airline food and airport security, the cabin air in jetliners is something we love to hate.
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 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.
AIR FROM THE ENGINE
On airplanes today, cabin air comes from an 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 percent of the U.S. commercial fleet is equipped with HEPA filters for cabin air.
But about 25 percent 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 on cabin air pollutants.
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 percent — to keep costs down. Siphoning off engine-bleed air also reduces the available engine power, increasing fuel consumption.
DIFFERENT ON THE 787
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. The composite material doesn't corrode like aluminum does. As a result, the 787 cabin air system will allow 15 percent humidity, a more comfortable level than the current 4 percent.
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 percent more oxygen for absorption into the blood. In studies, only 15 percent of passengers breathing 787-type air reported throat irritation vs. 30 percent 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 not only with the HEPA filters now in widespread use, but also a second filtering system that removes as many of the bad gases as possible.
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.