OEMs set sights on lowering cabin altitude

 - November 5, 2007, 10:40 AM

The business aviation industry is on a constant quest to create a more comfortable cabin environment for the traveler, but only in the past several years have aircraft manufacturers tackled the nagging issue of cabin altitude and its physiological effects, and with some success.

Before those efforts, virtually every aircraft cabin was pressurized to the equivalent of an 8,000-foot altitude, which equates to an atmospheric pressure of about 10.9 pounds per square inch. It wasn’t ideal in terms of human comfort, despite Air Force findings that an altitude of about 8,000 feet was tolerable by a normal healthy human. For the most part it was a compromise, considering that an 8,000-foot cabin was also about all the existing technology would allow without structural reinforcement that would make the airplane too heavy. Finally, FAR Part 25 set the upper limit of cabin altitude to the equivalent of 8,000 feet.

As time passed and passenger complaints about headaches, drowsiness, dizziness, nausea and mental and physical fatigue mounted, the atmospheric pressure of the cabin was assumed to be the culprit. Cabin pressure has even been known to exacerbate jet lag.

It isn’t that the human body isn’t capable of adapting to life at higher altitudes. Wenzhuan, China, at 16,730 feet (give or take a couple of feet) is the highest city in the world. In Lhasa, Tibet, a quarter of a million people live at more than 11,000 feet. But inhabitants at such altitudes were either born there or had adequate time to adapt.

However, the cabin of an aircraft takes the passenger from ground level to 8,000 feet in a matter of 30 or 45 minutes. Adaptation is not an option, so aircraft manufacturers have been hard at work reducing cabin altitude.

When the 737-based Boeing Business Jet was introduced in 2001 its cabin altitude was 8,000 feet at the airplane’s cruise altitude of 41,000 feet. In 2004 Boeing increased the pressure differential and reduced the cabin altitude to 6,500 feet at an altitude of 41,000 feet. Boeing reduced the cycle threshold of some maintenance inspections to accommodate the greater stresses, but even the reduced number of cycles “was plenty for a lifetime of flying the corporate/executive mission.”

Bombardier Business Aircraft has taken a particularly aggressive approach to cabin altitude. According to Vinod Mistry, air systems senior engineering specialist, the cabin altitude of the original Global Express was 7,500 feet at 51,000 feet. Now it is 5,600 feet. Subsequent derivative aircraft, the Global XRS and Global 5000, both have a cabin altitude of 4,500 feet at 45,000 feet and 6,000 feet at their 51,000-foot ceiling.

According to Cessna Aircraft new programs manager Dale Tutt, the Wichita-based maker of the Citation line has been examining a lower-than-the-norm cabin altitude for the new Large Cabin Concept jet it will likely soon launch.

While previous Falcons have a cabin altitude of 8,000 feet, Dassault’s new Falcon 7X features a cabin altitude of 6,000 feet at its 51,000-foot ceiling. There is also a flight-level select that will allow cabin altitude to remain at 1,000 feet up to 27,000 feet. That choice is also standard on the Falcon 900EX EASy and Falcon 2000EX EASy, up to 23,000 feet.

Every airplane in Gulfstream’s lineup has a cabin altitude of 6,000 feet at its ceiling.
And while the cabin altitude in the Eclipse 500 is 8,000 feet at 41,000 feet,
the airplane maintains sea-level cabin altitude up to 21,500 feet.

The Sino Swearingen SJ30 maintains a sea-level cabin all the way up to its 41,000-foot ceiling. Airliners are becoming more passenger-friendly too. The Airbus A380 has a 5,000-foot cabin at its normal cruise altitude of 43,000 feet. And Boeing’s 787 will employ electric compressors rather than bleed air to power the cabin pressurization system and maintain a 6,000-foot cabin at its service ceiling of 43,000 feet.

Plenty of Air, But Not Enough Oxygen?
Lower cabin altitude is well and good, but there is also the question of oxygen content in a pressurized cabin. A Boeing study seems to debunk allegations that airplane cabins have a below-normal oxygen content. According to a report titled “The Airplane Cabin Environment,” cabin pressurization systems provide about 20 cu ft per minute of air per passenger, resulting in a complete air exchange every two to three minutes. More important, according to the study, humans at rest consume about 0.015 cu ft of oxygen per minute, while cabin-pressurization systems provide approximately 4.19 cu ft of oxygen per minute per person, or about 279 times more oxygen per minute than can be physically consumed.

The oxygen content of the air at sea level is about 21 percent. But as altitude increases, the number of molecules of each gas–including oxygen–decreases. So at 8,000 feet, where the air is “thinner,” the actual oxygen content is less.

What’s more, the atmospheric pressure at 8,000 feet is about 74 percent of what it is at sea level. It is this pressure that helps force an intake of air into the blood across the alveoli of the lungs, sending oxygen throughout the body and keeping us conscious. It is the combination of less oxygen and lower pressure that contributes to passenger complaints.

Research by the National Academy of Sciences has concluded, “Pressurization of the cabin to an equivalent altitude of 5,000 to 8,000 feet is physiologically safe.” But this depends on how one defines “safe,” and certainly it might not be safe for people with certain medical conditions. And “safe” is not necessarily the same as “comfortable.”
On the Global line, Bombardier has a recirculating fan system that supplies 18 cu ft of fresh air per minute, per passenger, based on a maximum occupancy of 19 passengers in an aircraft that averages a passenger load of eight to 10.

Cabin Humidity Is a Challenge
Even as manufacturers are finding ways to lower the cabin altitude and thus ensure a greater percentage of oxygen, there remains the problem of aridity inherent in the air-exchange system. In the typical business jet cabin the humidity is about 2 or 3 percent–nice for the airplane itself, and even for the passengers in that it inhibits bacterial growth. But it also fosters dehydration that might increase the possibility of deep-vein thrombosis or aggravate blood-related health issues. This is why passengers are advised to avoid alcohol and other diuretics such as tea and coffee and drink plenty of hydrating fluids.

According to Mistry, Bombardier and Air Data engineers are studying a number of systems to manage condensation produced by the cabin humidifier that is an option on its Global series. “We are looking at different ways to keep the moist air inside the cabin inner shell,” said Mistry.

Dassault Falcon offers a cabin humidifier as an option on the Falcon 7X. The system, claims the company, will maintain 20-percent relative humidity at 75 degrees F.

There is also the matter of ozone, which can be harmful in sufficient amounts. According to Mistry, a cabin-pressurization system must take into consideration the increasing amount of ozone occurring in the atmosphere at altitudes higher than 27,000 feet. Dissolution and conversion of ozone to oxygen naturally occurs in the moist-air intake. There is also an in-line centrifuge bleed filter to neutralize ozone intake, and some aircraft flying at particularly high altitudes might carry an ozone converter. Boeing has also employed ozone converters as part of its cabin-pressurization system.

Boeing attributes its 6,000-foot 787 cabin to the greater structural integrity afforded by the airplane’s composite construction and to the choice of electric compressors for the cabin pressure system.

Manufacturers continue to make improvements in the field of cabin-pressurization systems. Some of these gains have made aircraft more efficient and reduced weight. But even better, they are creating a more comfortable environment for the passengers, who are increasingly faced with nonstop flights as long as 14 and 15 hours.