With ever larger numbers of ultra-long-range business jets taking to the skies, individual complaints about dry air in the cabin have grown to a chorus of demands for humidifiers. But while manufacturers would like to oblige, most have yet to come up with a system that is both airplane and people friendly.

Most cabin-environment specialists generally agree that for the health of the airplane, low or no humidity is best. Computers and avionics systems and corrosive metals like aridity. People don’t. They don’t find it particularly comfortable. More important, medical authorities agree that physical comfort is a minor inconvenience compared to the potential health hazards of an excessively dry environment, which may include:

• Aggravation of sinusitis, asthma, bronchitis and other respiratory problems.

• General dehydration caused by loss of body fluids during respiration.

• Skin irritation and itching due to evaporation of natural skin moisture.

• Eye itching due to loss of body fluids by general dehydration. Conditions that can lead to dehydration include:

• Disease of the adrenal glands, which regulate the body’s water and salt balance and the function of many organ systems.

• Diabetes mellitus.

• Eating disorders.

• Kidney disease.

• Chronic lung disease.

• Illness characterized by high temperature and diarrhea.

• Prolonged exposure to environment of high heat and low humidity.

Manufacturers of humidifier systems currently available for use in business aircraft and the aircraft manufacturers recommend an automatic setting that will maintain about 20-percent humidity, adequate in terms of both health and comfort.

Degrees of comfort vary with such factors as age, health, activity, clothing, body characteristics and even gender. But according to Carnegie Mellon University’s Lycos Internet site, “By increasing the relative humidity to above 60 percent within [the 71- to 77 deg F range], 80 percent or more of all average dressed persons would feel comfortable.” In fact, according to Lycos, “An apparent comfortable temperature can be maintained with a thermostat setting of 75 deg F with 20-percent relative humidity, or with a 70-deg F setting with 80-percent humidity.”

An ideal relative humidity in terms of general health considerations would be in the 50-percent range at about 71 deg F and applies to individuals in general good health.

Not a Dry Topic
A number of business aircraft manufacturers now offer the option of cabin-humidification systems. Others neither offer them nor recommend them.

A spokesman for Bombardier told AIN that early on almost half of its customers buying ultra-long-range Global Expresses were ordering the humidifier system provided by Liebherr-Aerospace of Toulouse, France. Bombardier has also installed the Humispace system produced by Le Bozec of Paris and marketed by partner Air Data of Montreal in its Challenger 604s, though in far smaller numbers.

Independent completion centers doing Boeing Business Jet interiors report that about half of the BBJs being finished are delivered with humidifiers. Ozark Aircraft Systems of Bentonville, Ark., recently delivered a BBJ with dual-zone humidification provided by two Le Bozec systems. And Raytheon Aircraft Integration Systems of Waco, Texas, has installed Le Bozec systems in 12 of the 21 BBJs it has delivered.

But while customer demand obviously exists, neither Gulfstream Aerospace nor Dassault Falcon Jet offer a cabin-humidification system– not even as an optional item.

Randy Grovenstein, project manager for completions at Gulfstream Aerospace, said the company does not recommend the installation of humidifiers and to date has installed only two, at the insistence of the customers.

“We did a lot of study and expected to find that it would be a significant comfort factor for passengers,” said Grovenstein. “While it appeared to have some desirable customer benefit, the long-term issues associated with other systems were a concern, including performance in an aircraft environment and its function as part of the existing environmental cabin-control system.”

The first Le Bozec humidifier was installed in a Gulfstream nearly two years ago and Grovenstein said feedback was “less than positive.” The crew had problems controlling humidity levels and “even with the system performing to spec, they found that customers didn’t know when it was operating and when it wasn’t.” He added that the company received similar feedback from the owner of the aircraft in which the second humidifier was installed.

Roy Elsasser, director of aircraft specification and design at Dassault Falcon Jet, said the company does not recommend humidifiers for its business jets. He added that several customers had them installed elsewhere, but as for Dassault itself, “We did an initial study and found moisture getting into areas where we felt it might promote corrosion.”

He admitted that Dassault may at times be “overly cautious,” but he also pointed out that not only would a humidifier (with an additional 8.4 gal of water) add about 90 lb to the airplane, but because the Falcons have 28-volt DC electrical systems, there would be the weight of a current inverter to consider as well.

Nevertheless, he said that Dassault continues to study the possibility, “especially for the FNX,” its under-development 5,700-nm trijet. First deliveries of the FNX are not expected until 2006.

Drip, Drip, Drip
Bombardier has been installing Liebherr-Aerospace humidifier systems as an option for its ultra-long-range Global Express since the company began doing the first interior completions in 1998. Late last year, however, after complaints from customers that during descent an annoying amount of condensation was dripping from openings in overhead panels, Bombardier advised customers to discontinue using the systems until it could find a solution to the problem.

A Bombardier spokesman said the company has taken several steps to manage the condensation problem, including changes to the insulation package and redesign of the drip trays. Tests of the modifications are continuing, but in the meantime the advisory remains in effect. Humidifier systems are being installed only on those aircraft already in production, and Bombardier is for the time being no longer listing the humidifier as an option.

“We plan to resume offering it,” said Marc Bouliane, manager for program planning for the Global Express, “as soon as we complete the [redesigned] water management system to our satisfaction.” The company is also considering how to deal with the mod as a retrofit for aircraft already in service.

Liebherr-Aerospace is one of Bombardier’s Global Express risk-sharing partners, with responsibility for the total cabin environmental control system.

Humidifier systems for business aircraft are typically installed to feed from the existing potable water system, but often require installation of an auxiliary tank as well. In an aircraft the size of a Global Express, a humidifier would typically use about 1.2 gal of water an hour to maintain a relative humidity of 20 percent. The unit itself weighs about 20- to 30 lb and includes a temperature/humidity sensor located in the existing air-conditioning/heating ductwork to maintain the desired humidity level. The price, uninstalled, is about $60,000, and the cost to the customer for a single finished humidifier system may run as high as $150,000.

In the BBJs delivered by Ozark, the systems were designed to shut off during descent at 24,000 ft. If water in the humidifier holding tank drops below a minimum level, a check valve opens to release additional water from the aircraft’s standard holding tanks.

The Liebherr Aerospace system mixes hot engine-bleed air with water and re-injects the resulting vapor created into the cabin air-distribution system. It uses a temperature sensor in the distribution duct and humidity feedback from a cabin sensor. The system is designed to activate automatically at about 30,000 ft, when the aircraft reaches a low-humidity flight level.

The Humispace system is the result of a partnership between Air Data and Paris-based Le Bozec. Air Data has a 25-percent interest in Le Bozec and is the North American distributor.

The Le Bozec unit itself weighs 22 lb. The vapor to be mixed with the circulating cabin air is created instantly as steam on contact with two electrodes in a “boiler” chamber. According to Air Data v-p Dominic Bruneau, an independent test lab in Paris showed that the resulting vapor contains “no contaminants, no minerals and no harmful bacteria.”

Allowed to return to its liquid state, the vapor produced by both systems would be accurately described as “distilled water.”

Meanwhile, customer health and comfort concerns are such that Elsasser and others admit that not only are some business airplane owners installing humidifiers through independent mod centers, some are even buying off-the-shelf household humidifiers for their airplanes. One business jet owner even bought a small personal humidifier and had it installed in the armrest of the divan. And a number of Global Express operators report that despite Bombardier’s advisory, they are continuing to operate their humidifiers.

Randy Grovenstein of Gulfstream does not recommend that operators bring household humidifiers aboard an aircraft for use. In fact, he pointed out that the American Society of Heating, Refrigeration and Air Conditioning Engineers recommends that the ideal home humidity level should range from 30- to 60 percent, which could cause damage to the high-gloss cabinetry and quality fabrics of an aircraft interior. Ozark’s Daron Dryer said a sustained humidity level of 30 percent or higher may also contribute to airframe corrosion.

As for the FAA, there is little it can do, as there are no regulations that prohibit aircraft owners from plugging pretty much anything they want into an outlet on their airplane. “One airworthiness inspector noted there is no regulation prohibiting the use of UL-certified humidifiers. “If I found a guy with one of these things, would I issue a violation? No,” he said. But he added that, as a pilot, he would also not approve. “There are good reasons why equipment is installed in an airplane and why STCs are issued for this equipment.”

Meanwhile, as demand for ultra-long-range business jets continues to grow, so does the demand for cabin humidifiers in an attempt to wring all the comfort possible out of a 10- or 12-hr trip. Is it a good idea? There are a lot of answers to that, depending on who’s doing the answering. But as one cabin-environment specialist put it, “If nobody wanted it and nobody needed it, why is it an option?”

Dehydration Is a Health Risk

While the concept of creating an ideal cabin environment in terms of temperature and humidity requires more fine tuning, there is little doubt among medical experts that prolonged exposure to dry air is a health risk. Among the risks often dismissed as minimal is dehydration, but it can be more serious than popular wisdom cares to admit.

The Gale Encyclopedia of Medicine description of dehydration should give frequent fliers–crew and passengers alike–pause to reflect. According to author Maureen Haggerty, it is “the loss of water and salts essential for normal body function.”

Dehydration occurs when the body loses more fluid than it takes in. It can be induced by a number of conditions, from exposure to a high, dry climate to the intake of alcohol or caffeine, or to the use of diuretics or other medications that cause increased urination. Certainly, long hours of exposure to a business jet cabin environment that typically contains 3- to 5-percent humidity can be included in the list, especially if added to the others. And older adults and younger children are more likely to become dehydrated.

How dangerous is dehydration? For most individuals, mild discomfort at worst. But it can also kill.

Mild dehydration is the loss of no more than 5 percent of the body’s fluid. Loss of 10 percent is considered moderate dehydration. Anything more than that is life-threatening and requires immediate medical care.

The first symptoms include dry or sticky mouth, cracked lips, lethargy, thirst and sunken eyes. [Sports trainers, in fact, recommend drinking fluids before one feels thirsty, because thirst is an early indication of dehydration.–Ed.] Severe symptoms may include confusion, drowsiness and fever.

Treatment of mild dehydration for average adults is simply a matter of increasing fluid intake and replacing lost electrolytes. Lost electrolytes can be replaced by drinking any of a variety of sports beverages created for that purpose. Severe dehydration may require hospitalization and intravenous fluid replacement.

Discomfort such as itching skin can be relieved by application of a moisturizing lotion and itching eyes by the use of drops for that purpose. But the best solution is not to allow the body to become dehydrated in the first place by maintaining a proper level of hydration and electrolytes. This means drinking plenty of water and juices and avoiding such common diuretics as alcohol and caffeine. For a normally healthy person, taking such preventive steps means you’re more likely to be both more comfortable and healthier on a long flight, with or without a humidifier.