The Quest for Lower Cabin Altitude

It was supposed to be a quick flight between two Colorado cities. But then afternoon thunderstorms moved in faster than predicted. I found myself diverting around the weather from my intended route to one that took me higher over the mountains, to outrun the thunderheads in my unpressurized light airplane. Relatively new to mountain flying at the time, I hadn’t planned on this and did not have supplemental oxygen onboard.

Big mistake.

As I ascended through 14,000 feet, the classic symptoms of hypoxia began to hit me: rapid heart rate, perspiration, anxiety, and tingling in the fingers, all caused by a lack of oxygen to tissues and vital organs—like my brain. The symptoms worsened as the climb continued. Fortunately, within a few minutes, I could see my destination airport through the windshield. I corkscrewed down and landed without incident. After securing the airplane, I got on the phone and ordered a portable, supplemental aviation oxygen system for overnight delivery. Lesson learned.

Pressurization is important partly because it keeps crew and passengers comfortable and safely breathing on their own. Any time a cabin pressurization system fails—a rarity—passengers and crew must quickly access supplemental oxygen and the aircraft must descend to 10,000 feet or lower before that oxygen runs out. Pilots can monitor pressurization controls from the cockpit and either manually set them or rely on automatic function.

Thanks to pressurization, aircraft can climb above most weather into smoother air, often with helpfully higher tailwinds when flying from west to east (in the northern hemisphere). The thinner air and colder temperatures up there allow jet and turboprop engines to operate more efficiently (i.e., burn less fuel) and reduce aerodynamic drag. (Piston engines are less happy in this environment and must be turbocharged to run well much above 12,000 feet.)

Passenger aircraft, mainly airliners, have been pressurized since the 1940s. Pressurization systems in turbine engine–powered aircraft historically have worked by automatically taking compressed air, called bleed air, from the compressor stage of a jet or turboprop engine. Those engines have four main phases of operation: suck, squeeze, bang, and blow. A fan sucks air into the engine; that air is compressed via pressure turbines; fuel is added and ignited; and thrust results out the back. Bleed air comes off the engine during the “squeeze” phase. It is then expanded and cooled for use in the cabin via a series of valves, and for other systems on the aircraft that rely on bleed air, such as ones for wing and engine anti-icing and pressurizing onboard water.

Newer aircraft, such as the Boeing 787, do not use bleed air for pressurization but rather rely on environmental-control systems powered by electrical generators driving adjustable-speed compressors, marginally improving fuel economy by eliminating drained-off engine energy used to supply bleed air.

Pressurization Problems

Most of the time, these systems work as advertised, but there have been some notable related accidents over the years.

Repeated pressurization and depressurization of an aircraft’s fuselage contributes to metal fatigue of the structure. The higher the pressurization differential between the outside and inside air, as measured in pounds per square inch (psi), the greater the likelihood of metal fatigue and the shorter the aircraft’s useful life. But a high differential also lowers cabin altitude. The cabin psi required to maintain a 6,000-foot altitude in that Boeing 787 is 9.06 while at 8,000 feet it is 8.11. With a rigid composite fuselage aircraft like the 787, this is rarely an issue, but with a metal one that expands and contracts with pressurization, it can be problematic.

During the early days of jet airline travel, two four-engine de Havilland Comets were lost due to metal fatigue, resulting in the temporary revocation of the model’s type certificate by U.K. authorities in 1954. Investigators conducted post-crash testing and found that after 3,057 actual and simulated flight cycles (each defined as one takeoff and landing) accompanied by pressurization and depressurization, the fuselage ripped open, failing far sooner than expected. The manufacturer subsequently adopted the use of thicker materials in the aircraft’s pressure vessel and strengthened the structure around the cabin windows, and the Comets were cleared again to fly.

In 1988, a 19-year-old Boeing 737 suffered explosive decompression at 24,000 feet on an Aloha Airlines flight in Hawaii from Hilo to Honolulu. The force of the explosion resulted in the departure of most of the external forward-cabin structure and ejected one flight attendant from the airplane. Amazingly, she was the only fatality, though 65 passengers were injured, and the pilots managed to make an emergency landing on Maui. Subsequent investigation revealed that the 35,000-hour aircraft had accumulated 90,000 cycles and corresponding cabin pressurizations/depressurizations—twice the model’s intended service life. The National Transportation Safety Board (NTSB) concluded that metal fatigue exacerbated by corrosion caused the accident.

Perhaps the best-known business jet accident involving cabin pressurization occurred in 1999, when the system apparently failed on a chartered Learjet 35, killing professional golfer Payne Stewart and all five others onboard. The aircraft had departed Sanford, Florida, en route to Dallas, and was cleared to a cruise altitude of 39,000 feet. Soon thereafter, air traffic control lost contact with it. Air National Guard F-16 fighters then scrambled to intercept it and followed it into South Dakota, where it apparently ran out of fuel and crashed. The NTSB concluded that the accident’s probable cause was a loss of cabin pressurization for unknown reasons and the incapacitation of the flight crew because they failed to receive supplemental oxygen.

Though long out of production, however, Learjet 35s like the kind Stewart flew on still are prized as air ambulances due to their ability to maintain sea-level cabin pressure to an aircraft altitude of 25,700 feet. While the model does burn more fuel at that altitude, it keeps medically compromised patients, especially those with pulmonary problems, comfortable.

Combating Jet Lag

For years, the industry standard for cabin pressurization was 8,000 feet while the aircraft is at its maximum cruising altitude, typically 41,000 to 43,000 feet. That standard, combined with a lack of humidification and changes in circadian body rhythms, accounts for the fatiguing sensation on longer flights known as “jet lag.”

One way to combat jet lag is to lower cabin altitudes. And this becomes a bigger issue for older passengers. I recall then-72-year-old Morgan Freeman, the actor and pilot, telling me in a 2009 interview for BJT that one of the main reasons he had ordered a particular business jet was that it maintained sea-level cabin pressure up to 41,000 feet. “So, you are a lot fresher when you hit the ground after a long flight,” he said.

Several studies bear him out. One in 2007 by the National Center for Biotechnology Information evaluated the impact of long-haul jet flights on 14 healthy male subjects using a hypobaric chamber to simulate an 8.5-hour flight in an aircraft pressurized to a cabin altitude of 8,000 feet. The researchers found that such flights can affect passengers’ levels of plasma creatinine, urea, uric acid, sodium, calcium, phosphorus, glycemia, and lipids.

A 2010 NASA study, meanwhile, found that “jet lag resulted in increased sleepiness for over half the participants and deterioration of cognitive functioning for approximately one-third. The morning following the flight, subjects experienced…disruptions in working memory, divided attention, and visual perceptual speed.”

A study by Boeing and Oklahoma State University’s Center for Aerospace and Hyperbaric Medicine found that “acute mountain sickness occurs in some unacclimatized persons who travel to terrestrial altitudes at which barometric pressures are the same as those in commercial aircraft during flight.” It noted that “the frequency of reported discomfort increased with increasing altitude and decreasing oxygen saturation and was greater at 7,000 to 8,000 ft than at all the lower altitudes [tested].” Blood oxygen saturation decreased by an average of 4.4 percent and “an increased prevalence of discomfort” was noted in passengers after three to nine hours at an 8,000-foot cabin altitude. The study concluded that a cabin altitude of 6,000 feet or lower on long flights “will reduce the occurrence of discomfort among passengers.”

How Airframers Have Responded

Boeing translated this information into action on the 787, giving it a maximum cabin altitude of 6,000 feet. However, airframers have made even lower cabin altitudes a key marketing tool and product differentiator. The new crop of large-cabin, ultra-long-range business jets are jousting for bragging rights to the lowest cabin altitude at cruising limits up to 51,000 feet.

Gulfstream boasts that at 41,000 feet its new flagship G700 delivers a cabin altitude of just 2,916 feet; Bombardier says its upcoming Global 8000 will have a cabin altitude of 2,900 feet; and Dassault maintains its big Falcon 10X will have a cabin altitude of 3,000 feet. Advanced metal bonding, increased use of composites, and other technologies allow these manufacturers to offer this performance.

Typically, cabin altitude increases with smaller airframes, but not always. As noted, Morgan Freeman’s SJ-30, a light jet that seats six, maintained a sea-level cabin up to 41,000 feet. But that’s an outlier.

The large-cabin Dassault Falcon 6X delivers a cabin altitude of 3,900 feet at 41,000 feet; Textron Aviation’s Cessna Citation Longitude super-midsize yields a cabin altitude of 4,700 feet at 40,000 feet; Embraer’s Praetor 600 super-mid offers a 5,500-foot cabin at 45,000 feet; and a light jet such as the discontinued Beechcraft Premier 1A has an 8,000-foot cabin at 41,000 feet. Pressurized turboprops usually don’t provide comfort anywhere along this continuum, but they’re generally flown at lower altitudes and for shorter durations. A Daher TBM 940 single-engine turboprop at 28,000 feet has a cabin altitude of 7,600 feet and a Beechcraft King Air 350 twin flown at its 35,000-foot ceiling has an 11,000-foot cabin altitude.

And remember, cabin pressurization is just one part of the triad needed to reduce jet lag. The other two are cabin humidification and interior lighting systems designed to mitigate disruptions to circadian rhythms. Higher-end business jets, like the big Gulfstreams and Bombardier Globals, have such technology. But when it comes to cabin altitudes, lower is always better. It’s one case where the race to the bottom is a good thing.

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Mark Huber has reviewed aircraft for BJT since 2005.