Low Pressure Flying

Defining how cabin pressure affects the body
technology, cabin, aviation, g650
Written By J. Mac McClellan

Simply stated, oxygen is the fuel that powers our body. Blood is the liquid delivery system that carries that fuel to every cell in our bodies. A certain amount of air pressure is required in order for our lungs to effectively oxygenate our blood, and that’s where aircraft cabin pressure comes into play.

The dynamics of the earth’s atmosphere are that air becomes less dense as you climb away from the surface of the planet. At 5,000 feet (1,524 meters), such as Guatemala City or the “Mile-High City” of Denver, people become more easily exhausted, golf balls travel 10 percent farther, and even food preparation requires special adjustment because the air is thinner. Above 12,500 feet (3,810 meters) humans require supplemental oxygen to survive. And this is why aircraft that routinely fly at high altitudes have pressurized cabins—so we can breathe.

But not all aircraft are designed equally, and the level of cabin pressurization has a direct correlation and impact on how fatigued (or well rested) you feel after a pressurized flight.

Most commercial airline traffic occurs between 30,000 feet (9,144 meters) and 45,000 feet (13,716 meters) with pressurized cabins so we mere humans can oxygenate our blood. Typically, commercial aircraft cabins are pressurized to an 8,000-foot (2,438-meter) altitude. So while the aircraft is physically at 30,000 feet (9,144 meters) or higher, inside the cabin, your body feels like you’re at 8,000 feet (2,438 meters) or roughly the altitude of Bogota, Colombia, or Aspen, Colo. Anyone who has spent any time in the mountains is aware of how long it can take your body to acclimate to the thinner air and lack of oxygen. This feeling of being easily fatigued results from the combination of air that is less dense and lower atmospheric pressure, which means that your heart and lungs have to work harder to deliver the fuel your cells need to remain powered up.

In contrast, Gulfstream aircraft are designed to cruise at very high altitudes above airline traffic and in calmer air. This allows the engines to operate more efficiently, enables the aircraft to reach higher speeds due to reduced drag from the thinner air, and helps provide passengers a smoother ride above most of the weather.

Aircraft certification regulations allow passenger cabins to maintain an altitude of 8,000 feet (2,438 meters). Yet unlike other business jets, Gulfstream has kept the cabin altitude at 6,000 feet (1,828 meters) or below for an improved passenger experience. And now the new G650 has a cabin altitude of 4,100 feet (1,250 meters) or below at typical cruise altitudes.

When a Gulfstream reaches a typical cruise altitude of 45,000 feet (13,716 meters), there is only 15 percent of sea level atmospheric pressure outside the cabin. But inside the new G650, passengers will enjoy the comfort of 4,100 feet (1,250 meters) atmospheric pressure, which is like standing on the ground in western Kansas.

And because the large-cabin aircraft can cruise for nine hours, 15 hours and more, maintaining a lower cabin pressure means that passengers will arrive at their destination feeling more well rested and less fatigued than they would on any other aircraft. Passengers will literally breathe a sigh of relief.

How it Works

A jet cabin is pressurized by tapping high-pressure air from the engine compressors. The air is then cooled to a comfortable level and piped into the cabin. The desired cabin pressure level is maintained by outflow valves that continuously adjust and allow air to flow through the cabin and be vented outside. Gulfstream aircraft use only fresh air, as opposed to the recirculated air common on other aircraft.

To build both the ideal cabin shape for passengers and still contain the high pressure necessary for a low cabin altitude, Gulfstream engineers used new materials and new analytical tools.

Increasing cabin pressure and thus lowering the cabin altitude is a structural challenge. Higher pressures inside the cabin impose greater loads on the fuselage structure so it must be stronger and heavier. The lower the cabin pressure, the higher the stress.

Any structure—think scuba tank or propane gas bottle—that contains pressure is round. Pressure inside a container forces the structure to become circular because the forces are exerted equally and uniformly in all directions. That’s why jet aircraft fuselages are round—to more easily contain the cabin pressure.

A circular cabin is not, however, ideally suited for the people inside, so Gulfstream engineers created a new cabin shape for the G650. The cabin floor is flat, and because the fuselage is not a circle there is more headroom on both sides of the cabin, creating plenty of space for people to move about.

To build both the ideal cabin shape for passengers and still contain the high pressure necessary for a low cabin altitude, Gulfstream engineers used new materials and new analytical tools. Most of the cabin floor is made from high-tech carbon fiber composite material, and so are the bulkheads that contain cabin pressure at the ends of the fuselage. The cabin pressure tries to push outward and thus tries to pull on the floor—tension loads, engineers call this—and carbon fiber is extremely strong in tension. Additionally, carbon fiber is lightweight compared to metal with comparable tensile strength.

To design the metal cabin frames, Gulfstream used recently advanced computerized analytical tools to determine exactly where the stress of the noncircular cabin occurs. With these computerized tools Gulfstream engineers designed in the extra structure—and extra weight—where the analysis showed it was needed. Design of such an advanced fuselage frame would not have been possible even a few years ago because the computerized capability to analyze the loads did not exist. So the G650 gives passengers the lowest cabin altitude of any business jet.

And to be certain the new cabin was up to the stress of increased cabin pressures, engineers dialed up pressure to 18.37 pounds per square inch (psi), in a test rig subjected to normal flight loads. Bear in mind that the maximum normal operating pressure differential from inside the cabin to outside ambient is only 10.7 psi, so you can get a sense of the extreme nature of the test scenario. Further structural analysis based on actual test results demonstrated that the fuselage could withstand an astonishing 27.56 psi.

The cabin environmental system must protect passengers from an extremely hostile environment outside the aircraft. Air temperatures can be -94 F (-70 C) or lower but the air temperature inside the G650 is the temperature passengers select for two different cabin zones. Air coming from the engine compressors for the cabin leaves the engine with a maximum temperature of around 765 F (407 C). That temperature is sufficient to kill any possible bacteria or contaminant, so air entering the cabin is extremely pure. A series of cooling devices drop the air temperature to the desired level before it flows into the cabin.

Air enters the G650 cabin at a rate that exchanges the entire cabin volume every 3.2 minutes at a typical cruise altitude of 45,000 feet (13,716 meters). Compare that air exchange rate to the circulation in a well-insulated office building or residence and you see that the cabin environment of a G650 is superior to what we experience in our everyday lives on the ground.

Because Gulfstream engineers are fanatical about reducing drag and increasing efficiency for greater speed and range, it wasn’t long before they started to think about managing the high volume of air that flows into, and then out of, the Gulfstream as a mechanical advantage.

Through a very careful design Gulfstream engineers are able to direct air flowing out of the cabin so that it behaves like a mini-jet engine delivering approximately 30 pounds of thrust propelling the aircraft forward. That’s not a lot, but over a 12- or 14-hour trip it adds up. And in other aircraft that force is simply wasted or even a disadvantage creating inefficiency.

It stands to reason that the aircraft that boasts the industry’s highest speed, longest range and largest cabin should also offer the most comfortable cabin environment for passengers. Once again Gulfstream delivers The World Standard™.

A circular cabin is not ideally suited for the people inside, so Gulfstream engineers created a new cabin shape for the G650.

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