Clouds

Breathe easy

The outside atmosphere doesn’t appear particularly scary from the window of a jet airplane. Clouds look puffy and sun-dappled in daytime, while stars above and glistening cities below fill the night sky with twinkling jewels of light. But the air we fly through way up there is as hostile as the ocean’s deepest depths. Not only is the average temperature 70 degrees below zero at 38,000 feet, but there isn’t nearly enough oxygen to sustain fragile human life.

For some context on just how fragile we humans are, consider that the entire atmosphere surrounding Earth is only about 300 miles thick. For perspective, if our planet were the size of an apple, the atmosphere would be about the thickness of that apple’s skin. And it’s only at the lowest 1 percent of that thin membrane where we oxygen breathers can survive comfortably.

That’s because the 300-mile-thick atmosphere has significant heft and at ground level, squeezes the air to a consistency we can breathe, compressing the gases so that there’s enough oxygen to sustain us. Above 15,000 feet, however, air is thin enough that the concentration of life-sustaining oxygen in each lungful is much lower than at the surface.

Just as submarines are safe havens for their occupants, our airplanes are our life-sustaining “vessels” on high. One big difference is that submarines must bring their own air with them when they submerge (though I understand they can chemically extract some oxygen from the ocean waters—kind of like a fish’s gills). Our airplanes are surrounded by the stuff, but we need to artificially pressurize and warm the thin, frozen air to create our life-sustaining cocoon.

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Perhaps they’d be successful if we adjusted our expectations.

Early airplanes didn’t fly high enough to need pressurization. But with the advent of the jet age, pressurization became a necessity. Drawing power from the engines, pressurization systems “inflate” the inside of a pressurized airplane, forcing in enough air for passengers to be comfortable. Relief valves open and close to regulate pressure levels. The fuselage must not only be sealed to hold in the air, but also be strong and flexible enough to withstand the strain of being inflated and deflated with every flight—not an inconsequential design limit.

And the engine power it takes to pump up the cabin comes at the cost of performance—sapping a small amount of speed and range, and burning that much more fuel. In fact, the air pumped in by engine power is called “bleed air.” So, ever-cost-conscious airlines have traditionally been stingy with pressurization requirements for the airplanes they buy.

The way to measure how pressurized an airplane gets is to calculate “cabin altitude.” That is the equivalent height above sea level that the outside air would be. For example, if the cabin altitude is 5,000 feet, that means the air density (and oxygen level) inside it is similar to what you would experience in Denver. Until recently, airliners typically had cabin altitudes equivalent to around 8,000 feet or more when flying at their 30,000-plus-foot cruising altitudes. While that is certainly a safe environment, it can be physically taxing—especially on flights that can last up to 17 hours. This can lead to fatigue and headaches for some passengers.

In the competition to sell airliners, passenger comfort levels are finally being addressed, thanks in part to new technology. The long-range Boeing 787 Dreamliner, for example, has maximum cabin altitudes closer to 6,000 feet. That’s possible partly due to the Dreamliner’s mostly composite construction, which allows for a stronger but not heavier aircraft. The Dreamliner’s popularity has led Boeing to re-engineer and adopt the 6,000-foot maximum cabin altitude for its newer 777 aluminum models, as well.

For private fliers, though, such levels are nothing new: most business jets have long had robust pressurization systems that make long flights relatively comfortable. So as you look out the window on your next flight, take a deep breath and appreciate the view—as well as your hospitable cabin.    

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