Back in the 1930s, aviation manufacturer Boeing came up with a new airliner, the Model 307 Stratoliner, which featured a game-changing innovation. It was equipped with a pressurized cabin, which enabled the plane to fly more swiftly and safely at altitudes above the weather, without causing passengers and crew to have difficulty getting enough oxygen from breathing the thinner air at 20,000 feet (6,096 meters).
Since then, cabin pressurization has become one of those technologies that most of us who fly probably take for granted.
Cabin pressurization works so well that passengers barely even notice it, in part because it gradually adjusts the air pressure inside the plane as it climbs in altitude, and then adjusts it again on the way down, explains Chuck Horning. He's been an associate professor in the aviation maintenance science department at Embry-Riddle Aeronautical University in Daytona Beach, Florida, since 2005 and before that, a mechanic and maintenance instructor at Delta Airlines for 18 years.
"It's not a terribly complex system," says Horning, who explains that the basic technology has pretty much stayed the same for decades, though the advent of electronic, computerized controls has made it more precise. Essentially, the aircraft uses some of the excess air that's pulled in by the compressors in its jet engines. "The engines don't need all that air for combustion, so some of it is tapped off and used both for air conditioning and pressurization."
The excess air from the compressors is cooled, and then pumped into the cabin. It's regulated by a device called the air cabin pressure controller, which Horning describes as "the brains of the pressurization system."
"That controller automatically regulates the pressurization," Horning explains. "It knows from information that the flight crew enters in what the cruising altitude is. It schedules the pressurizing so that as the airplane climbs and the external pressure goes down, it goes to work."
Pressurizing an aircraft too much could put its fuselage under too much stress from differential pressure as the plane climbs, Horning says. To avoid that, airliners don't try to duplicate the air pressure at sea level. Instead, at a cruising altitude of 36,000 feet (10,973 meters), most commercial jets simulate the air pressure at an elevation of 8,000 feet (2,438 meters), about the same as Aspen, Colorado.
The Boeing 787 Dreamliner, which has super-strong carbon fiber in its airframe, is able to get that down to the equivalent of air pressure at 6,000 feet (1,829 meters). "That's better, because as the cabin altitude goes up, you have less oxygen in your blood," Horning explains. "That's why when you get off a plane, you may feel tired."
How much air needs to be added to pressurize depends on the volume of the cabin, Horning says. Because the aircraft's pressurization system works in combination with the air conditioning system, it's also continuously cycling that air through the cabin, recirculating some of it and venting the rest as it draws in fresh air from the engine compressor.
Most airplanes will completely exchange the air inside the cabin in three to five minutes, according to Horning.
Gradual Pressurization Is Key
Airliners have to be careful to pressurize gradually as they ascend and depressurize just as gradually when they descend toward the destination airport, because humans are pretty sensitive to changes in air pressure — something anyone who's ever suffered from airplane ear already knows. That's one reason why the air pressurization system has automated controls. As Horning explains, if the controller were to malfunction, the aircraft's pilot could manually depressurize the aircraft during the descent, but it might be an uncomfortable experience for passengers and crew, since it's tough to do it as deftly by hand.
The air pressurization system also contains safety mechanisms designed to ward off mishaps. The positive pressure release valve will pop open if inside pressure gets too high because too much air is being pumped in the cabin. It will relieve that pressure. There's also the negative pressure valve, which protects the aircraft from the effects of a shift in which the outside pressure would become greater than inside the cabin. (This might occur during a sudden descent, as Aerosavvy details.)
"Airplanes are not designed to be submarines," Horning says. "They're designed to have a higher inside pressure than the outside. That's why that negative pressure relief valve is much more sensitive." As a result, when you're on a plane that's descending, once in a while you actually hear a loud rush of air. That's the negative pressure valve kicking in.
In the rare event that depressurization fails during a flight, there are other safeguards, Horning notes. There's a sensor that detects when the pressure declines to the equivalent of 12,000 feet (3,658 meters) in elevation. That switch automatically drops oxygen masks into the cabin, so that the passengers can continue to breathe without difficulty. In some aircraft, the oxygen comes from cylinders, while others get it from generators that release oxygen through a chemical reaction.