This month, in 1921, a modified Airco DH-9-A aircraft (a late WWI light bomber single-engine biplane) took flight with the first pressurized cabin. The achievement would lead engineers to adjust, modify, fail, and eventually succeed in the quest to bring breathable air to commercial airplanes.
Airplanes need air with less oxygen. Pilots are at altitudes where they lose consciousness and would ultimately die without oxygen. Early fliers coped by filling tanks with pressurized oxygen and inhaling the gas through rubber tubes. Later, they used form-fitting face masks. In many high-flying light airplanes and military aircraft, oxygen systems and face masks are still used to keep the pilot alive and conscious.
Federal aviation regulations require pilots to have oxygen above 12,500 feet for more than 30 minutes, and passengers need it continuously above 15,000 feet. For airliners operating at altitudes more than 15,000 feet, everyone must be supplied with 10 minutes of oxygen in the event pressure cannot be maintained.
Air is pressurized by the engines. Turbofan engines compress intake air with vaned rotors right behind the fan. At each stage of compression, the air gets hotter, and at the point where the heat and pressure are highest, some air is diverted. Some of the hot, high-pressure air, called bleed air, is sent to de-ice wings and other surfaces. The remaining air goes to systems operated by air pressure, and some is sent to the cabin.
Air headed to the cabin is cooled by the intercooler- much like a car radiator. From there, it travels further into the plane where air packs will cool the air even more, using a refrigeration process. The air packs compress the air to heat it before it goes to another intercooler to remove the heat from the plane.
The air then expands through an expansion turbine, which cools it, making it ready to mix with the air in the cabin. Moved by fans and regulated by systems to keep the air at a comfortable temperature, the pressure in the cabin is equal to that at a low altitude. For this stabilization to take place, the incoming air is held within the cabin by opening and closing an outflow valve (an opening about the size of a briefcase), regulated by pressure sensors. The entire system is much like an inflated balloon with a leak continuously being inflated.
As an airplane descends, the pilot sets the system controller to the altitude of the destination airport, and the process works in reverse. Once the plane has landed, the outflow valve is wide open.
After the Army’s accomplishment in 1921, the U.S. Army Air Corps began researching flights in a modified Lockheed Electra. The XC-35 was the first airplane built with a pressurized cabin. In 1937, the XC-35 earned the Air Corps the Collier Trophy for the most significant development of the year.
In 1940, Boeing’s 307 Stratoliner began flying passengers at 20,000 feet in pressurized cabins. Today almost all pressurized airliners have universal systems with only slight variations in details among them.
How much pressure a cabin can tolerate is dependent on the structural strength of the airplane. Planes built today are tested thousands of times to ensure they operate correctly.
If you would like more information about how pressurization works visit Aircraft pressurization beginner’s guide – AeroSavvy