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What Are the Functions of the Outflow Valve in an Airplane?

Until the 1930s, aircraft could not venture into the less turbulent altitudes above 10,000 feet because the air at that height is too thin to support life. In 1940, a decade of technological advances culminated in the Boeing 307 Stratoliner, the world's first pressurized airliner. The 307 could fly passengers up to 20,000 feet yet maintain a cabin air pressure equivalent to a trip to the local mountains. The technology behind pressurization has advanced as jets replaced propeller-driven airliners such as the Stratoliner, but the basic theory remains the same. The outflow valve is one of the critical components that maintains cabin pressure and makes flying safe in the smooth air above 10,000 feet.

Engine Pressure
- Air to pressurize an airliner is compressed by its engines. Jet turbofans produce thrust for flight by compressing air, injecting fuel and igniting the mixture in the combustion chamber. Some of that compressed air, known as "bleed air," is diverted from the turbofan's high-pressure compressor stage just before fuel is injected. Air straight from a jet engine is hot, so before being utilized for cabin pressurization, it's routed through an intercooler and expansion turbine to lower its temperature.
Cabin Pressure Controller
- The temperature-controlled compressed air is then channeled to a manifold where it enters the cabin for pressurization. The amount of pressurization is set by the cabin pressure controller in the cockpit. Before taking off, the pilot enters the cruising altitude into the controller. As soon as the plane is airborne, the controller gradually modulates cabin pressure so there are no abrupt changes even as the aircraft quickly gains altitude. The cabin pressure controller accomplishes this by adjusting the position of the outflow valve.
Outflow Valve
- A structurally reinforced, rectangular port usually located on the lower rear quarter of the aircraft, the outflow valve vents air pressure from within the cabin. As the plane gains altitude, it incrementally closes to build pressure within the cabin. When the plane is at a typical cruising altitude of 35,000 feet, the cabin pressure controller keeps the output valve positioned to maintain cabin pressure -- also known as "cabin altitude"-- at the equivalent of 6,000 to 8,000 feet, depending on the aircraft. This is comparable to breathing the air in a mountainous area on the ground and easily tolerated by most passengers.
Landing
- As the airliner begins its descent for landing, the pilot enters the destination airport's altitude into the cabin pressure controller. The controller incrementally opens the outflow valve to bleed out pressure as the plane descends so the cabin altitude matches the altitude of the destination as the plane touches down. The adjustment is so uniform that, except for some occasional ear-popping, the transition back to ground-level pressure is unnoticeable to most passengers.

Aviation