Aircraft have so many systems allow the aircraft to fly properly to its next destination. Of those systems one of the more important for high altitude flight is the pressurization system. Pressurization systems are designed to maintain the proper pressurization needed at high altitudes during flight. Usually the pilot will set the pressurization to the altitude they are taking off from and maintain that during flight and will adjust the pressurization during descent to the destination they will be going to. An aircraft pressurizes its cabin by taking in bleed air from its engines, sends it to compressors, then is cooled, and then is distributed to the cabin. The cabin holds in the air and has a relief valve system in the rear of the aircraft that releases over flow air to maintain regulated pressurization (Cabin Altitude) that is set by the pilots.
All systems require maintenance. This opens the possibility of maintainer errors that can cause system failure. As an example, a Boeing 737 experienced a loss of cabin pressurization and immediately initiated a descent and emergency landing to avoid an accident or injury.
"It is estimated that this serious incident would have occurred through the following process: the aircraft encountered turbulence when flying at FL370, quickly reduced the engine power in order to avoid excessive airspeed, and this in turn caused a change of the source of bleed air, which resulted in bleed air with higher temperature flowing into the pre-coolers, but the bleed air was not cooled sufficiently, and the overheat switches activated, closing the bleed valves for both systems and thus preventing the air supply necessary for pressurization of aircraft, ultimately resulting in an abnormal cabin depressurization... It is estimated that contamination deposits on the bleed related valves and other components resulted from incomplete draining of water and detergent which entered these components in large quantities during water washing conducted for engine gas path cleaning."
(SKYbrary - B735, en-route, SE of Kushimoto Wakayama Japan, 2006 (AW). (n.d.). Retrieved from http://www.skybrary.aero/index.php/B735,_en-route,_SE_of_Kushimoto_Wakayama_Japan,_2006_(AW))
In this scenario, the systems automatically shut themselves down preventing continued pressurization to the cabin. Because the pilots noticed this malfuction early on, they were able to descend and land the air craft to prevent injury to the crew and passengers. In other events like the Helios accident that I previously posted about, where the cabin pressurization system was not functioning because it was set in the manual operation in which the pilots were unaware of, all crew and passengers became hypoxic after running out of oxygen supplied by the oxygen masks.
Provided is a link from the FAA that better explains cabin pressurization.
Here is an example of how the air flows in a cabin pressurization system.
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