Flying higher is almost always better than flying lower. Getting above the weather is always more comfortable than going through it, and taking advantage of jet-stream tailwinds and increased engine performance can save hours of flight time. A pressurized aircraft makes all of this possible. Basic pressurization concepts are included in the Commercial Pilot Practical Test Standards. The pilot must exhibit knowledge of the elements related to pressurization and the physiological hazards associated with high altitude flight and decompression.
When discussing pressurization, think of the aircraft as consisting of two sections: pressurized vessel and nonpressurized vessel. The pressurized vessel will generally encompass the passenger cabin and cockpit, as well as the baggage compartment (we don’t want Fido getting hypoxia). This section will be able to hold air under pressure in order to simulate a suitable altitude for comfort, temperature, and oxygen saturation. Most pressurized aircraft operate between 6,000 and 8,000 feet of cabin altitude.
This pressure is maintained by using bleed air from some form of a compressor on the aircraft engine (“How It Works: Pressurization,” March 2012 Flight Training). Whether that is a piston-powered, turbocharged engine or a turbine-powered jet engine, compressed air is drawn into the cabin and the pressure is maintained by controlling the outflow valve. The outflow valve usually is installed on the aft pressure bulkhead of the fuselage. It controls the amount of pressurized air allowed to escape the pressure vessel, thus controlling the cabin altitude.
In most modern aircraft, the outflow valve is automatically modulated to maintain a desired cabin altitude. Previous designs required constant monitoring and adjustment of the outflow valve to prevent the cabin altitude from climbing too high or the aircraft from landing at sea level while the cabin was still at 6,000 feet. Advances in technology allow the pilot to simply set the landing elevation and let the automatic controller do the work.
When that system fails, problems can occur quickly. Rapid or even explosive decompression at 35,000 feet can render you unconscious in as little as 60 seconds. That time drops to a mere 15 seconds at 43,000 feet. For these reasons each pilot station is equipped with an oxygen mask, and it almost always is a quick-donning style that can be opened and put on with one hand in five seconds. Just as you practice and train for crosswind landings, airline pilots are routinely trained and checked on rapid decompression and emergency descents.
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