Cabin pressurization

Airliners developed since the 1940s have had pressurized cabins (or more accurately, pressurized hulls including baggage holds) to enable them to carry passengers safely at high altitudes where low oxygen levels and air pressure would otherwise cause sickness or death. High altitude flight enabled airliners to fly above most weather systems that cause turbulent or dangerous flying conditions, and also to fly faster and further as there is less drag due to the lower air density. Pressurisation is applied using compressed air, in most cases bled from the engines, and is managed by a environmental control system which draws in clean air, and vents stale air out through a valve. Pressurization presents design and construction challenges to maintain the structural integrity and sealing of the cabin and hull and to prevent rapid decompression. Some of the consequences include small round windows, doors that open inwards and are larger than the door hole, and an emergency oxygen system. To maintain a pressure in the cabin equivalent to an altitude close to sea level would, at a cruising altitude around 10,000 m (33,000 feet), create a pressure difference between inside the aircraft and outside the aircraft that would require greater hull strength and weight. Most people do not suffer ill effects up to an altitude of 18002500 m (60008000 feet), and maintaining cabin pressure at this equivalent altitude significantly reduces the pressure difference and therefore the required hull strength and weight. A side effect is that passengers experience some discomfort as the cabin pressure changes during ascent and descent to the majority of airports, which are at low altitudes. Cabin pressurization is used to create a safe and comfortable environment for aircraft passengers and crew flying at high altitude by pumping conditioned air into the cabin. This air is usually bled off from the engines at the compressor stage. The air is then cooled,humidified, mixed with recirculated air if necessary and distributed to the cabin by one or

more environmental control systems.[1] The cabin pressure is regulated by the outflow valve. Pressurization becomes necessary at altitudes beyond 12,500 feet (3,800 m) to 14,000 feet (4,300 m) above sea level to protect crew and passengers from the risk of a number of physiological problems caused by the low outside air pressure above that altitude; it also serves to generally increase passenger comfort. The principal physiological problems are as follows: Hypoxia. The low partial pressure of oxygen at an altitude reduces the alveolar oxygen tension in the lungs and subsequently in the brain, leading to sluggish thinking, dimmed vision, loss of consciousness, and ultimately death. In some individuals, particularly those with heart or lung disease, symptoms may begin as low as 5,000 feet (1,500 m), although most passengers can tolerate altitudes of 8,000 feet (2,400 m) without ill effect. At this altitude, there is about 25% less oxygen than there is at sea level.[2] Hypoxia may be addressed by the administration of supplemental oxygen, either through an oxygen mask or through a nasal cannula. Without pressurization, sufficient oxygen can be delivered up to an altitude of about 12,000 meters (about 40,000 feet). That is because a human being that is used to living at sea level needs about 0.20 bar partial oxygen pressure to function normally. That pressure can be maintained up to about 12,000 meters by increasing the mole fraction of oxygen in the air that is being breathed. At about 12,000 meters ambient air pressure is about 0.2 bar and partial oxygen pressure of 0.2 bar can therefore just be maintained by enriching the air to 100% oxygen. A diluter-demand mask is able to do this. Emergency oxygen supply masks in the passenger compartment of airliners do not need to be pressure-demand masks, as most flights stay below 40,000 feet. Rising above that altitude the air, be it in an oxygen mask or in the cabin, needs to be pressurized more and more if it wants to maintain normal partial oxygen pressure and avoid hypoxia.