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Can you explain pressurized airplane cabins? Is the same supply of air used over and over? How frequently does air flow into the cabin? What happens when the air filters get dirty? Aren't viruses too small to be captured by the high efficiency filters? How long have recirculation systems been used on passenger airplanes? How does the air-flow rate on current jetliners compare to earlier models? Doesn't the recirculated air just keep recirculating? Do pilots turn off air conditioning units to save fuel? Does combustion air make it into the supply air? If recirculated air is filtered, why isn't bleed air off the engine filtered before it comes into the passenger cabin? What happens if fumes from jet fuel or oil get into the passenger cabin? How dangerous are the fumes from jet fuel or oil that sometimes get into the passenger cabin? |
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Can you explain pressurized airplane cabins?
When you blow up a tire on a car or a bike, you use a pump to increase the pressure inside a closed space. A car tire typically runs at 30 psi, and a bike tire might run at 60 psi. There is no magic here -- the pump simply stuffs more air into a constant volume so the pressure rises. A plane flies at about 30,000 feet. The air pressure at 30,000 feet is significantly lower than at sea level (4.3 psi versus 14.7 psi). High-pressure air is used to "pump up" the cabin in much the same way that a tire is pumped up. The high-pressure air on most planes comes from the compression stage of the jet engines. Is the same supply air used over and over? No. Approximately 50 percent of the supply air is outside air and 50 percent is filtered recirculation air How frequently does air flow into the cabin? Ventilation is continuous. Air is constantly flowing in and out of the cabin. What happens when the air filters get dirty? As filters get dirty, two things happen: 1. 1. The particulate capture efficiency increases because the trapped particles make it more difficult for other matter to pass through, and 2. 2. The filter resistance increases, which leads to a reduction in recirculation flow. Boeing specifies a scheduled replacement interval for the High Efficiency Particulate Air (HEPA) filters to assure ventilation performance is maintained. Aren't viruses too small to be captured by the high efficiency filters? The HEPA filters are rated according to their ability to remove particulates measuring 0.3 microns, an industry standard. Because of the way these filters are designed, their efficiency actually increases for particles both smaller and larger than the most penetrating particle size, which is about 0.1 to 0.2 microns. The efficiency of HEPA filters to remove bacteria and viruses (.01 to .1 microns) is greater than 99 percent. How long have recirculation systems been used on passenger airplanes? Recirculation was in use before the jet age began. For example, the Boeing Stratocruiser of the late 1940s was equipped with an air recirculation system but it did not include HEPA filters. In jet airplanes, filtered or recirculated air combined with outside air came into use principally with the introduction of high-bypass-ratio fan engines. At Boeing, this began with the 747 back in 1970. Keep in mind that air recirculation is common in building ventilation systems. How does the air-flow rate on current jetliners compare to earlier models? Each Boeing airplane model, from the earliest to the latest, have been designed to deliver approximately the same total ventilation rate per passenger. The principal difference is that on newer versions, the cabin air is a mixture of about 50 percent outside air and 50 percent filtered/recirculated air. Among the benefits of this design is a lower potential exposure to atmospheric ozone and reduced fuel burn and associated engine emissions. Doesn't the recirculated air just keep recirculating? No. Outside-air mixing replenishes the cabin air constantly. Replenishment assures that the recirculated portion does not endlessly recirculate but is rapidly diluted and replaced with outside air. During cruise or on the ground, the outside air is drawn in at the same rate that cabin air is exhausted out of the airplane. Do pilots turn off air conditioning units to save fuel? Pilots have the ability to turn off air conditioning units but this is intended only as a safety feature in the event of an equipment failure and is not intended as a means to save fuel. If one air conditioning unit must be turned off during an equipment failure, the remaining unit or units on most jetliners will increase flow to partially recover the total air ventilation rate. Older 747 model aircraft did have an economy feature for lightly loaded flights. Does combustion air make it into the supply air? In systems that use air from engines, the air is taken from the engine compressors that are well upstream of any combustion. The air is simply compressed outside air. If re-circulated air is filtered, why isn't bleed air off the engine filtered before it comes into the passenger cabin? The ambient air outside the airplane at altitude cruise levels is very clean, cold (below -35 F/-37 C) and low in partial pressure of oxygen, too low to sustain life. Consequently, the air must be compressed to a density that is healthy for passengers and crew. Airplanes with a traditional bleed air system "bleed" or divert air from the airplanes' engine compressors to accomplish the task of warming and pressurizing the air. The air taken from the engine compressors is upstream of the combustion chamber where fuel is added. The bleed air is essentially dry, sterile and dust free. It is cooled in air conditioning packs and is then mixed with approximately 50 percent filtered recirculated air. The mixed air is then supplied to the airplane cabin at the proper temperature. What happens if fumes from jet fuel or oil get into the passenger cabin? How dangerous are the fumes from jet fuel or oil that sometimes get into the passenger cabin? On the very rare occasions where bleed air contaminants may enter the cabin, the contaminant levels are expected to be lower than occupational health thresholds established by toxicologists who have studied these contaminants extensively. Cabin Air Systems The cabin air system in today's jetliners is designed to provide a safe, comfortable cabin environment at cruising altitudes that can reach upwards of 40,000 feet. At those altitudes, the cabin must be pressurized to enable passengers and crew to breathe normally. By government regulation, the cabin pressure cannot be less than the equivalent of outside air pressure at 8,000 feet. Here's briefly how the system works: Cabin Air System Operation Pressurized air for the cabin in today's jetliners comes from the compressor stages in the aircraft's jet engines. Moving through the compressor, the outside air gets very hot as it becomes pressurized. The portion drawn off for the passenger cabin is first cooled by heat exchangers in the engine struts and then, after flowing through ducting in the wing, is further cooled by the main air conditioning units. The cooled air then flows to a chamber where it is mixed with an approximately equal amount of highly filtered air from the passenger cabin. The combined outside and filtered air is ducted to the cabin and distributed through overhead outlets. Inside the cabin, the air flows in a generally circular patterns and exits through floor grilles on either side of the cabin and on airplanes with overhead recirculation, the air may exit through the overhead. The exiting air goes below the cabin floor into the lower lobe of the fuselage. The airflow is continuous and is used for maintaining a comfortable cabin temperature, pressurization and overall air quality. About half of the air exiting the cabin is exhausted from the airplane through one or more outflow valves in the lower lobe, which also controls the cabin pressure. The other half is drawn by fans through High Efficiency Particulate Air (HEPA) filters under the cabin floor, and then is mixed with the outside air coming in from the engine compressors. (Some airplanes are equipped with an overhead recirculation system in combination with the under floor system). The HEPA filters are very effective at trapping microscopic particles such as bacteria and viruses and can provide essentially particle free air in the recirculation system. Key Characteristics and Overall Effectiveness There are several characteristics of the cabin air system that deserve special emphasis: Air circulation is continuous. Air is always flowing into and out of the cabin. Outside-air mixing replenishes the cabin air constantly. The outside-air content keeps carbon dioxide and other contaminants below standard limits and supplies oxygen far faster than the rate at which it is consumed. There are multiple factors associated with the aircraft cabin environment that can influence comfort. Symptoms occasionally reported by passengers and crew, including respiratory irritation and fatigue, can be caused by complex interactions of cabin environmental factors (cabin altitude, low humidity, noise, motion and temperature, odors and contaminants (personal hygiene, perfumes, etc), among other factors), individual health factors and work-related factors for cabin crew (duty-schedule impacts, jet lag, etc.). Differences Between Older and Newer Cabin Air Systems Engines that produced all or most of their thrust directly from the engine core powered early-generation jetliners. Air extracted from the compressor in these older aircraft provided the cabin with 100 percent outside air with only a modest impact on fuel economy. But by today's standards, the early-generation engines themselves were very noisy, emitted much higher levels of pollutants into the atmosphere and were much less fuel-efficient. By contrast, most newer jetliners are powered by high-bypass-ratio fan engines which are much quieter, much cleaner burning, more powerful and much more efficient. At the front end of this engine type is a large-diameter fan, which is powered by the core. The fan moves a large volume of air past the core rather than through it, and actually generates most of the thrust. By providing the cabin with a mixture of about 50 percent outside air taken from the compressor and 50 percent recirculated air, a balance has been achieved that maintains a high level of cabin air quality with respect to particulates, good fuel efficiency and less impact to our environment. Cabin pressurization 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. Need for cabin pressurization Pressurization becomes necessary at altitudes above 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 lower partial pressure of oxygen at 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 40,000 feet (12,000 m). This is because a person who is used to living at sea level needs about 0.20 bar partial oxygen pressure to function normally and that pressure can be maintained up to about 40,000 feet (12,000 m) by increasing the mole fraction of oxygen in the air that is being breathed. At 40,000 feet (12,000 m) the ambient air pressure falls to about 0.2 bar and to maintain a minimum partial pressure of oxygen of 0.2 bar requires breathing 100% oxygen using a oxygen mask. Emergency oxygen supply masks in the passenger compartment of airliners do not need to be pressure-demand masks because most flights stay below 40,000 feet (12,000 m). Above that altitude the partial pressure of oxygen will fall below 0.2 bar even at 100% oxygen and some degree of cabin pressurisation or rapid descent is essential to avoid the risk of hypoxia. Altitude sickness. Hyperventilation, the body’s most common response to hypoxia, does help to partially restore the partial pressure of oxygen in the blood, but it also causes carbon dioxide (CO2) to out-gas, raising the blood pH and inducing alkalosis. Passengers may experience fatigue, nausea, headaches, sleeplessness, and (on extended flights) even pulmonary oedema. These are the same symptoms that mountain climbers experience, but the limited duration of powered flight makes the development of pulmonary oedema unlikely. Altitude sickness may be controlled by a full pressure suit with helmet and faceplate, which completely envelops the body in a pressurized environment; this is impractical for commercial passengers. Decompression sickness. The low partial pressure of gases, principally nitrogen (N2) but including all other gases, may cause dissolved gases in the bloodstream to precipitate out, resulting in gas embolism or bubbles in the bloodstream. The mechanism is the same as for compressed-air divers on ascent from depth. Symptoms may include the early symptoms of "the bends"—tiredness, forgetfulness, headache, stroke, thrombosis, and subcutaneous itching—but rarely the full symptoms of the bends. Decompression sickness may also be controlled by a full-pressure suit as for altitude sickness. Barotrauma. As the aircraft climbs or descends, passengers may experience discomfort or acute pain as gases trapped within their bodies expand or contract. The most common problems occur with air trapped in the middle ear (aerotitus) or paranasal sinuses by a blocked Eustachian tube or sinuses. Pain may also be experienced in the gastrointestinal tract or even the teeth (barodontalgia). Usually these are not severe enough to cause actual trauma but can result in soreness in the ear that persists after the flight and can exacerbate or precipitate pre-existing medical conditions, such as pneumothorax. Pressurization of the cargo hold is also required to prevent damage to pressure-sensitive goods that might leak, expand, burst or be crushed on re-pressurization. Cabin altitude The pressure inside the cabin is technically referred to as the equivalent effective cabin altitude or more commonly as the cabin altitude. The cabin altitude is the equivalent altitude having the same atmospheric pressure, so that if the cabin altitude were set to zero then the pressure inside would be the pressure found at sea level. In practice, it is almost never kept at zero, in order to keep within the design limits of the fuselage and to manage landing at airfields higher than sea level. The cabin altitude of an aircraft planning to cruise at 40,000 ft (12,000 m) is programmed to rise gradually from the altitude of the airport of origin to around a maximum of 8,000 ft (2,400 m) and to then reduce gently during descent until it matches the ambient air pressure of the destination. Mechanics of pressurization Pressurization is achieved by the design of an airtight fuselage engineered to be pressurized with a source of compressed air and controlled by an environmental control system (ECS). The most common source of compressed air for pressurization is bleed air extracted from the compressor stage of a gas turbine engine, from a low or intermediate stage and also from an additional high stage, the exact stage can vary, depending on engine type. By the time the cold outside air has reached the bleed air valves it is at a very high pressure and has been heated to around 200 °C (392 °F). The control and selection of high or low bleed sources is fully automatic and is governed by the needs of various pneumatic systems at various stages of flight.[15] The part of the bleed air that is directed to the ECS is then expanded and cooled to a suitable temperature by passing it through a heat exchanger and air cycle machine known as the packs system. In some of the larger airliners hot trim air can be added downstream of air conditioned air coming from the packs if it is needed to warm a section of the cabin that is colder than others. At least two engines provide compressed bleed air for all of the plane's pneumatic systems, to provide full redundancy. Compressed air is also obtained from the auxiliary power unit (APU), if fitted, in the event of an emergency and for cabin air supply on the ground before the main engines are started. Most modern commercial aircraft today have fully redundant, duplicated electronic controllers for maintaining pressurization along with a manual back-up control system. All exhaust air is dumped to atmosphere via an outflow valve, usually at the rear of the fuselage. This valve controls the cabin pressure and also acts as a safety relief valve, in addition to other safety relief valves. In the event that the automatic pressure controllers fail, the pilot can manually control the cabin pressure valve, according to the backup emergency procedure checklist. The automatic controller normally maintains the proper cabin pressure altitude by constantly adjusting the outflow valve position so that the cabin altitude is as low as practical without exceeding the maximum pressure differential limit on the fuselage, which varies between different aircraft types but is normally around 8.6 psi. At 39,000 feet, the cabin pressure would be automatically maintained at about 6,900 ft (450 feet lower than Mexico City), which is about 11.5 psi of atmosphere pressure. Unplanned decompression Unplanned loss of cabin pressure at altitude is rare but has resulted in a number of fatal accidents. Failures range from sudden, catastrophic loss of airframe integrity (explosive decompression) to slow leaks or equipment malfunctions that allow cabin pressure to drop undetected to levels that can lead to unconsciousness or severe performance degradation of the aircrew. Any failure of cabin pressurization above 10,000 feet (3,000 m) requires an emergency descent to 8,000 feet (2,400 m) or the closest to that while maintaining terrain clearance (MSA), and the deployment of an oxygen mask for each seat. The oxygen systems have sufficient oxygen for all on board and give the pilots adequate time to descend to below 8,000 ft (2,400 m). Without emergency oxygen, hypoxia may lead to loss of consciousness and a subsequent loss of control of the aircraft. The time of useful consciousness varies according to altitude. As the pressure falls the cabin air temperature may also plummet to the ambient outside temperature with a danger of hypothermia or frostbite. |