AIRCRAFT SYSTEMS II
5.0 AIRCRAFT ENVIRONMENTAL SYSTEM (ATA 21)
ATA 21 – Air Conditioning and Pressurization (environmental control)
5.1 INTRODUCTION:
Environmental control system (Pressurization and Air Conditioning System) is a system responsible for
ensuring that passengers in an aircraft flying at high altitude are supplied with air (oxygen) at the right
temperature, amount and pressure. It also ensures that the general environment of the cabin is habitable to
a human being i.e., not very cold or very hot.
Nitrogen is an inert gas that is not used directly by man for life processes; however, many compounds
containing nitrogen are essential to all living matter.
The small quantity of carbon dioxide in the atmosphere is utilized by plants during photosynthesis.
Thus, the food supply for all animals, including man, depends on it. Carbon dioxide also helps control
breathing in man and other animals.
The amount of water vapor in the atmosphere is variable but, even under humid conditions at sea level, it
rarely exceeds 5 percent. Water also occurs in the atmosphere as ice crystals. All forms of water in the
atmosphere absorb far more energy from the sun than do the other gases. Water plays an important role in
the formation of weather.
Ozone is a form of oxygen. It contains three oxygen atoms per molecule, rather than the usual two. Most
of the atmosphere’s ozone is formed by the interaction of oxygen and the sun’s rays near the top of the
stratosphere in an area called the ozone layer. This is important to living organisms because ozone filters
out most of the sun’s harmful ultraviolet (UV) radiation. Ozone is also produced by electrical discharges,
such as lightning strikes. It has a faint odor, somewhat like that of weak chlorine, that may be detected
after a thunderstorm. Auroras and cosmic rays may also produce ozone. Ozone is of great consequence to
living creatures on earth and to the circulation of the upper atmosphere.
HUMAN RESPIRATION AND CIRCULATION
Oxygen and Hypoxia - The second most prevalent substance in the atmosphere, oxygen, is essential
for most living processes. Without oxygen, humans and animals die very rapidly.
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, A reduction in the normal oxygen supply alters the human condition. It causes important changes in body
functions, thought processes, and the maintainable degree of consciousness. The resultant sluggish
condition of mind and body produced by insufficient oxygen is called hypoxia.
There are several scenarios that can result in hypoxia, one of these is:
Altitude - During aircraft operations, it is brought about by a decrease in the pressure of oxygen in the
lungs at high altitudes. The air contains the typical 21 percent of oxygen, but the rate at which oxygen can
be absorbed into the blood depends upon the oxygen pressure. Greater pressure pushes the oxygen from
the lung alveoli into the bloodstream. As the pressure is reduced, less oxygen is forced into and absorbed
by the blood.
Below 7,000 feet above sea level, the available oxygen quantity and pressure remain sufficient for
saturation of the blood with oxygen. Above 7,000 feet, however, the oxygen pressure becomes
increasingly insufficient to saturate the blood. At 10,000 feet mean sea level (MSL), saturation of the
blood with oxygen is only about 90 percent of normal. Long durations at this altitude can result in
headache and fatigue, both symptoms of hypoxia. At 15,000 feet MSL, oxygen transfer to the
bloodstream drops to 81 percent of saturation. This typically results in sleepiness, headache, blue lips and
fingernails, and increased pulse and respiration. Worse yet, vision and judgment become impaired and
safe operation of an aircraft becomes compromised. Higher in the atmosphere, decreasing pressure
causes even less oxygen to enter the bloodstream; only 68 percent saturation at 22,000 feet MSL.
Remaining at 25,000 feet MSL for 5 minutes, where oxygen transfer to the blood is reduced to
approximately 50 percent saturation, causes unconsciousness.
Hyperventilation - Another physiological phenomenon of interest to aviators is hyperventilation. Its
symptoms greatly resemble hypoxia. When various cells in the body use oxygen and food delivered to
them by the blood, carbon dioxide is a by-product. Blood carries this carbon dioxide to the lungs where it
is exhaled. Carbon dioxide functions in the body to regulate the depth and frequency of breathing. A high
level of carbon dioxide in the blood triggers rapid, deep breathing to expel it. This also promotes the
intake of a greater amount of oxygen for active cells to use. A low carbon dioxide level causes more
relaxed breathing resulting in less oxygen intake. Therefore, an oxygen/carbon dioxide balance exists in
the blood. Occasionally, fear, panic, or pain triggers excessive rapid breathing in a person. With it comes
a reduction of carbon dioxide in the blood, even though the body does not need this. The lower carbon
dioxide level signals the body that there is enough oxygen available and blood vessels constrict, causing
hypoxia-like symptoms because insufficient oxygen is being delivered to the cells. Note that the onset of
hypoxia described in the previous section occurs without the rapid breathing that accompanies
hyperventilation. Hyperventilation can often be alleviated by having the person calm down and breathe
normally, which restores the oxygen/carbon dioxide balance in the bloodstream.
Carbon Monoxide Poisoning - Carbon monoxide is a colorless, odorless gas produced by incomplete
combustion of hydrocarbon fuels, such as those used in aviation. The human body does not require this
gas to function. Its presence, however, can prevent a sufficient level of oxygen to be maintained in the
body, resulting in hypoxia. This is also known as carbon monoxide poisoning. As with all forms of
oxygen deprivation, extended exposure to carbon monoxide can result in unconsciousness and even
death.
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5.0 AIRCRAFT ENVIRONMENTAL SYSTEM (ATA 21)
ATA 21 – Air Conditioning and Pressurization (environmental control)
5.1 INTRODUCTION:
Environmental control system (Pressurization and Air Conditioning System) is a system responsible for
ensuring that passengers in an aircraft flying at high altitude are supplied with air (oxygen) at the right
temperature, amount and pressure. It also ensures that the general environment of the cabin is habitable to
a human being i.e., not very cold or very hot.
Nitrogen is an inert gas that is not used directly by man for life processes; however, many compounds
containing nitrogen are essential to all living matter.
The small quantity of carbon dioxide in the atmosphere is utilized by plants during photosynthesis.
Thus, the food supply for all animals, including man, depends on it. Carbon dioxide also helps control
breathing in man and other animals.
The amount of water vapor in the atmosphere is variable but, even under humid conditions at sea level, it
rarely exceeds 5 percent. Water also occurs in the atmosphere as ice crystals. All forms of water in the
atmosphere absorb far more energy from the sun than do the other gases. Water plays an important role in
the formation of weather.
Ozone is a form of oxygen. It contains three oxygen atoms per molecule, rather than the usual two. Most
of the atmosphere’s ozone is formed by the interaction of oxygen and the sun’s rays near the top of the
stratosphere in an area called the ozone layer. This is important to living organisms because ozone filters
out most of the sun’s harmful ultraviolet (UV) radiation. Ozone is also produced by electrical discharges,
such as lightning strikes. It has a faint odor, somewhat like that of weak chlorine, that may be detected
after a thunderstorm. Auroras and cosmic rays may also produce ozone. Ozone is of great consequence to
living creatures on earth and to the circulation of the upper atmosphere.
HUMAN RESPIRATION AND CIRCULATION
Oxygen and Hypoxia - The second most prevalent substance in the atmosphere, oxygen, is essential
for most living processes. Without oxygen, humans and animals die very rapidly.
This study source was downloaded by 100000858663287 from CourseHero.com on 12-09-2022 20:29:53 GMT -06:00
https://www.coursehero.com/file/178778247/AIRCRAFT-SYSTEMS-II-AIRCRAFT-ENVIRONMENTAL-SYSTEM-ATA-21docx/
, A reduction in the normal oxygen supply alters the human condition. It causes important changes in body
functions, thought processes, and the maintainable degree of consciousness. The resultant sluggish
condition of mind and body produced by insufficient oxygen is called hypoxia.
There are several scenarios that can result in hypoxia, one of these is:
Altitude - During aircraft operations, it is brought about by a decrease in the pressure of oxygen in the
lungs at high altitudes. The air contains the typical 21 percent of oxygen, but the rate at which oxygen can
be absorbed into the blood depends upon the oxygen pressure. Greater pressure pushes the oxygen from
the lung alveoli into the bloodstream. As the pressure is reduced, less oxygen is forced into and absorbed
by the blood.
Below 7,000 feet above sea level, the available oxygen quantity and pressure remain sufficient for
saturation of the blood with oxygen. Above 7,000 feet, however, the oxygen pressure becomes
increasingly insufficient to saturate the blood. At 10,000 feet mean sea level (MSL), saturation of the
blood with oxygen is only about 90 percent of normal. Long durations at this altitude can result in
headache and fatigue, both symptoms of hypoxia. At 15,000 feet MSL, oxygen transfer to the
bloodstream drops to 81 percent of saturation. This typically results in sleepiness, headache, blue lips and
fingernails, and increased pulse and respiration. Worse yet, vision and judgment become impaired and
safe operation of an aircraft becomes compromised. Higher in the atmosphere, decreasing pressure
causes even less oxygen to enter the bloodstream; only 68 percent saturation at 22,000 feet MSL.
Remaining at 25,000 feet MSL for 5 minutes, where oxygen transfer to the blood is reduced to
approximately 50 percent saturation, causes unconsciousness.
Hyperventilation - Another physiological phenomenon of interest to aviators is hyperventilation. Its
symptoms greatly resemble hypoxia. When various cells in the body use oxygen and food delivered to
them by the blood, carbon dioxide is a by-product. Blood carries this carbon dioxide to the lungs where it
is exhaled. Carbon dioxide functions in the body to regulate the depth and frequency of breathing. A high
level of carbon dioxide in the blood triggers rapid, deep breathing to expel it. This also promotes the
intake of a greater amount of oxygen for active cells to use. A low carbon dioxide level causes more
relaxed breathing resulting in less oxygen intake. Therefore, an oxygen/carbon dioxide balance exists in
the blood. Occasionally, fear, panic, or pain triggers excessive rapid breathing in a person. With it comes
a reduction of carbon dioxide in the blood, even though the body does not need this. The lower carbon
dioxide level signals the body that there is enough oxygen available and blood vessels constrict, causing
hypoxia-like symptoms because insufficient oxygen is being delivered to the cells. Note that the onset of
hypoxia described in the previous section occurs without the rapid breathing that accompanies
hyperventilation. Hyperventilation can often be alleviated by having the person calm down and breathe
normally, which restores the oxygen/carbon dioxide balance in the bloodstream.
Carbon Monoxide Poisoning - Carbon monoxide is a colorless, odorless gas produced by incomplete
combustion of hydrocarbon fuels, such as those used in aviation. The human body does not require this
gas to function. Its presence, however, can prevent a sufficient level of oxygen to be maintained in the
body, resulting in hypoxia. This is also known as carbon monoxide poisoning. As with all forms of
oxygen deprivation, extended exposure to carbon monoxide can result in unconsciousness and even
death.
This study source was downloaded by 100000858663287 from CourseHero.com on 12-09-2022 20:29:53 GMT -06:00
https://www.coursehero.com/file/178778247/AIRCRAFT-SYSTEMS-II-AIRCRAFT-ENVIRONMENTAL-SYSTEM-ATA-21docx/
