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Airplanes, which do not have pressurization and air conditioning systems, are usually limited to the lower altitudes. They need to stay below 8,000 feet. A cabin pressurization system accomplishes several functions in providing passengers comfort and safety. It maintains a cabin pressure altitude of approximately 8,000 feet at the maximum designed cruising altitude of the airplane, and prevents rapid changes of cabin altitude, which may be dangerous to the passengers and crew. In addition, the pressurization system permits a reasonably fast exchange of air from inside to outside the cabin.

This is necessary to eliminate odors and to remove stale air. Pressurization of the airplane cabin is now the accepted method of protecting persons against the effects of hypoxia. Within a pressurized cabin, people can be transported comfortably and safely for long periods of time, particularly if the cabin altitude is maintained at 8,000 feet or below, where the use of oxygen equipment is not required. In this typical pressurization system, the cabin, flight compartment, and baggage compartments are incorporated into a sealed unit, which is capable of containing air under a pressure higher than outside atmospheric pressure.

Pressurized air is pumped into this sealed fuselage by cabin superchargers, which deliver a relatively constant volume of air at all altitudes up to a designed maximum. Air is released from the fuselage by a device called an outflow valve. Since the superchargers provide a constant inflow of air to the pressurized area, the outflow valve, by regulating the air exit, is the major controlling element in the pressurization system. It is necessary to become familiar with some terms and definitions to understand the operating principles of pressurization and air conditioning systems. These vocabulary terms are essential to know:       1.

Ambient pressure. The pressure in the area immediately surrounding the airplane. 2. Cabin altitude. Used to express cabin pressure in terms of equivalent altitude above sea level. 3. Differential pressure. The difference in pressure between the pressure acting on one side of a wall and the pressure acting on the other side of the wall. In aircraft air conditioning and pressurizing systems, it is the difference between cabin pressure and atmospheric pressure. The cabin pressure control system provides cabin pressure regulation, pressure relief, vacuum relief, and the means for selecting the esired cabin altitude in different ranges. In addition, dumping of the cabin pressure is a function of the pressure control system. A cabin pressure regulator, an outflow valve, and a safety valve are used to accomplish these functions. When the airplane reaches the altitude at which the difference between the pressure inside and outside the cabin is equal to the highest differential pressure for which the fuselage structure is designed and further increase in airplane altitude will result in a corresponding increase in cabin altitude.

This differential pressure is determined by the structural strength of the cabin and often by the relationship of the cabin size to the probable areas of rupture, such as window areas and doors. The cabin air pressure safety valve is a combination pressure relief, vacuum relief, and dump valve. The pressure relief valve prevents cabin pressure from exceeding a predetermined differential pressure above ambient pressure. The vacuum relief prevents ambient pressure from exceeding cabin pressure by allowing external air to enter the cabin when ambient pressure exceeds cabin pressure.

Several instruments are used in conjunction with the pressurization controller. The cabin differential pressure gauge indicates the difference between inside and outside pressure. This gauge should be monitored to assure that the cabin does not exceed the maximum allowable differential pressure. A cabin altimeter is also provided as a check on the performance of the system. In some cases, these two instruments are combined into one. A third instrument indicates the cabin rate of climb or descent. A cabin rate of climb instrument and a cabin altimeter.. [pic]

Decompression is defined as the inability of the airplane’s pressurization system to maintain its designed pressure differential. This can be caused by a malfunction in the pressurization system or structural damage to the airplane. Decompressions fall into two categories:       1. Explosive Decompression. Explosive decompression is defined as a change in cabin pressure faster than the lungs can decompress. Therefore, it is possible that lung damage may occur. 2. Rapid Decompression. Rapid decompression is defined as a change in cabin pressure where the lungs can decompress faster than the cabin.

Therefore there is no likelihood of lung damage. During decompression there may be noise, and for a split second one may feel dazed. Flights above 3,000 m (10,000 ft) in unpressurized aircraft put crew and passengers at risk from four separate sources, hypoxia, altitude sickness, decompression sickness and barotrauma as follows: Hypoxia. The low 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

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