a device called an outflow valve. By regulating the air exit,  the outflow valve allows for a constant inflow of air to the  pressurized area.  [Figure 7-40]  A cabin pressurization system typically maintains a cabin  pressure altitude of approximately 8,000 feet at the maximum  designed cruising altitude of an aircraft. This prevents rapid  changes of cabin altitude that may be uncomfortable or cause  injury to passengers and crew. In addition, the pressurization  system permits a reasonably fast exchange of air from  the inside to the outside of the cabin. This is necessary to  eliminate odors and to remove stale air.  [Figure 7-41]  Pressurization of the aircraft cabin is necessary in order to  protect occupants against hypoxia. Within a pressurized  cabin, occupants 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. The flight crew in this  type of aircraft must be aware of the danger of accidental  loss of cabin pressure and be prepared to deal with such an  emergency whenever it occurs.  The cabin pressure control system provides cabin pressure  regulation, pressure relief, vacuum relief, and the means  for selecting the desired cabin altitude in the isobaric and  differential range. 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.  The cabin pressure regulator controls cabin pressure to a  selected value in the isobaric range and limits cabin pressure  to a preset differential value in the differential range. When an  aircraft 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, a further increase in aircraft altitude will result  The following terms will aid in understanding the operating  principles of pressurization and air conditioning systems:  Figure 7-41.  Standard atmospheric pressure chart.  Figure 7-40.  High performance airplane pressurization system.  7-35  • Aircraft altitude—the actual height above sea level at  which the aircraft is flying  • Ambient temperature—the temperature in the area  immediately surrounding the aircraft  • Ambient pressure—the pressure in the area  immediately surrounding the aircraft  • Cabin altitude—cabin pressure in terms of equivalent  altitude above sea level  • 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.  Altitude (ft)  Pressure (psi)  Atmosphere pressure  Sea level  2,000  4,000  6,000  8,000  10,000  12,000  14,000  16,000  18,000  20,000  22,000  24,000  26,000  28,000  30,000  14.7  13.7  12.7  11.8  10.9  10.1  9.4  8.6  8.0  7.3  6.8  6.2  5.7  5.2  4.8  4.4  At an altitude of 28,000  feet, standard atmo­ spheric pressure is 4.8  psi. By adding this pressure to the cabin  pressure differential of  6.1 psi difference (psid),  a total air pressure of  10.9 psi is obtained.  The altitude at which the  standard air pressure is  equal to 10.9 psi can be  found at 8,000 feet.  CODE  Ambient air  Compressor discharge air  Pressurization air  Pre-heated ambient air  Conditioned pressurization air  Pressurized cabin  Air scoops  Turbocharger  compressor  section  Heat shroud  Heat exchanger  Forward  air outlets  Cabin heat  valve  Floor level outlets  Safety/dump valve  Flow control  venturi  To cabin altitude  controller  Outflow valve 