Aircraft Systems

Figure 7-40. High performance airplane pressurization system. 7-35 EDOC a device called an outflow valve. By regulating the air exit, Atmosphere pressure the outflow valve allows for a constant inflow of air to the Altitude (ft) Pressure (psi) pressurized area. [Figure 7-40] Sea level 14.7 2,000 13.7 A cabin pressurization system typically maintains a cabin 4,000 12.7 pressure altitude of approximately 8,000 feet at the maximum The altitude at which the 6,000 11.8 standard air pressure is designed cruising altitude of an aircraft. This prevents rapid 8,000 10.9 equal to 10.9 psi can be changes of cabin altitude that may be uncomfortable or cause found at 8,000 feet. 10,000 10.1 injury to passengers and crew. In addition, the pressurization 12,000 9.4 system permits a reasonably fast exchange of air from At an altitude of 28,000 14,000 8.6 the inside to the outside of the cabin. This is necessary to feet, standard atmo- 16,000 8.0 spheric pressure is 4.8 18,000 7.3 eliminate odors and to remove stale air. [Figure 7-41] psi. By adding this 20,000 6.8 pressure to the cabin 22,000 6.2 Pressurization of the aircraft cabin is necessary in order to pressure differential of 24,000 5.7 6.1 psi difference (psid), protect occupants against hypoxia. Within a pressurized 26,000 5.2 a total air pressure of cabin, occupants can be transported comfortably and safely 10.9 psi is obtained. 28,000 4.8 for long periods of time, particularly if the cabin altitude 30,000 4.4 is maintained at 8,000 feet or below, where the use of Figure 7-41. Standard atmospheric pressure chart. oxygen equipment is not required. The flight crew in this type of aircraft must be aware of the danger of accidental The following terms will aid in understanding the operating loss of cabin pressure and be prepared to deal with such an principles of pressurization and air conditioning systems: emergency whenever it occurs. • Aircraft altitude—the actual height above sea level at which the aircraft is flying • Ambient temperature—the temperature in the area Cabin heat Air scoops valve immediately surrounding the aircraft Heat exchanger • Ambient pressure—the pressure in the area Heat shroud immediately surrounding the aircraft Turbocharger • Cabin altitude—cabin pressure in terms of equivalent compressor Forward section altitude above sea level air outlets • Differential pressure—the difference in pressure Flow control between the pressure acting on one side of a wall venturi and the pressure acting on the other side of the Floor level outlets wall. In aircraft air-conditioning and pressurizing To cabin altitude systems, it is the difference between cabin pressure controller and atmospheric pressure. 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 Safety/dump valve Outflow valve 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. Ambient air Compressor discharge air The cabin pressure regulator controls cabin pressure to a Pressurization air selected value in the isobaric range and limits cabin pressure Pre-heated ambient air to a preset differential value in the differential range. When an Conditioned pressurization air aircraft reaches the altitude at which the difference between Pressurized cabin 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