Control of Cabin Pressure  Pressurization Modes Aircraft cabin pressurization can be controlled via two  different modes of operation. The first is the isobaric mode,  which works to maintain cabin altitude at a single pressure  despite the changing altitude of the aircraft. For example, the  flight crew may select to maintain a cabin altitude of 8,000  feet (10.92 psi). In the isobaric mode, the cabin pressure is  established at the 8,000-foot level and remains at this level,  even as the altitude of the aircraft fluctuates. The second mode of pressurization control is the constant  differential mode, which controls cabin pressure to maintain  a constant pressure difference between the air pressure inside  the cabin and the ambient air pressure, regardless of aircraft  altitude changes. The constant differential mode pressure  differential is lower than the maximum differential pressure  for which the airframe is designed, keeping the integrity of  the pressure vessel intact. When in isobaric mode, the pressurization system maintains  the cabin altitude selected by the crew. This is the condition  for normal operations. But when the aircraft climbs beyond  a certain altitude, maintaining the selected cabin altitude  may result in a differential pressure above that for which the  airframe was designed. In this case, the mode of pressurization  automatically switches from isobaric to constant differential  mode. This occurs before the cabin’s max differential  pressure limit is reached. A constant differential pressure  is then maintained, regardless of the selected cabin altitude.  In addition to the modes of operation described above, the  rate of change of the cabin pressure, also known as the cabin  rate of climb or descent, is also controlled. This can be done  The most common method of pressurizing turbine-powered  aircraft is with an air cycle air conditioning and pressurization  system. Bleed air is used, and through an elaborate system  including heat exchangers, a compressor, and an expansion  turbine, cabin pressurization and the temperature of the  pressurizing air are precisely controlled. This air cycle system  is discussed in greater detail in the air conditioning section  of this chapter.  [Figure 16-50] Figure 16-47.  A centrifugal cabin supercharger.  Figure 16-48.  A jet pump flow multiplier ejects bleed air into a venturi which draws air for pressurization from outside the aircraft. 16-28 Bleed air Jet pump Pressure vessel Outflow valve Turboprop engine Flush air inlet Outside skin