system (FMS). The controllers process the information and  send electric signals to motors that directly position the  outflow valve(s).  [Figure 16-53] Modern pressurization control is fully automatic once  variable selections are made on the pressurization control  panel if, in fact, there are any to be made. Entering or selecting  a flight plan into the FMS of some aircraft automatically  supplies the pressurization controller with the parameters  needed to establish the pressurization schedule for the entire  flight. No other input is needed from the crew. Pneumatic operation of outflow valves is common. It is  simple, reliable, and eliminates the need to convert air pressure  operating variables into some other form. Diaphragms,  springs, metered orifices, jet pumps, bellows, and poppet  valves are used to sense and manipulate cabin and ambient  air pressures to correctly position the outflow valve without  the use of electricity. Outflow valves that combine the use  of electricity with pneumatic operation have all-pneumatic  standby and manual modes, as shown in  Figure 16-52. The pressure regulating mechanism can also be found as a  separate unit. Many air transport category aircraft have an  outflow valve that operates electrically, using signals sent  from a remotely located cabin air pressure controller that  acts as the pressure regulator. The controller positions the  valve(s) to achieve the settings on the cockpit pressurization  panel selectors according to predetermined pressurization  schedules. Signals are sent to electric motors to move the  valve as needed. On transports, often AC motors are used  with a redundant DC motor for standby or manual operations.  [Figure 16-55]  All pressurization systems contain a manual mode that can  override automatic control. This can be used in flight or on the  ground during maintenance. The operator selects the manual  mode on the pressurization control panel. A separate switch  is used to position the outflow valve open or closed to control  cabin pressure. The switch is visible in  Figure 16-53,  as well  as a small gauge that indicates the position of the valve. Aircraft pressurization systems incorporate various features  to limit human and structural damage should the system  malfunction or become inoperative. A means for preventing  overpressurization is incorporated to ensure the structural  integrity of the aircraft if control of the pressurization system  is lost. A cabin air safety valve is a pressure relief valve set to  open at a predetermined pressure differential. It allows air to  Controlling cabin pressurization is accomplished through  regulating the amount of air that flows out of the cabin. A  cabin outflow valve opens, closes, or modulates to establish  the amount of air pressure maintained in the cabin. Some  outflow valves contain the pressure regulating and the valve  mechanism in a single unit. They operate pneumatically in  response to the settings on the cockpit pressurization panel  that influence the balance between cabin and ambient air  pressure.  [Figure 16-54]  16-31 Figure 16-53.  This pressurization panel from an 800 series Boeing 737 has input selections of flight altitude and landing altitude. 30 0 35 40 5 10 20 25 15 4 0 5 2 1 8 6 3 7 9 10 CABIN ALT D I F F P R E S S S I AUTO FAIL OFF SCHED DESCENT ALTN MANUAL V A L V E 0 0 0 1 4 0 0 8 AUTO MANUAL FLT ALT LAND ALT ALTN AUTO MAN O P E N C L O S E COCKPIT VOICE RECORDER ALT HORN CUTOUT HEADSET 800 OHMS 4 2 3 2 1 0 1 3 .5 .5 CABIN ALT UP DN C A BI N  CL I$_{M}$$_{B}$ Press diff  Limit: Take off & LDG .125 PSI Cabin Air Pressure Safety Valve Operation Cabin Air Pressure Regulator and Outflow Valve