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Assuming cabin differential pressure has reached the required value in normal flight conditions, if flight altitude and air-conditioning system setting are maintained the..
  • A
    outflow valves will move to the fully closed position.
  • B
    pressurisation system ceases to function until leakage reduces the pressure.
  • C
    mass air flow through the cabin is constant.
  • D
    outflow valves will move to the fully open position.

During flight there will be always fresh air coming in the aircraft and once the aircraft is in level flight the pressure in the cabin is maintained at a constant level by the position of the outflow valve which will not be fully closed nor open but somewhere in between.

System control methods
The way the pressure in the aircraft is controlled is by having a constant mass of air entering the cabin through the airconditioning system. This mass of air is kept constant by the mass flow controller. The pressure is regulated by the amount of air which is discharged back to the atmosphere by the outflow valves. So the more open these valves are, the lower the pressure will be in the aircraft. There must be two safety measures in place with these kind of systems to avoid a too high pressure or too low when the outside pressure is higher than that of the cabin.

Safety valve
This is a mechanical pressure relief valve which should open when the cabin exceeds the maximum pressure differential to which the aircraft is designed.

Inward relief valve
This valve works the other way around than the safety valve and will open once the pressure outside of the aircraft is higher so there will be no negative pressure differential.

These two valves may be combined in the same unit or may be two separate units, but there must be at least two of each installed.

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