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The pilot puts the cruise altitude to the FMC and the plane auto adjusts the cabin pressure. I know that but I once heard the cabin pressure is set to three thousand feet.
Is it true? Why is the cabin pressure set to 3000 feet? How does plane designers of Boeing/Airbus/Bombardier etc. choose which altitude the cabin pressure will be set to?
How do manufacturers select the cabin pressure?
In principle, the optimal cabin pressure for people on board of an aircraft would be sea level pressure. However, this would create very large pressure differentials between the inside of the aircraft and the outside at high altitudes, which puts a lot of stress on the fuselage. One could build an aircraft that can withstand these pressure differentials, but this would make the aircraft very heavy, something that is not generally desirable when designing an aircraft. It would also require a lot of bleed air from the engines reducing available thrust. That is why the cabin pressure is typically lowered when climbing to cruise altitude.
How much the pressure needs to be lowered depends on the maximum pressure differential allowed between inside and outside. Newer aircraft designs using carbon composite materials like the Boeing 787 Dreamliner or the Airbus A350 can withstand higher pressure differentials and thus allow a higher cabin pressure (lower equivalent cabin altitude) than older aircraft at the same cruise altitude (see also: Do people get less airsick if cabin pressure is at 6000 ft.?).
The cabin altitude must not exceed 8,000 ft at maximum operating altitude on a commercial aircraft. To make sure that the pressure differential does not limit the maximum altitude an aircraft can climb to, the fuselage should be designed strong enough to maintain at least this cabin pressure at the aircraft's service ceiling.
What do pilots select?
The pressurization systems on modern aircraft are largely automatized. The system only needs to know the cruise altitude and the landing elevation. From the cruise altitude it can calculate the target cabin altitude for the climb and cruise segments, which keeps the pressure differential below the maximum allowed value. From the landing elevation it can calculate the outside pressure at the destination airport and then start to approach this pressure during the descent.
On a Boeing 737 these two values have to be selected manually by the pilots on
the overhead panel using the dials below FLT ALT and LAND ALT:
(source: http://www.b737.org.uk/pressurisation.htm)
You can also see a switch to the right (currently set to AUTO), which puts the system into automatic mode (it is also possible to control cabin pressure manually, but that is out of scope for this question).
On most modern aircraft however, the flight management system (FMS) will automatically pass on the two required values to the pressurization system. Assuming the system is in automatic mode, no pilot input is necessary.
As a generic answer, the only setting the crew inputs in most pressurization systems is the destination field elevation. This determines the pressure schedule on descent, and the system will use a cabin altitude descent rate that will put the pressure differential to near zero, usually slightly positive (that is, a hundred feet or so below field elevation) upon landing before the outflow valves are opened fully to vent the cabin to atmosphere.
So if the crew sets the field elevation at 3000 ft, the system will use a pressure differential reduction schedule on descent that gets the pressure hull close to zero differential when at 3000 ft, so there is minimum effect on people when the system goes into dump mode to vent the cabin.
