Ask the Captain: How does cabin pressurization work in commercial jets?

Upon takeoff, is pressurization of commercial passenger aircraft cabins automatic or a switch has to be flipped or activated? Is it fixed at a particular equivalent altitude level?

– Ric Guy, Mt. Pleasant, Michigan

On modern airliners, it is usually an automatic process and the airplane begins to pressurize as soon as it takes off. 

On most modern jets, the pressurization system gets the airplane’s intended cruising altitude from the flight management system. Once the cruise altitude is known, the automatic pressurization system calculates the proper cabin altitude not to exceed the maximum differential pressure. The pressurization system then climbs the cabin to the calculated altitude.

Once descent begins, the pressurization system then descends the cabin to the destination airport elevation. 

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