|Quoting readytotaxi (Thread starter):|
From the ground to our cruising height and then the decent are the crew adjusting the cabin pressure as we go or is this computer generated? How does it work pls.
A modern automated system takes three inputs...takeoff field altitude, landing field altitude, and cruise altitude. These are used to design the pressurization schedule. The nice systems pull all this information from the flight management computer, so there's no extra inputs for the flight crew (which means "nothing" is happen in the cockpit vis a vis pressurization during the flight).
The system will start out at the ambient altitude of the takeoff airport (outflow valves full open). Some time during the takeoff roll the system will pressurize slightly to reduce pressure transients as the ECS system fires up during takeoff. From there, it will increase cabin altitude at some fraction of the climb rate so that you end up at the target cabin altitude when you hit cruising altitude. Depending on the nuts and bolts of the pressurization controller, this may stay constant for cruise or may change slightly with step climbs. The process is reversed on descent, ramping from the cruise cabin altitude to the landing field altitude.
In the infrequent case that the landing altitude is actually higher than the normal cabin altitude (i.e. airports above about 8000') the cabin actually *climbs* during the descent to match up to the landing altitude. Extra steps are necessary if the landing airport is above the altitude that triggers cabin warnings or oxygen mask deployment.