How exactly does the cabin pressure of an airliner change during ascent and descent?

Question

At cruise altitude, airliners' cabin altitudes are usually at 6000-8000 ft, but how and when does it get there respectively how does it behave from ground until cruise altitude?

Is the cabin unpressurized until the plane is at 8000 ft and from there on it maintains the 8000-ft-cabin altitude while further ascending? Or does the cabin altitude change all the time just much more slowly than the actual altitude so that the 8000-ft-cabin altitude is reached only at the plane's cruise altitude?

It's a long time I flew on an airliner the last time so I don't remember properly when my ears started/ceased to feel it but I think they were registering changing pressure when the airplane was above 8000' as well.

Answer 1

8,000’ is the maximum cabin pressure altitude allowed, and will usually only happen if the aircraft is at it’s maximum altitude, and the maximum pressure differential for that aircraft. Airliners don’t often cruise at their maximum certified altitude. Many long haul airliners are unable to climb higher than about 29,000’ feet for the first few hours until sufficient fuel is burned off. In those cases the cabin pressure altitude will start off much lower, and then slowly climb each time the aircraft climbs.

In general, most airliners at typical cruising altitudes will have a cabin pressure altitude of about 4,000’ to 6,000’.

There are many different design philosophies regarding aircraft cabin pressurization systems. Some are fully manual, some are fully automated, and lots in between.

In all cases the cabin pressure altitude is designed to be slowly raised as the aircraft climbs, and to slowly descend as the aircraft descends.

As an example, an aircraft may take 10 minutes to climb from sea level to 30,000 ft at 3,000’ per minute, while the cabin climbs to 4,000’ in 10 minutes at 400’ per minute.

If there is a system fault, or pilot operating error, the cabin pressure altitude can sometimes climb or descend at the same rate as the aircraft altitude.

Answer 2

Cabin pressure is typically regulated from the moment power is set for takeoff (on the runway), where cabin altitude goes slightly below the runway elevation. As the aircraft climbs, the cabin altitude is climbed at a proportional rate respecting passenger comfort and aircraft design limits.

On the older aircraft types, a pilot or flight engineer would set the planned cruise altitude during the preflight setup. When the plane gets to that altitude, pressurization would go into cruise mode, keeping the altitude steady even if the aircraft makes slight moves above and below the associated flight (pressure) altitude. On newer aircraft, the pressurization system would get the planned cruise altitude from the flight management computer.

Before descent, on older planes, the landing airport elevation would need to be set. As you may expect, modern planes would get landing field elevation from the FMC.

The pressurization system would recognize movement away from the cruising level, and if the movement is a descent, would switch into a mode proportionally reducing the cabin altitude as the aircraft makes its way down to land. Ideally, the cabin altitude at touchdown would be slightly below the runway elevation, then gradually equalize during the landing rollout.

This is a simplified version of airliner pressurization. Things can become more complicated, as when the flight never arrives at the planned cruise altitude or diverts to a field with a different elevation from the planned destination.

(grin) I once flew a turboprop with overpressure protection based on spring loaded outflow valves. If you climbed above the altitude corresponding to the max internal / external differential, cabin pressure would crudely push open those spring loaded valves and the result was wild "pressure bumps." Instead of a nice 500 to 1000 fpm change in cabin alt, it would be a sudden and uncomfortable 1500 fpm or more.