Hi,

 

a question concerning cabin pressurization:

 

I have both bleeds on, both packs auto, pressurization set to auto. Flight level and dest aerodrome alt are set.

Now, whenever I climb above FL370, the pressure diff will hit the yellow area. Is that normal?

I remember having seen a vid with an NG cruising at FL410 with the oressure diff just about to hit the yellow area.

 

What gives?

If cruise altitude is above 37000 diff pressure sets up at 8.35 psid. That's right near yellow zone.

Interesting. Apparently that's OK in real life since airlines are still striving for high flight levels, e. g. Ryanair typically at FL380, my last Air Berlin flight at FL390.

I would have thought that this is putting too much wear on the cabin.

It is still safe and still in limits.

It is how the system works.

The controller will generate a cabin climb profile depending on what you put on the panel (if you then go out from the profile, the cabin will always try to mantain the differential within limits, regardless of the cabin rate, this can lead in a controller fault (all dashes) in case maximum cabion differential is passed.)

These are the conditions:

If you set less than 18500ft on the panel it will command the pressurization to mantain the landing field elevation for the entire flight (obviously if you fly at 18500 or until you reach max. diff. pressure wich is 8.45 psid. If reached the cabin will start to climb at the same rate of the plane.

If you set between 18500 to less than 28000 the pressure will be scheduled for 7.45 PSID

Between 280 to fl 370 7.8 will be scheduled,

37000 to above 8.35.

The 8.35 is not yet the maximum as the maximum is set to 8.45, and can be reached if climbing more than scheduled altitude.

8.45 is however not the maximum structural limit, the mechanical protection of the positive pressure relief valves will start at 8.95PSID wich is quite higher than the maximum value you can reach if system works.