I seem to recall seeing in tutorials that the cabin altitude is set to the final cruising altitude on the pressurisation panel. If this is the case then I don't understand why this is required. Surely the cabin altitude is slowly increased/ decreased to a maximum of around 8000 feet (to avoid passengers passing out). Why would the system need to know the maximum cruising altitude in that case?What am I missing?ThanksGraeme

I seem to recall seeing in tutorials that the cabin altitude is set to the final cruising altitude on the pressurisation panel. If this is the case then I don't understand why this is required. Surely the cabin altitude is slowly increased/ decreased to a maximum of around 8000 feet (to avoid passengers passing out). Why would the system need to know the maximum cruising altitude in that case?What am I missing?ThanksGraeme

The final cabin altitude is proportional to the cruising altitude. I.e. the higher the cruising level the higher the cabin altitude. The pressurisation system will keep the cabin altitude at 0 feet at all flight altitudes below 18,000 feet. Flight altitudes greater than 18,000 feet set in the flight altitude window will cause the cabin to commence climbing at takeoff, the rate of climb being proportional to the figure set in the flight altitude window. The pressurisation controller needs to know the cruising altitude in order to set the correct rate of climb for the cabin during the climb in order to reach the correct cabin altitude as the cruising level is reached. This is in order to achieve/maintain the correct differential pressure between the inside of the aircraft & the outside. Cheers, Doug

The cruise cabin altitude is set to maintain a particular cabin differential pressure, programmed as described in the FCOM, Vol 2, P2.40.6. It's something of an urban myth that airliner cabins are pressurised to an altitude of 8,000 ft. It might be quite a lot less than that. Many modern airliners use FMS flight plan data to determine cruise and landing elevation, but the 737 retains a manual input for both. Kevin Hall

Makes sense now. ThanksGraeme