TP
Without referring to my manuals (and not having flown the B744 for nearly 3 years!):
When the LDG ALT switch is pushed in (in AUTO), it takes the elevation of the destination airfield as programmed in the FMC and uses that known elevation to schedule the pressurisation profile for the cabin.
1. eg Depart LHR elevation 80ft, destination MEX elevation ~7500ft amsl.
the press controller will climb the aircraft to a cabin altitude of 7500ft even if that means the aircraft is below max pressure differential for the airframe. ie - if the aircraft is flying at 35000ft and it could maintain a cabin altitude of 5600ft, it would still schedule an altitude of 7500ft as that is the destination elevation.
2. eg Depart LHR elevation 80ft, destination JFK elevation ~20ft amsl.
the press controller will climb the aircraft to a cabin altitude appropriate to max press diff eg at 35000ft to a cabin altitude of 5600 ft.
So, what the QRH is doing is to say to exit AUTO mode and instruct the pressurisation schedule to set a cabin altitude of 8000-8500ft to keep it safe for pax whilst depriving any fire source of as much oxygen as possible.
Later, before descent, it is set back to AUTO to schedule proper pressurisation prior to landing to eliminate any unwanted pressrisation differential between the aircraft and the landing environment.
Last edited by TopBunk; 27th June 2012 at 19:56.