Dvlcom...
Basically pressurisation is pumping the air press inside the cabin to an equivalent pressure/ altitude.
How...early a/c used engine driven air pumps, nowadays air is tapped off the engine compressors and pumped into the cabin. An outflow valve is used to restrict the exit of the air thereby creating a back pressure. The difference between this press and the ouside air press is referred to as cabin differential press (Diff press).
Each airframe has a maximum press diff that can be achieved.
Most a/c are designed to have a max diff equal to a
cabin alt of around 7-9000 ft at normal cruising altitude.
"Do you change the press during flight"...only if the a/c changes its cruising altitude.
"Does the cabin press change as the a/c ascends"....yes. The cabin is usually climbed/ descended at 400/500 feet per minute (fpm) for pax comfort whereas the a/c may be climbing at up to 3-4000 fpm initially.
During descent much the same happens.
" Pax fainting"....never had any problems over 30 yrs. System designed to avoid that sort of thing.
I always used to think of the system as operating like a piece of string! The lower end was the air pressure in the cabin and the upper end was the air press at the operating altitude of the a/c. That piece of string could only be as long as the max diff of the a/c. As long as the piece of string was slack (below max diff) the cabin rate of climb/descent could be controlled. If the string was tight the cabin rate of change would be the same as the a/c.
Confused?
