"running with scissors" wrote in
message om...

"they" dont. cabin pressurization is normally at 8000ft though crew
may elect to use different cabin pressures under certain circumstances

Actually, it depends on the aircraft. Aircraft are certified to certain
"differential pressures" which is the difference between in the inside cabin
pressure and the outside ambient air pressure. An airframe must be designed
to handle the differential pressure required to maintain acceptable cabin
altitude up to the max flight level the aircraft is certified to.

As an example, the King Air I fly has a normal differential pressure of 6.6
psid. This provides for a cabin altitude of 2,700 up to 20,000 cruise
altitude, an 8,700 foot cabin altitude at 31,000 cruise altitude, and a
10,200 foot cabin altitude at 35,000 cruise altitude.

In contrast, the Lear 60 I fly has a normal psid of 9.5 (9.7 max) which
typically results in a cabin altitudes of around aprox 6,500 up in the
40,000 ish cruise altitudes.