Originally Posted by
Wheelnut69
You'll have to forgive my ignorance, but I always thought that one of the reasons aircraft were pressurised to 7-8000 feet was weight.
Air at sea level is 14.7psi, what is it a 8000ft, around 11psi? Does that not make a big difference to the weight of the aircraft and therefore fuel use?
You're exactly correct - it's all about the weight of the
aircraft. Every pressurized aircraft has a maximum pressure differential (between outside pressure and cabin pressure) that reflects the maximum load the pressure vessel can withstand. For instance, in a 747-400 it's 9.4 psi, which is enough pressure to fly at its max certified altitude of 45,100' with an 8000' cabin altitude (the highest cabin altitude allowed by law on commercial aircraft).
Could they pump it down to sea level? Sure, all they'd have to do is add a bunch of structural weight and either lose around 75 seats or 3 hours worth of fuel. Not gonna happen. Most people have little or no discomfort with an 8000' cabin, and it'll rarely be that high in flight, as they've got to burn off most of their fuel before they can get to their max altitude anyway. The pressurization system is designed to keep the cabin at the lowest possible cabin altitude (and thus, the max allowable pressure differential) automatically.