# Question about aircraft cabin pressure.

Why is the cabin pressure in aircrafts flying at high altitudes maintained at a level equal to atmospheric pressure at 8,000 feet? Since they already use a system to keep the cabin pressure higher than outside, why this figure of **8,000 ** feet? Why don’t they pressurize it to the regular atmospheric pressure that exists outside the aircraft when it takes off, so that the passenger does not feel any difference at all?

WAG: Because 8000 is good enough and doesn’t cost as much as sea level?

Pressurizing to sea level increases the stress on the fuselage, which would cause it to fail or wear-out more quickly.

It’s set at 8,000 feet to reduce the stresses on the airframe while still maintaining a comfort level for passengers and crew. If they kept it at sea level pressure, they’d have to design the airframe to handle the additional stress–remember that every pound of pressure means 144 pounds of force acting on every square foot of surface area. In a large aircraft this can mean hundreds of thousands of additional pounds of force in total.

This wiki article deals with pressurization and notes that:

This implies that few to no current airliners can do much better than 8000’ - or to be more precise, can maintain a maximum pressure difference no greater than that between 8000’ and cruise altitude (around 40000’);

This goes along with what I read from an airline pilot in another thread (debunking explosive decompression, I think it was). The pilot said that airliners leak like a sieve… Which supports the “8000 is as good as they can do” theory.

J.

If you walk round an aircraft on the ground when it’s in the process of doing a pressurisation check i.e. the cabin is being pressurised, then you will find most of the doors and hatches will have at least a small leakage of cabin air.

In the old days when smoking was more generally allowed on aircraft you could easily spot where the leaks were by the nicotine stains on the aircraft skin.

Cabin pressure is usually maintained by how fast you allow the air to escape via the outflow valves. I’m pretty sure they could do better than 8000 ft. but as stated above it causes more strain on the airframe.

Another reason is that there are several airports located at fairly high altitude. The bleed air from the engines is a pretty high flow source, and needs to be to keep up with the leaks. Pressurizing an aircraft to 30"Hg on the apron in Denver or Albuquerque could be fairly painful for some or the occupants.

Standardizing on an altitude above the expected operating airports allows a single common procedure to work in all circumstances.

FWIW, Boeing is designing the 787 for a cabin altitude of 6000’, in the interest of passenger comfort (and market preference). But 8000’ does the job with minimal expense.