[stomachmonkey]

Originally Posted by IROC

The internal pressure of the aircraft, I would presume. At 35,000 feet, the outside air pressure is something like 3.5 psi. If they pressurize the inside of the aircraft to something comfortable for humans (12 psi or so I would guess), you can see that the differential pressure works out to around 9 lbs of force per square inch on the inside of the fuselage. There's lots of square inches on the inside of a plane that large, so the forces are incredible.

Edit: 9 psi is 1296 lbs per sq foot, so section of the plane that large had literally tons of force acting on it.

Wow.