Because the pressure at 42k is about 15% that at sea level, it’s only about 15% more stress on the plane to remain pressurized in a vacuum. Which is nothing compared to safety margins. (Yes, I know that it’s not 1 ATM inside at 24k feet, but the principle still applies). They're probably using a factor of safety of 1.1 on the pressure shell. That would be typical for the aerospace industry with load conditions well established. But your point is well taken that if the aircraft could maintain lift, the pressure shell will likely hold.
They’re probably using a factor of safety of 1.1 on the pressure shell. That would be typical for the aerospace industry with load conditions well established. But your point is well taken that if the aircraft could maintain lift, the pressure shell will likely hold.
Also keep in mind that the pressure differential need not even change as the aircraft goes from 49k to 50k, ambient drops from 19% of sea level to 11%. The pressure inside can drop by 8% (of 1 ATM) which means that the cabin pressure drops from an effective 10kft to 13kft (no big deal, with zero effect on the pressure stress on the AC.