I was always irrationally nervous flying into SFO, worried about precisely what ended up happening. We should all be glad the plane mostly held together through the cartwheel, saving many lives.
True, it's not part of the certification process, but I do know that modern jets airframes are made to be as... um... "flexible"* (? searching for the word here...) as possible in catastrophic situations. An older airframe probably would have disintegrated under the same forces.
Luck and good fortune also played a huge role of course. What the pilots did was unconscionably neglectful. CFIT is often (usually?) fatal for all passengers.
*EDIT: It's similar to software that's written to be extremely robust that encounters an unexpected error (or set of errors). The software wasn't necessarily designed to handle it, but sometimes it can nonetheless.
That isn't really something engineers can design to. What happens is the aerodynamics group calculates the maximum loads on the airplane. This is increased by 50% and called the "ultimate load", and the parts are all designed to not break up to that load.
Parts stronger than that are overweight, and weight is the enemy of all airplanes.
After parts are designed, they go through an independent "stress" group which verifies that the parts meet the strength requirements.
I think what he is referring to is the modern use of composites and carbon fiber which are both stronger and lighter than older aluminum or titanium components. Titanium, for example, is a very hard metal that will fracture more readily than a softer composite that is able to dissipate the stress better.
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