[WalterBright]

High reliability of airliners is achieved by having redundancy of all critical parts. The idea is no single failure can cause a crash.

For example, if system A has a failure probability of 10%, if A is redundant with another A', the combined failure probability is 1%.

That of course presumes that A and A' are not connected.

[wat10000]

Yes for systems, not always for structure. A failed wing spar means everybody dies. For real-world examples, there were two 747 crashes caused by improper repairs to a rear pressure bulkhead or aircraft skin. When the repairs eventually failed, the explosive decompression caused catastrophic damage to the tail in one instance, and total structural failure resulting in a mid-air breakup in the other.

The response to this was to make sure repairs are carried out correctly so the structure doesn’t fail, not to somehow make two redundant bulkheads or two skins.

[WalterBright]

The wing spar is dual, too.

The idea is to design the airplane to survive an explosive decompression failure, not pretend that explosive decompression doesn't happen. For example, on the DC-10, the floor collapsed from explosive decompression, jamming the control cables and causing a horrendous crash.

The fix was not preventing explosive decompression. The fix (on the 757) was to locate the redundant set of control cables along the ceiling. Also, blowout panels were put in the floor so the floor wouldn't collapse.

It's not always practical to fix an older design like the 747. When it isn't practical, a stepped-up inspection protocol is added.

P.S. The 747 was designed to survive a decompression. The oversight was nobody realized that a failure of the rear bulkhead could destroy the tail section. Things like that happen in complex systems, and an airliner is incredibly complicated.

P.P.S. When I was a newbie at Boeing, I asked about the wing spar, too. That's how I know it is dual!