Six months after a UPS cargo jet fell out of the sky above Louisville and killed 15 people, federal investigators gathered in Washington this week to start answering the hardest question: how does something like this happen?

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The National Transportation Safety Board wrapped up two days of public hearings on Wednesday, taking testimony from witnesses at UPS, Boeing, and the FAA. What came out of those hearings paints a troubling picture- not just of what went wrong on November 4th, but of a system that had warning signs for years and didn't act on them.

Here's what you need to know.

The plane that crashed wasn't supposed to be the plane

This detail got somewhat buried in the flood of technical testimony, but it matters. The crew of UPS Flight 2976 didn't originally board the MD-11 that went down. They switched planes shortly before takeoff because their original aircraft had a fuel leak.

That last-minute swap cleared by maintenance and operations put them on a plane heading to Honolulu with a full tank of fuel- and, as we now know, a critical structural flaw that nobody caught.