Forever is a long time. But with proper engineering you have a setup which can be either guaranteed for the designated life time (using loctite or safety pins) or inspection intervals which would ensure tightness. Assuming of course, the bolts were installed correctly in the first place.

Castellated nuts and wire?

Nothing can be “guaranteed” which is why proper procedure for inspection and maintenance exists. This whole controversy isn’t around random acts of god but the corruption of one of aviation’s most sacred and fundamental rules: that procedures are followed and improved, not stretched and bypassed for profit.

It’s unlikely in this situation that the bolts were ever tightened correctly the first time.

There are plenty of ways to ensure bolts stay tight that work, and these plug doors have been in service on other planes for a long time with no issues.

The entire rotor assembly (literally what holds it up) on some helicopters is held on by a single nut, for example. [https://en.m.wikipedia.org/wiki/Jesus_nut]

This doesn’t appear to be a case of the engineers not designing it safely - but it not being installed correctly and no one catching it before it rolled out the door.

> How can one guarantee once tightened bolts remain tight forever?

Safety wiring: https://en.wikipedia.org/wiki/Safety_wire

This is actually required for some bolts per aviation regulations. I safety wired nearly every suspension/chassis bolt on my race car when I built it.