Bill said

Yep, from time to time the vibration around those inspection/access covers will cause the pin in the hinges to wear down and break the end where it is secured in the hinge. Before long the pin vibrates out and the cover is free as a bird. By the time the wear is that bad, usually it's time to drill out the attaching rivets and replace the hinge to fix it properly. Some mechanic has a nice little fabrication project to day replacing that access cover.

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