I’ve seen this mentioned a few times already: people wondering why certain communities don’t show up in their instance. You need to first search for that community via the magnifying glass icon and type in “!communityname@instance.domain”
Notice the exclamation mark at the beginning. Might take a few seconds. This will teach your instance about the remote community.
This has been changed with the latest version of Lemmy. Try this link: /c/aviation@lemmy.ml and check over at lemmy.ml/instances to make sure your instance is not blocked. (It’s at the very bottom, I just checked for you and it doesn’t look like it).
Edit: your instance is using an older version. Searching should still work. Try a couple of times. Make sure the search page is /search and use no filters