Every 5 seconds? That seems excessive. But you could do it if you wanted. You could also update a timestamp everytime they refresh the page, assuming that someone who has been away for 5 or 10 minutes is gone offline, and remove their fighter then.
Here's a thought, you mentioned fighters are available when the user is logged in? For other users to fly or shoot down, or just a list of active players?
You could add to the drama of the game, by not removing the fighter from the available list unless the user logs out (you could call it docking/landing etc...). Their fighter would then be up for grabs to other players (to pilot or shoot), and it would make the game a little bit more realistic.
If you are trying to keep track of active players, then I would say just count the active sessions. It's only for viewable statistics and not too important.
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