Depends what you mean by a fight. Two idiots who've come to blows about something? No chance, unless maybe they look like they're just going to go on an indiscriminate rampage.

Someone being attacked who is obviously at a high risk of being seriously injured or killed, e.g. a child, an old person, someone who is much smaller than their attacker, etc.? Yes, I would probably try to intervene even if the best I can do is just to get in the way and take the beating for them. I think trying to help in that case is the right thing to do, although I don't think trying to avoid getting yourself hurt is wrong.

To be honest, if for one of those reasons I did feel absolutely compelled to get involved, I'd be looking around to see who else might be thinking about jumping in to see if I can give them the nod.

The other thing is that if you intervene, even if you're legally in the right (definitely far from being a given even in the best of circumstances), there's a pretty good chance you're getting taken off that plane in handcuffs along with whoever else was involved, so probably say goodbye to your trip and possibly any future trips too.