Someone you know is making a scene on a plane. They don’t see you. Yet. As of now they think they are making a scene only in front of total strangers who they will never see again. It might be awkward if they knew you were a witness. Should you avert your eyes in hopes they won’t see you seeing them?
If they are really making a scene it is highly unlikely that you didn’t notice. So if eventually he does see you and sees that you are looking the other way he is still going to know that you saw him. So in fact it’s not really possible to pretend.
Moreover if he sees that you were trying to pretend then he will infer that you think that he was behaving inappropriately and that is why you averted your eyes. Given that he’s going to know you saw him you’d rather him think that you think that he was in fact in the right. Then there will be no awkwardness afterward.
However, there is the flip side to consider. If you do make eye contact there will be higher order knowledge that you saw him. How he feels about that depends on whether he thinks his behavior is inappropriate. If he does then he’s going to assume you do too. Once you realize you can’t avoid leaving the impression that you knew he was behaving inappropriately, and the unavoidable mutual knowledge of that fact, the best you can do is avoid the higher-order knowledge by looking the other way.
So it all boils down to a simple rule of thumb: If you think that he knows he is behaving inappropriately then you should look away. You are going to create discomfort either way, but less if you minimize the higher-orders of knowledge. But if you think that he thinks that in fact he has good reason to be making a scene then, even if you know better and see that he is actually way out of line, you must make eye contact to avoid him inferring that you are being judgmental.
Unless you can’t fake it. But whatever you do, don’t blog about it.