Part way through this question I got a bit side-tracked. I'll try and express my problem better here.
I think that the conclusion to the previous question was that 飛行機が混みそうです means "It seems the plane will be crowded". Here 'seems' implies that after examining the evidence (e.g. of previous flights) there is a reasonable probability that the plane will be crowded.
How would I translate "The plane will seem crowded"? In this case 'seem' is expressing visual perception while on the plane. e.g. it will seem crowded because all the passengers that have checked in are very fat and will take up a lot of room.
Would that also be 飛行機が混みそうです? Do I just have to infer the meaning from context or is there a different grammatical construction?