Very true. I've just checked in the book, p.5. it says:
"Ships may never be stacked on top of one another. A player may never end his movement 'on top' of another ship."
So, no mention of planes then.
With regards to vessels moving on to other vessels, any ship should have adequate turning capability to avoid a collision and even if they do not, no sane captain (i.e. one who wants to keep their command) is going to put their ship into a position where they are going to be rammed, even by a smaller vessel.
What I'm trying to say in a round about way is, do not permit players to move their ships into this situation in the first place (threat of smacked bottom usually works unless they enjoy that sort of thing) or if you have some fundamental objection to this, wiggle the counters around a bit to simulate the invariable panic arising out of the "what the f***, Sh*t! Sh*t! Sh*t!'" effect.
Let's face it, what two consenting adults do within the privacy of their own home/club is up to them. Hang on a minute, what's this sticking from the spine of my rulebook. A bug! Oh no the rules police are coming for me again! :wink: