By now I've checked seven dictionaries and usage manuals, and I'm still not sure I understand "onto" correctly, nor am I sure I understand it incorrectly.
"Onto," I learn here, means "on top of." Okay, we don't hang a picture onto the wall; we hang it on the wall. The picture isn't on top of the wall. So far, so good.
But if "onto" always means "on top of," you can spread frosting onto the top of a cake, but you can't spread frosting onto the sides of a cake, right? This is one application of "onto" where my understanding departs from the "on top of" restriction. It's second nature to me to say you spread frosting onto the sides (or on them).
In the example "I walked on to the tarmac," "on" is an adverb meaning "farther"; "to the tarmac" is a prepositional phrase meaning "as far as the tarmac." You could put a comma between them. "I walked on, to the tarmac."
In sentences like "Tarzan held onto the vine" or "The sailor held onto the rope," I want to ask why "onto" won't do. Should it really be "held on to" because the relation of vertical placement isn't there? You couldn't use a comma: "The sailor held on, to the rope." If "on to" is required, what kind of "to" is this? Not the "as far as" kind; the sentence doesn't mean "The sailor held on as far as the rope." Not the "purpose" kind; it doesn't mean "The sailor held on to accomplish rope" (like "The sailor held on to escape drowning").
That is, I understand "onto" as part of a phrasal verb when used with "hold," "hang," or "seize" – there may be others – when the meaning is "grasp," even when nothing is on top of anything else. Is this use improper?