One caveat: Know when passengers must be back on board the plane.
I find this sentence really weird. It's an example sentence explaining the word "caveat". I couldn't get what "the plane" does there.
Know when passengers must be back (why?) to board the plane.
Know when passengers must be back (where?) onto the board of the plane.
Unlike these versions, the original one doesn't make sense to me. Is it a typo or is there any semantic or grammatical point I failed to notice?
I suppose on board is a prepositional phrase used as an adverb here. How can a phrase answering the "where" question can be followed by a noun without any prepositions between them (as in "the board of the plane")