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")

  • 1
    Your second sentence is wrong. "on board of the plane" isn't standard English.
    – pboss3010
    Apr 30, 2019 at 16:17
  • @pboss3010 yes, I understand it. I was trying to make sense out of it and suggested alternatives which might explain why I am struggling with the original sentence. Would "to/onto the board of the plane" work? Apr 30, 2019 at 16:28
  • 2
    the phrase is "on board the [vehicle]", I don't know where the confusion is coming from.
    – pboss3010
    Apr 30, 2019 at 16:55
  • 1
    It's not the board of the plane. It's simply on board the plane. You need to stop adding an article, breaking up the expression, and treating board is a noun. On board is a synonym for aboard. May 1, 2019 at 6:00

1 Answer 1


It is perfectly grammatical. Compare:

While on board the plane, please don't use your cell phone.

I assume you'd be happier with this sentence, right?

One caveat: Know when passengers must be back on the plane.

In fact, back on board the plane is just like back on the plane, a prepositional phrase. The syntax tree would look something like this (I will wait for BillJ to critique my diagram):

enter image description here

The prepositional phrase (PP) on board is part of a longer PP back on board the plane and functions similarly to preposition on. The whole PP back on board the plane in turn functions as a nominal (predicative).

You seem to be having issues with the lack of a preposition between "board" and "the plane". "On board" is a set phrase, so it is no longer "the board of the plane".

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