On Board Meaning

(idiomatic) Agreeing or supporting.

Example:   Even when I am on board the plane, I can never feel secure that my luggage is, too.‎
1915, George A. Birmingham, Gossamer, chapterI:
  There is an hour or two, after the passengers have embarked, which is disquieting and fussy. Mail bags, so I understand, are being put on board. Stewards, carrying cabin trunks, swarm in the corridors. Passengers wander restlessly about or hurry, with futile energy, from place to place.
  Is that new teammate properly on board yet?‎
  It's a good idea, but let's see if we can get a few more of the management team on board.‎