When are the two used?Do they mean the same? Flight hostesses and pilots use either, as i have noticed. Is there any difference? And if there is, what is the difference? " Welcome on board!" or " Welcome aboard!"???
But the adverb aboard (without reference to the vehicle boarded) has a sense of direction - you welcome someone aboard, but you don't 'have drinks aboard'* (although you do have drinks 'aboard the plane'). Similarly, 'Is there a doctor on board?' or 'Was there a doctor aboard the plane?'
In brochures, 'on board' (usually an adverbial phrase) can be used as an adjectival phrase as well: 'There is an extensive on board bar.'
* At least, it sounds odd to me.