Is it grammatically correct for a pilot or airline cabin crew to say "welcome on board", rather than "welcome aboard?" Is there a difference?
On board describes that something is aboard a vessel, i.e., the location of something or someone.
Onboard is one word (sometimes hyphenated—on-board) when it comes before the noun it modifies (e.g., onboard radio, onboard computer). Elsewhere, writers usually make on board two words. For instance, one might write, “We brought a radio on board so we could have an onboard radio.” Reference: "Onboard vs. on board", from grammarist.com
It's rather an idiomatic phrase (or a quasi-adverb).
- There are no medical physicians on board.
- Smoking is not allowed on board.
Aboard modifies an action and is often followed by reference of the vessel, e.g. to step aboard a something.
Grammarist supplies several examples (same reference as above):
Justin boasts an onboard 3-D camera system for analyzing points in space. [Wired News]
Although the airline grabs headlines for threatening to charge people to use onboard toilets or save money by dumping co-pilots, it normally turns to conventional ruses. [Guardian]
At one highway fill-up, the onboard computer showed I had a range of 880 km. [National Post]
President John F. Kennedy called Shepard after he was taken on board the aircraft carrier that retrieved him from the ocean. [USA Today]
San Pietro was being sailed by the remaining crew on board. [Stuff.co.nz]
We are on board the plane because we have boarded the aircraft.
Aboard the ship, we watched the waves.
Perhaps a bit silly, but:
As you are now on board, welcome aboard the our vessel.
As you are now aboard (the vessel) let me say welcome on board!
I don't see any grammitical difference, rather it seems to be usage as Fumble (hi ya Fumble :) says. (And there's Amid ship - center of ship.)
It would be even more silly, but grammatically correct, to say:
As you are now aboard, let me say welcome aboard.
I wonder if the word 'board' derives from the planks / boards used from dock to ship to enter the vessel?
"Arrr matey, Howard, walk the plank off board!"