I used 'plane' for short, but another solution included 'airplane'. This is OK (apparently this is correct spelling in the US), but is 'aeroplane' also among the correct solutions? Aeroplane is the correct spelling for British English.
Just checked it with AeroplaneS (because the sentence is in plural), it is accepted.
While there's nothing wrong with localized forms, there's little chance of all of them, from every region of the English-speaking world, being recognized in a beta Duolingo course :). You should submit it as an acceptable answer.