Hi again Dirty D. ;-)
If the answer to 17d is Airplanes as AJH suggests,then the clue does work in a(pretty poor)way.
(airlines/airplanes - 8 and 9 letters, which is correct?)
Diana and Trev-ignore my last post it is AIRLINES -I think this crossword is the worst one I can remember I put engine as the answer but unless I am mistaken(not impossible) it is a pretty poor link