Originally posted by Beckles:
MD-80 - looks like a DC-9, but is noticeably larger
MD-90 - looks like an MD-80, but its engines are noticeably larger
717 - looks like a DC-9 (noticeably smaller), and it's engines are noticeably larger.
I find the easiest way to tell a -80 and a -90 apart is to look at the top of the tail fin. Seeing it from the side, the -80 is curved while the -90 is straight and horizontal. Same thing seeing it from the rear or front, the -80 is curved and the -90 is flat. Once you get the hang of it, you can tell them apart from across the airfield (which is not always possible by just looking at the engine size).
While you are looking at the tail fins, the F-100 is even more curved than the MD-80 so that could give you a clue to recognize the F-100 as well.
I would say the 717 is a miniature version of the -90, not the DC-9.
/Pete
[This message has been edited by L-1011 (edited 01-03-2001).]