If I were you, I'd hold out for the Canon 70-200 f 2.8 lense. They now have on that has IS (image stabilization) which would be great for airline shots. I have Canon gear, 1 EOS 1VHS, 2 630's and only use Canon lenses. Others on this board will tell you that after-market lenses such as Sigma, Tamron, Tokina, and Quantaray are good, but in my personal opinion, don't listen to them. Only buy Canon lenses!! They are made exclusively for Canon cameras. Others may flame me on my opinion, but oh well, I've got thick skin. I had a Tamron 200-400mm and hated it! Sold it within a week. Your camera body (The Rebel) is only a light box, spend your money on lenses !!
Yes, I have a top of the line and very expensive EOS 1VHS, but it still is only a light box.......my Canon 630's take just as quality photos as the 1VHS, but the 1VHS has more frames per second, more focusing points, etc....
Good luck !