Well, I haven't had any luck with those shots so just as an observer, i would think 200mm would be enough to see a contrail. i can see them with my bare eyes. But if you have any hopes of seeing any detail on the plane, like a winglet, or logo, you'd better have a 500mm lens or more. I shot one contrail once, at the full 300mm, and all i could see was that it was a fairly long fuselage twin-engine with a "light colored" bottom...like a 757.
Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens