I'd definitely recommend a crop sensor over full frame for the purpose of getting more reach with whatever lens you use. With Canon you get a 1.6x crop on a lot of different bodies (EOS 80D included) and with Nikon a 1.5x crop. To get the aircraft to fill the frame you'd need a huge lens of 2000mm+ focal length which is probably best achieved with a telescope of some kind which I don't have any experience with. I do have some experience in the research of lenses (Nikon) for doing contrail photography myself which include things like a 300mm + 2x, 400mm + 2x, 500mm + 1.4x or 2x, 600mm + 1.4x or 2x but the bigger lenses can be quite expensive! Also compared to telescopes these telephoto lenses plus the teleconverter won't get the aircraft to fill the frame. If you don't mind not filling the frame completely then maybe consider an old manual focus prime lens with teleconverter. An AF prime would be quite expensive but with a MF lens at these distances of aircraft at cruise altitude having the lens set to the infinity focus point should be sufficient.
Canon have manual focus FD 300mm, 400mm, 500mm, 600mm and 800mm lenses which could be used with a teleconverter.
https://en.wikipedia.org/wiki/Canon_FD_ ... #TelephotoNikon also have MF lenses of 300mm, 400mm, 500mm, 600mm and 800mm which can be used with teleconverters
https://en.wikipedia.org/wiki/Nikon_F-m ... ime_lensesThere are also other brands of lenses but I've not done research on them.
I was considering a Nikon 600mm f/5.6 Ai/Ais lens with a 2x teleconverter for a 1200mm f/11 lens but I didn't go for it because we don't have too much big aircraft traffic passing overhead where I live!
If you like Canon then maybe an FD 500mm f/4.5 L lens plus a 2x teleconverter for a 1000mm f/9 lens?
Like I mentioned earlier there are AF verions but they're more expensive! Less costly AF versions like 300mm f/4 with 2x only get you 600mm.
Here are some examples with an 80-400mm zoom at the 400mm end. Taken with D80/D200 so a 1.5x crop sensor. Images are uncropped right from the camera only resized. It's not really much of a view but imagine with 800mm the aircraft would be twice as big and 1000mm slightly bigger etc. Hope all of this helps a little!