who shot down the plane?

You don't just gun down Apollo Creed and get away with stuff like that. Carl Weather's "Sundog" is easily the most likable character in the whole film, and it loses a bit of it's human element after his character is so needlessly killed. That's a weird scene too, where they chase Bronson to the cliff and he jumps in the tree just before the pilot makes his strafing run. If you look closely at Bronson once he's in the tree they reflected light onto him with some sort of kukolaris type device: It looks unnatural as all hell, but they had to reinforce the image of his being in the tree and wearing the kind of jacket he (or his stuntperson, more likely) was wearing he sort of blends into the branches & snow.

They also cheated a bit with some pretty lame continuity errors: Bronson clearly drops his rifle onto the ground BEFORE jumping into the tree, so it shouldn't have been conveniently lying next to his pack after he shimmies down to the riverside below. And also his snowshoes disappear, then re-appear lying neatly on top of his pack, though they clearly were not attached to the pack when he tossed it over the cliff, and he is not wearing them when he jumps into the tree. But then there they are, right next to the rifle too. Amazing that they messed up so badly during filming and never caught the error before the editing phase.