Unfortunately, not only was it raining, but the sky was almost totally white, which made for rather boring footage.
Luckily, I'd shot almost everything on a tripod, so I didn't have to deal with any motion of the foreground subject. My sky was blue (with clouds) and the blue just looked completely wrong - particularly as there was rain clearly falling down! But adding the sky using Apple Motion, and changing the sky to black and white produced a very good effect (and only took a few minutes to create the mask shape of the plane!)
You'll notice that the front of the plane has a clear perspex dome. At first I masked around that, but I found a much more realistic solution was to not mask around it; but to make the sky overlay semi-transparent. The dome was still visible, but the sky is visible "through" it. Accelerating the sky slightly to make it more dramatic, and adding a small push-in to the frame during the sequence added some extra impact.