I reached the castle in 45 days. It was fun, and I think with some tweaks I could spend a lot more time playing. The controls are quite subtle, and the pacing quite fast, so I ended up mostly playing it as a game of luck, i.e. select upgrades, launch and hope for the best. But the basic gameplay appears similar to flying a glider, you're trying to spend as much time as possible in the favorable weather and as little as possible in the unfavorable weather. If that's your intention, I think you're almost there and can get the rest of the way mostly by tweaking the speeds and accelerations.
One nitpick: It takes a while for the plane to slow to a stop and recognize the landing, would it be possible to add more friction when we collide with the ground?
Nice job!