>>Have you added in the Earth's velocity vector around the Sun? How about the velocity of the solar system around the Milky Way galaxy?
No, I didn't add this, though I did consider and is the reason I did add the phrase "Due to lack of time, I didn't have the time to think this through for 100%, though I do expect this to be relevant."
And indeed at first glance those aspects do have to be taken into account, though I -think- this is not relevant because the airplane is not acting in the sun's gravitational field. The earth is, but the airplane isn't (because of its low mass).