In Toaru Hikuushi e no Koiuta, ep 8: A seagull-crested aquamarine plane piloted by a ridiculously skilled person helps Kal-el. (screenshot)
We later learn that Levamme is coming to help them. (I have not seen the last episodes yet).
Is the pilot of that plane Charles, from The Princess and Pilot?
I think the light novels might shed some light on it, but my Japanese reading skills suck.