Is Charles the pilot of the seagull plane? 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.
I haven't read the Koiuta light novels, but according to this guy on Chiebukuro and the Wikipedia article for Koiuta, there are strong indications that the pilot of the seagull plane is, in fact, Charles KARINO from The Princess and the Pilot. However, this is never made explicit. There is apparently a scene in the fourth volume where he and Kal-el have a conversation, but despite this, his name is never revealed. 

The Koiuta light novels are complete (no further volumes are expected to be published), so this is probably as good an answer as you're going to get.