Not to assume, but looking at the textures of the ground in this stage, it seems that the plane is taking off from a lower resolution area... that is, crisp in the games graphics, but the pixels are visible is all, even from this screenshot. Perhaps the plane starts out on the 16 bit version of the Pilot Wings environment and... well... it may just be pointing out the obvious, but... goes off somewhere else. Now, this isn't an assumption by any means, but... what if attacking the plane were to cause it to lose altitude or fuel, and thus, crash...?
Uhm... that would, in this end's opinion, certainly make for some kind of stage hazard... BUT that's just what ifs