The area which forces the icon to fade out, in the above picture, is pretty large, since the gunner is directly looking at the sun.
If the sun was obscured by a hand or something, he'd be able to spot the Spitfire icon in real life.
So, I'm suggesting to implement icon fade, which is determined in relation to the intensity of sunlight effect to the pilot's point of view.
The reason that the icon fades back in to visible, even when the sunlight is still intense, is my suggestion of gameplay concession - making it possible to spot the plane at the periphery of the screen.
So, it's sort of a mixed system, where the intensity of the sunlight first and foremostly fades all icons to a point, but the when at the periphery of the screen, still visible a bit. If this is too gamey, the icon can just stay invisible all the way, if the sunlight is that intense, as shown above.