Complex sentence: They are successful in shooting down a plane in their next skirmish, but Collins's plane is damaged and he is forced to ditch in the Channel, with Farrier assuming Collins is fine after seeing his waving hands sticking out of the plane.
Split into simpler sentences: They are successful in shooting down a plane in their next skirmish, but Collins's plane is damaged. After Collins' plane comes to a halt on the surface, Farrier assumes Collins is fine after seeing his waving hands sticking out of the plane.