When two aircraft are converging at approximately the same altitude, the aircraft that has the other on its right shall give way, except as follows: 
(a) a power-driven heavier-than-air aircraft shall give way to airships, gliders and balloons; 
(b) an airship shall give way to gliders and balloons; 
(c) a glider shall give way to balloons; and 
(d) a power-driven aircraft shall give way to aircraft that are seen to be towing aircraft or other objects.