RAF A400 just went south over here out of Waddington, leaving the most beautiful twin vortices amidst its vapour trail several miles behind - very reminiscent of the Smokewinder trails left at airshows by various Sea Furies. Several complete loops formed then broke up, intersections left sudden gaps, other sinuous shapes formed... amazing. Not sure if it was the vapour trail itself coalescing or additional vapour formed by the interaction of the props on each side - as I recall, inner and outer turn in opposite directions.