Hello!
I was thinking the same whilst at this years Download Rock Festival at Castle Donnington. The position of the main stage was directly beneath the flight path, and when Metallica were playing they used a lot of pyro's.
Coudn't help but think, what would have happened if a plane was going over the stage as they started the intro to "One" (Fans of Metallica would know what I'm on about!!)
Best Wishes
Adam