I just returned from vacation in Seaside, FL. There were multiple times when military helicopters and Osprey (the plane, not the bird) flew along the beach and I know they were below 500'. I estimate around 300-350' based on perspective relative to how high I flew my drone and the height of a 20 story condominium complex slightly down from me. They were very low and in at least one case, they had to be right along the water line. I even commented to my wife on one of the fly-bys that it was a good thing I wasn't flying the drone. It definitely made me nervous and more alert to plane/heli rotor noise when I was flying. All of this begs the question, who is at fault and liable when a plane hits a drone flying under 400'?