When you fly around under 400 feet, are you at all concern with colliding with these low flying single prop engine plane (like Cessna)? Yesterday while at a friend's place I saw 2 single engine plane flew overhead of me. They were flying in "formation" and doing some sort of aerial maneuvers of some sort. One in front, one in the back, the gap between the two plane were less then the length of the plane. From the ground, the gap looked to be about 1/5 of the plane length. Anyway, the size of plane looks huge and I can clearly see the tail numbers so I am guessing they are not flying all that high. I believe these planes were flying out of a small airport about 3 miles away (as the crow flies) with one 4000 ft asphalt runway. Are these planes allow to be flying that low? Are there any safe height to fly around in that area? On a broader note, I often see news helicopter and other helicopter flying overhead and they don't look at all flying that high. Are there any concerns with colliding with these helicopter even at height below 400 ft?