Chairs are ok.. but you just need to remember to remove them from the flight line when done. A couple of years ago, one of our instructors took a chair to the line to relax while a new member worked over the buddy box. They finished up and went back to the pits. Bit later, one of the members was on the line flying, and I was watching his plane as he made a low pass over the field. Suddenly the plane made some erratic up and down manuvers. On the last oscillation the plane went down behind some small trees and scrub brush, that wasn't any taller that 6-7 ft. For an instant all seemed doomed. Then just as suddenly it popped up from behind the brush and went vertical. Best save I've ever seen. Turns out, as the pilot was flying and watching his plane, he was slowly walking down the flight line. Yeah.. he didn't see it and tripped over the chair!