i take your point and understand where you are coming from but the answer was that the plane flies at different heights dependant on various factors so it would need to be a very clever"terrorist" to be able to decide exactly what height he/she could set off a potential device and i think most people who fly have a rough idea of the maximum heights which they fly at so that is no secret either
thanks for putting a different perspective on this topic as it never even occurred to me.