Thank you r75 a point that has been missed by many. The very essence of our profession is to look to prevent a system failure that will cost lives. We rely heavily on testing,procedures,technology,science and experience. Each of these is important in what we call preventative maintenance. Those in the industry understand the term redundant system but even the maintaining of these systems follow the same principle. We don't just rely on the design, every element is important and that is why we will learn from this event and our experience and awareness will profit.