6.1 Introduction

An airplane has been described as 4 million parts flying in close harmony. Each of these components can generally fail in anyone of several ways. A simple nut may come undone because it was not tightened properly or it was not "locked" correctly or it was over-tightened and its thread was stripped or it was put on the bolt crookedly causing it to cross thread. It could have been the wrong size nut for the bolt or the wrong type of thread, the wrong type of material or the nut and bolt were too small or weak for the task. In each case, when the two parts being held together exert undue strain on the nut, it will not be able to withstand the stress and come off. The time when this happens may be the first time the aircraft takes off or it could happen after several successful flights. The time it lasts may be affected by the number of take-offs (and landings which should be the same number), the actual time in the air (flying hours), the number of times the door is opened and closed, the number of times the engine fairing is opened and closed and, if it survives long enough, the amount of corrosion it may have undergone.

In some cases, these failure modes will be competing, whilst in others they will be mutually exclusive. Unless the designer or maintenance manual...