Study of INspection Intervals for Fail-Safe Structures

Based on a method of evaluating the reliability of fail-safe structures, developed earlier at FFA, a theoretical investigation, applicable to an aircraft wing, has been made of the influence of the length of inspection intervals which are both constant and varying. An optimum study shows that the least number of regular inspections during the service life is obtained by making especially the first interval considerably longer than the following ones. This result presupposes that all fatigue cracks occurring may be statistically anticipated. Unscheduled inspections, randomly distributed in time, decrease the risk of total failure but are shown to be much less effective than regular inspections. This is also demonstrated in a numerical example assuming cyclic inspection on a fleet of 100 aircraft with a sampling ratio of 0.25 for the regular inspections and further extra random inspections. It is concluded that unscheduled inspections should not be carried out, unless a completely unexpected crack has been found, which will mainly happen during the first part of the service life. (Author)