write:However errors do occur and fatigue cracks do grow unnoticed to critical size as their fatigue life is used up and fatigue crack-induced accidents do occur.E. InSPECtIon AS A Cont

000 cycles
N2 = 100
000 cycles
and the percentage of the fatigue life consumed during cycles to 60% strength (stress level two) is: (12)Likewise
n3 = 350
000 cycles
N3 = 1
0
000 cycles
and the percentage of the fatigue life consumed during cycles to 40% strength (stress level three) is: (13)And the percentage of the total fatigue life consumed during the 382
500 stress cycles would be: (14)The percentage of the components fatigue life remaining would be: (15)This 10 percent could be used up in several different ways. 250 cycles at 80% strength
or 3
000 cycles at 60% strength
or 35
000 cycles at 40% strength would do it. Or lots of other different combinations.Finally
it should be remembered an aircraft should not be expected to fall apart when its fatigue life is used up. Because of the large scatter in the fatigue life of components
conservative estimates are normally used. In ad-dition
if adequate inspections have identified critical fatigue cracks and have been tracking their growth
the decision to replace cracked components before they reach critical size or retire the aircraft should be an economic decision. However
errors do occur and fatigue cracks do grow unnoticed to critical size as their fatigue life is used up and fatigue crack-induced accidents do occur.E. InSPECtIon AS A ContRoL AGAInSt FAtIGuE FAILuRES. One of the baseline defenses against catastrophic fatigue failures is periodic in-spections which detect fatigue cracks and allow their repair before the cracks be-come a safety issue. Several questions need to be answered in order to ensure that fatigue cracks are detected before they approach a critical length. Where do we look
when do we start to look
how often do we look
how do we look
and has anything changed since we answered the first four questions? If a fatigue crack-induced structural failure is involved in an accident causal sequence
then one or more of the above questions was not answered correctly. It’s the investigator’s job to find out which of the five questions were blown and why.Where do we look? During a modern aircraft’s design
the loads on individual structural components are analyzed and tested to ensure that they have suf-ficient fatigue life to withstand the planned mission load cycles. Potentially critical components are analytically identified and watched during subsystem and full-scale fatigue tests. During structural qualification tests
one air-frame is exposed to the number and amplitude cyclic load cycles anticipated during the aircraft’s life cycle. During frequent inspections
critical components are checked for the presence of fatigue cracks and their rate of growth. When unanticipated cracks are occasionally discovered
they should be critically evaluated and countermeasures developed. After the aircraft’s safe fatigue life has been demonstrated
“lead-the-fleet” aircraft are often used to accumulate “real” load cycles while undergoing close surveillance. But
humans are human and humans make mistakes which occasionally result in accidents. Then it is the investigator’s job to find out if we were looking in wrong place and if so why. Figure 35-61 may help.•Figure 35-61. Maximum Inspection Interval as a Function of Cycles Required to Grow to Critical Length.