Fatigue Inspection Interval for a Repeated Load Path
An engineering team is setting an inspection interval for a critical wing attachment fitting. Fatigue testing shows a crack initiates at 18,000 flight cycles and grows to the critical size at 26,000 cycles. The fleet leader has flown 9,500 cycles. The inspection method has a 90% probability of detecting a 1.2 mm crack, and the crack reaches 1.2 mm at 21,000 cycles. Regulatory guidance requires that the inspection interval be no greater than one-third of the detectable crack growth life, and that the first inspection occur before the detectable crack could reach critical size. The team proposes a 4,000-cycle interval starting at 12,000 cycles. Maintenance capacity allows inspections every 3,000 or 4,000 cycles.
1. Calculate the detectable crack growth life and test whether the proposed 4,000-cycle interval satisfies the one-third rule.
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The detectable crack growth life is the interval between detection at 1.2 mm and critical size. Detection occurs at 21,000 cycles and critical size at 26,000 cycles, so the detectable growth life is 26,000 minus 21,000 = 5,000 cycles.
The one-third rule caps the interval at one-third of that life: 5,000 / 3 = 1,667 cycles. The proposed 4,000-cycle interval is more than double the allowed maximum, so it fails the rule.
Even the 3,000-cycle option fails, because 3,000 exceeds 1,667. The team cannot satisfy the guidance with either offered interval; it must either improve detection capability (which would extend the detectable life and raise the cap) or accept a shorter interval than maintenance capacity currently allows. The calculation, not intuition, drives the answer.
2. Determine the latest permissible first inspection and recommend an interval consistent with the constraints.
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The first inspection must occur before the detectable crack reaches critical size, and practically before it grows past the detectable threshold with margin. Critical size is at 26,000 cycles and detection at 21,000, so the first inspection must be well before 21,000 cycles to leave room for repeated checks within the detectable window.
With a cap of 1,667 cycles, the fleet leader at 9,500 cycles is already past the point where a single inspection is enough. The team should schedule the first inspection immediately and repeat at intervals no greater than 1,667 cycles. Since maintenance capacity offers only 3,000 or 4,000, neither is compliant, so the recommendation is to escalate for a shorter interval or a better inspection method. The key judgement is that the fleet leader's current cycles make delay indefensible.