Fatigue failure of an aircraft wings due to fitting error in hi-locks

Two hi-locks were fractured during a flight of a carrier aircraft, causing the leakage of fuel in the right wing section. The failed and a new hi-lock were subjected to failure investigation. al of the hi-locks was AMS-6270. The hardness of both hi-locks was also comparable to each other. The investigations entailed that the failed hi-locks were loosened or improperly preloaded. Thence the fastened sections exerted bending fatigue loads on the hi-locks. Therefore, two propagating beach mark fields were observed on the fracture surface, which were advancing towards each other under high cycle fatigue (HCF). When the crack reached at the threshold level, the hi-lock(s) failed catastrophically.