3D fatigue crack growth (FCG) performance under variable-amplitude loading was experimentally studied for the attachment lug of undercarriage retractile system. An initial corner crack of 0.6 × 0.6 mm was introduced at the pin-loaded hole, with the initial cracked section perpendicular to the loading direction. However, fatigue cracks initiated along the sections about 30° and 159° differentiated from the initial crack section for the specimen with zero clearance copper bushing. For the specimen without bushing at the pin-loaded hole, a fatigue crack initiated along the section about 151° differentiated from the initial crack section, and the initially introduced crack also propagated till specimen fracture. All of the new fatigue cracks originated on the inner face of the lug hole, but grew to quarter- or half-elliptical front depending on the originating position. Microscopic fractography was conducted to find possible reasons for the unexpected cracks.