WPR20LA306

After landing, as the pilot was taxiing back to the runway for another takeoff, the right landing gear collapsed, and the right wing struck the runway resulting in substantial damage. Examination of the landing gear bolts revealed they failed due to reverse bending fatigue cracking. The size of the fatigue cracking area on the fracture surfaces versus the overstress area indicated the fatigue likely grew over time under low-to-moderate stress. The occurrence of the fatigue cracking on all the hex-head bolts and the similarities of the fatigue cracking areas on the fracture surfaces were consistent with some amount of movement on that side of the landing gear, which indicates there was likely an issue with preloading of the bolts.

On August 17, 2020, at 2005 Pacific daylight time, a Piper PA-28-181 airplane, N79MS, sustained substantial damage when it was involved in an accident near Upland, California. The two pilots were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. According to both pilots, the private pilot was undergoing a check flight for his commercial certificate. After the private pilot completed a landing on runway 24 at Cable Airport (CCB), he was taxing back to the runway when the airplane’s right landing gear collapsed and folded under the wing. As a result, the wing dropped to the ground and the airplane slowly veered to the right and skidded to a stop. The right wing sustained substantial damage to the wing spar. Nine pieces of bolts from the collapsed right landing gear were submitted to the NTSB Materials Laboratory for further examination. The pieces included one full round head bolt, one separated round head, two shanks from round-head bolts, two separated hex heads, two shanks from hex heads, and one hex head with a portion of shank still attached. The examination revealed the fracture surfaces of the four separated hex-head bolts were relatively flat and had crack arrest marks (aka beach marks) consistent with fatigue cracking. Small circular areas of rougher features consistent with overstress from the final fracture of the bolts were also observed. Ratchet marks consistent with multiple fatigue cracking initiation sites were observed around the outer diameter of the shanks. The fatigue cracking progressed toward the center of the bolts. Two main areas of fatigue cracking propagating from opposite sides of the bolt outer diameters were observed on the fracture surfaces, which is consistent with reverse bending fatigue cracking. Three of the bolts had one fatigue cracking area larger than the other, which indicated the fatigue cracking on those bolts grew from one side of the bolts initially before the opposite side fatigue cracking began progressing. The fourth bolt had opposing fatigue cracking areas of approximately the same size, indicating the fatigue cracking on the two sides started progressing around the same time.