Accident Description
On August 17, 2020, at 2005 Pacific daylight time, a Piper PA-28-181 airplane, registration N79MS, sustained substantial damage during a ground incident at Cable Airport (CCB) near Upland, California. The airplane was operating under Title 14 Code of Federal Regulations Part 91 as a personal flight. The two pilots on board were not injured.
According to the pilots, the private pilot was conducting a check flight for his commercial certificate. After completing a landing on runway 24, the pilot was taxiing back to the runway when the right landing gear collapsed and folded under the wing. The wing subsequently dropped to the ground, and the airplane slowly veered to the right before skidding to a stop.
Post-Accident Examination
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 examination. The pieces comprised 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.
Examination revealed that the fracture surfaces of the four separated hex-head bolts were relatively flat and exhibited crack arrest marks (beach marks) consistent with fatigue cracking. Small circular areas with rougher features, consistent with overstress from the final fracture, were also present. Ratchet marks, consistent with multiple fatigue cracking initiation sites, were observed around the outer diameter of the shanks. 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 noted on the fracture surfaces, consistent with reverse bending fatigue cracking. Three of the bolts had one fatigue cracking area larger than the other, indicating that fatigue cracking on those bolts grew from one side initially before the opposite side cracking began. The fourth bolt had opposing fatigue cracking areas of approximately equal size, suggesting that fatigue cracking on both sides started progressing around the same time.