Piper PA 30 N8244Y accident near Salem, Oregon

Probable cause

A fatigue failure of the right main landing gear trunnion.

NTSB analysis

The pilot reported that, after departure, he raised the landing gear and heard an unusual "clunk"; however, he did not observe any anomalies at the time and continued the flight. As the airplane neared the destination airport, the pilot extended the landing gear, but the landing gear did not fully extend. After troubleshooting, including attempting the emergency extension procedure, the pilot landed the airplane with the gear partially extended, during which the airplane sustained substantial damage to the keel beam and the right-wing spar.

Postaccident examination revealed that the right main landing gear trunnion was fractured as the result of two fatigue cracks that had developed at a welded gusset joined to a protruding boss. Once these fatigue cracks had propagated far enough into the boss cross-section, the remainder of the part fractured by overstress, likely during the last landing. The presence of two opposite-facing fatigue cracks was consistent with reverse bending fatigue. The fatigue cracks initiated at a tapered neck-shaped area, where the gusset and trunnion were joined. The cross-sectional thickness at this location was half the thickness of the gusset and a quarter of the diameter of the boss. This configuration would introduce a stress concentration at the locations of the fatigue cracking.

The trunnion boss was consistent with a heat-treated aluminum alloy. The hardness approaching the welded joint decreased, consistent with softening from the heat input. The reduced hardness would increase susceptibility to fatigue crack initiation at the areas of stress concentration.

It is likely that, over time, the fatigue cracks propagated as the joint flexed or bent back and forth, and the stress concentrations at the neck-shaped region were exacerbated by the reduced mechanical properties of that region. The fracture surfaces were damaged after the accident, and, due to the amount of post-fracture damage, no reliable fatigue analysis could be performed.

Text from the NTSB. Open the NTSB docket for WPR20LA252 for the full record.