What happened
During a scheduled annual inspection, maintenance personnel discovered a crack in the lower wing spar cap of the Piper aircraft, registration N27PG. The discovery was reported to the NTSB's Northwest Regional Office via email. At the time the crack was detected, the aircraft had accumulated 5,273 total flight hours, including approximately 1,100 hours of low-level pipeline patrol missions.
The investigation
The investigation focused on a fatigue crack that originated at a rivet hole on the horizontal surface of the right wing lower spar cap. This crack had propagated through roughly 80 percent of that surface. Testing conducted by the NTSB materials laboratory found no evidence of material defects, corrosion pitting, inclusions, or discontinuities at the point of origin. Additionally, all dimensional measurements met engineering specifications, and the hardness and conductivity of the aluminum alloy were within established tolerances.
The investigation also examined the aircraft's maintenance history regarding Piper Service Bulletin 796B, Part One. This bulletin was issued by the manufacturer to address loose and working rivets along the lower spar cap, warning that uncorrected issues could compromise structural integrity. While loose and working rivets were found at the site of the crack on the lower skins of both wings, the specific actions required by this service bulletin had not been performed on N27PG.