Discovery
During an annual inspection, a crack was identified in the lower wing spar cap of the subject aircraft. The finding was reported via email to the NTSB's Northwest Regional Office.
Investigation
Subsequent analysis determined that a fatigue crack had initiated at a rivet hole on the horizontal surface of the right wing lower spar cap. The crack had propagated through approximately 80 percent of the spar cap surface. The NTSB materials laboratory examined the crack origins and found no evidence of inclusions, discontinuities, corrosion pitting, or material defects. All dimensional measurements were within the engineering drawing specifications, and the hardness and conductivity parameters for the aluminum alloy used in the wing spar cap were within material tolerances.
At the time of detection, the aircraft had accumulated 5,273 total flight hours. Of these, approximately 1,100 hours were flown as part of a low-level pipeline patrol mission.
Certification Considerations
During the investigation, it was determined that Piper Aircraft, as part of the certification process, had established a "general use" safe time in service (TIS) of 10,723 hours for the wing structure, adjusted with a scatter factor of eight. However, the certification process did not include a determination of safe TIS for a "pipeline survey" mission profile.
Service Bulletin
The two rivet holes through which the crack passed (including the origin point) were locations where rivets secured the lower wing skin. This area was addressed by Piper Service Bulletin 796B, Part One, which was published to address "loose and working rivets along the lower spar cap." The bulletin stated: "If this condition exists, and is left uncorrected over an extended period of time, the structural integrity of the wing could be compromised."
Despite the presence of loose and working (smoking) rivets at the subject location on the lower skins of both wings, the actions prescribed in the service bulletin had not been performed on the aircraft (registered as N27PG).