On June 2, 2005, approximately 0600 Pacific daylight time, the NTSB Northwest Regional Office was notified via email that a significant crack had been found in the right wing spar lower cap of a Piper Malibu PA-46-350P, N27PG, during an annual inspection at Tacoma Narrows Airport, Tacoma, Washington. The crack in the spar cap, which had progressed through approximately 80 percent of the horizontal surface of the cap, was discovered in a follow-up inspection by a maintenance technician at PAVCO Flight Center, after that individual had found loose lower wing skin rivets directly below the area where the crack was located. The aircraft, which previously had been owned by Pacific Gas and Electric Company (PG&E), and had been occasionally used for low-level pipeline visual observation patrol, had accumulated 5,273 hours on its airframe. The current owner, who had operated the aircraft for the last five years, stated that he was not aware of any significantly hard landing since he had purchased the aircraft. A review of the log books did not show any indication of other airframe damage from any previous hard landing, and there was no other entry that indicated that the aircraft had been involved in any accidents or events that might be expected to have produced such a crack from a one-time overload. The wing spar lower cap was removed from the aircraft and submitted to the NTSB laboratory to determine the extent and mode of the failure. The NTSB Materials Laboratory inspection of the lower wing spar cap determined that the crack surface features were consistent with a fatigue crack that had initiated at both the aft and forward surfaces of the aft rivet hole on the horizontal surface of the cap, at a location 28.63 inches from its outboard end. The crack was 1.53 inches in length on the cap's upper surface, and 1.53 inches in length on its lower surface. The surface of the crack was relatively flat, with smooth, well defined, curving arrest lines. There was no evidence of inclusions, discontinuities, corrosion pitting, or material defects at the origins of the crack. All dimensional measurements were within engineering drawing specifications, and the hardness of 81.3 HRB, and conductivity of 39.7 percent IACS were within the typical range for the aluminum alloy specified for the wing spar cap material. During the investigation it was determined that at the time the crack was detected, the aircraft had flown 5,273 hours. Of that total time, approximately 1,100 hours were flown as part of a low-level pipeline patrol mission. It was further determined that as part of the certification process, Piper Aircraft had determined that the "general use" safe time in service (TIS), adjusted with a scatter pattern of eight, was 10,723 hours for the wing structure. The certification process did not include determination of a safe time in service when any portion of the aircraft's life had involved a "pipeline survey" mission profile. The investigation also determined that the two rivet holes through which the crack passed (one of which contained the points of origin) was the location where two of the rivets that held the lower wing skin on in an area addressed by Piper Service Bulletin 796B, Part One. This service bulletin was published by Piper to address "loose and working rivets along the lower spar cap." In part, the bulletin states, "If this condition exists, and is left uncorrected over an extended period of time, the structural integrity of the wing could be compromised." The actions delineated in this service bulletin had not been performed on this aircraft (N27PG) even though loose and working (smoking) rivets were found at the subject location on the lower skins of both wings.